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Institute
- Institut für Physik (160) (remove)
Recent experimental campaigns in the Wendelstein 7-X stellarator, a
plasma-confining device designed to investigate the Magnetic Confinement Fusion
(MCF) approach to generating electrical power, have shown that the injection of
fuelling pellets had an unexpected and considerable impact on the performance of
the plasma. Rather than simply refuelling the device and `diluting' the plasma
energy, pellet injection is followed by a significant increase in the ratio of
the ion temperature to the electron temperature. It has been suggested that this
is not merely due to the improved confinement following the reduction of
turbulent transport after the pellet material has homogenised with the bulk
plasma, but also due to a direct transfer of energy from electrons to ions. The
proposed mechanism for this energy transfer is the ambipolar expansion of the
pellet plasmoid, the localised plasma structure produced by the
ionisation of ablated pellet material, along magnetic field lines.
Early work on pellet plasmoid expansion predicted that half the heating power
deposited in plasmoid electrons by collisions with hot ambient electrons is
transferred to plasmoid ions in the form of flow velocity as the plasmoid
expands. The complicated nature of the system of the pellet plasmoid embedded in
the ambient plasma, particularly the behaviour of electrons, which experience
many collisional and collisionless phenomena on multiple disparate timescales,
means that early models of the expansion were not wholly self-consistent, but
rather made use of strong approximations that apply in some regions of the
plasmoid but not in others. For example, only electrons and ions associated with
the plasmoid were rigorously treated, meaning that the framework was one of
`expansion into vacuum'. Combined with the assumption of Maxwellian electrons,
this led to an electric potential that was unbounded at infinity. Naturally, the
validity of the conclusions of such a model are called into question because the
approximations lose their validity far from the plasmoid and as time advances,
yet predictions about the final state of the plasma are desired. A deeper
investigation is required: careful consideration of the phenomena in question
and the timescales (and lengthscales) on which they act must be made in order to
rigorously construct a model that is valid throughout the entire expansion.
The first two papers presented in this thesis iterate on the model established
in the paper that first predicted the electron-to-ion energy transfer; their aim
was to find out how the character of the expansion changes with a more
sophisticated and accurate description of various phenomena, while remaining
within the existing framework of expansion into vacuum. Ultimately, we find that
the qualitative character is unchanged, and that approximately half the heating
power deposited in plasmoid electrons is transferred to ions.
Two other papers in this thesis address the limitations of the original model.
This is achieved by properly considering the electron kinetic problem in a
plasmoid. One paper considers the electron kinetic problem when electrons are
highly isotropised. In this case the kinetic equation can be integrated to
remove all but two independent variables, which is the maximum possible
reduction considering it is a time-dependent problem. The full nonlinear
integro-differential Landau self-collision operator is integrated exactly and
few approximations are made, leading to a rather general kinetic equation.
However, for fuelling pellets some anisotropy in the electron distribution is
expected. Another paper considers the electron kinetic problem (and the entire
plasmoid expansion) allowing for electron anisotropy. Careful consideration of
the ordering of timescales of electron phenomena in a pellet plasmoid leads to a
steady-state kinetic problem that we call collisional quasi-equilibrium (QE). QE
appears in many ways similar to the collisional steady-state characterising a
true thermal equilibrium. It was found that the time-dependent kinetic problem
of the earlier paper, with isotropic electrons, produces the QE distribution
function, corroborating the existence of the QE state. We then take moments of
the electron kinetic equation that is valid on the expansion timescale, assuming
that the electron distribution is that given as the solution to the QE kinetic
problem. This is completely analogous to what is done to obtain the Braginskii
equations or any Chapman-Enskog theory. The result is a set of equations for the
long-term evolution of the macroscopic quantities that describe the distribution
function existing in a quasi-steady-state at each point in time. It is from this
point that one may feasibly describe the plasmoid expansion with an accurate
picture of the electron kinetics and finally obtain the electron-to-ion energy
transfer so desired in a rigorous model of the expansion.
From a broader point of view, the two frameworks provided by these rigorous
investigations of the electron kinetic problem serve as a basis for the future
study of plasmoids. Such a `first-principles' approach to plasmoid dynamics is
novel and interesting in its own right, but it will be demonstrated that such an
approach is essential for pellet plasmoids owing to the fact that they are
poorly described by the `standard tools' of plasma physics.
Using the QE framework it was found that, once more, about half the heating
power experienced by plasmoid electrons is transferred to plasmoid ions. The
incredible robustness of the prediction of such an energy transfer is, in the
author's opinion, the result of the self-similar nature of the expansion found
as a solution to the original model. As a rule, the profiles of self-similar
solutions tend to be attractors for the `real', more complicated, system, and
the qualitative predictions involving no parameters, of which the
electron-to-ion energy transfer is one, tend to be very sturdy.
Aside from fuelling pellets, composed of hydrogen or deuterium, one paper in
this thesis investigates the physics of high-Z pellets that are designed to
terminate the plasma safely in the event of a `disruption', where much of the
magnetic field energy is channelled into a runaway electron beam with
potentially disastrous consequences if the beam encounters a plasma-facing
component. The paper draws on the work carried out in the paper concerning the
kinetic problem of isotropised electrons in a plasmoid.
This thesis is `cumulative'; the vast majority of the work carried out is
described within a set of Papers, labelled A-E, placed at the back of the text.
There is a preceding `wrapper text' (given in numbered Sections) tasked with
introducing the reader to the topic, guiding the reader through the papers, and
expounding some of their main results. Some amount of material not present in
the papers is also provided in the wrapper text. Naturally, the wrapper text
mainly focusses on the results of the papers which are under my first
authorship. In the course of publishing papers over an extended period of time
the nomenclature is bound to vary. Although it is mostly consistent between the
papers, a few difference do arise, and the section `Common symbols and
subscripts' is provided in the frontmatter to alleviate confusion. Particular
care should be taken with the symbols x and z; both can refer to the
coordinate parallel to the magnetic field line, but in papers where z is used
for this purpose x tends to have another definition. In the wrapper text the
choice of symbols is generally chosen to reflect those in the corresponding
paper.
The combination of the Layer-by-Layer (LbL) method, a nano-material such as carbon nanotubes (CNTs), and charged polyelectrolytes (PEs) is a reliable approach to produce highly functionalized surface coatings. These coatings are stable, controllable, ultra-thin, and most importantly, biocompatible. The ability to tune their properties by varying the preparation conditions and the terminating layer opens up a wide range of applications in the fields of biology and medicine. Here, the goal was to create electrically conductive coatings on which cells grow and proliferate. To achieve this goal, a coating with a stable conductive film structure, a suitable film surface topography, and suitable surface potential (and 𝜁-potential) must be prepared.
At the beginning of this thesis, the focus was on the fabrication of electrically conductive multilayer films, whose electrical properties should be stable and adjustable in a controlled manner (Article 1). The combination of chemically modified CNTs as polyanions, a strong linear polycation like poly(diallyldimethylammonium chloride) (PDADMA), and the LbL-method allowed us to prepare such films. Their characterization was carried out in air at ambient conditions. Since PDADMA is non-conductive, the charge transfer within the film and thus the electrical conductivity itself depends mainly on the CNTs and their arrangement. It was found that four CNT/PDADMA bilayers (BL) were always necessary to create a lateral network structure with multiple CNT crossing points to enable and support electron transport within the film. Moreover, additional CNT/PDADMA BL resulted in decreasing sheet resistance, while the conductivity remained constant at ≈ 4 kS/m regardless of the number of bilayers. Increasing the PDADMA molecular weight (Mw) from 44.4 kDa to 322 kDa did not affect film properties such as thickness or electrical conductivity.
However, increasing the CNT concentration from 0.15 mg/ml to 0.25 mg/ml in the deposition suspension resulted in thicker and less conductive films. This is attributed to a faster adsorption process of the CNTs leading to more adsorption sites for the polycation. We found an increased PDADMA monomer/CNT ratio compared to films prepared with the lower CNT concentration in the deposition suspension. The electrical conductivity decreased by a factor of four down to 1.1 kS/m, which can be attributed to fewer contact points between the CNTs. Overall, we were able to prepare stable and electrically conductive multilayer films. Additionally, by varying the preparation conditions tuning of the electrical conductivity is possible.
To fulfill requirements regarding i.e., medical implants, film properties not only have to be stable and controllable in a dry state (described in Article 1) but also in a biological aqueous environment. Therefore, in Article 2 we immersed our coated samples in three different solutions usually employed in biological research and compared their properties with their dry state, respectively. Also, hydration/swelling effects that normally occur for polyelectrolyte multilayer films (PEMs) in solutions were investigated.
For the film preparation, PDADMA (Mw = 322 kDa) and a deposition suspension of modified CNTs with two different concentrations (0.15 mg/ml and 0.25 mg/ml), which aged for two years, were used. Independent of the CNT suspension concentration, it turned out that the film thickness of the samples, prepared from the aged suspension, decreased significantly compared to the film thickness previously measured in Article 1. As a cross-check a new and fresh CNT suspension was made, which allowed us to reproduce the film thickness described in Article 1.
These results indicated that something happened with the CNT suspension over a two-year period. An analysis via X-ray photoelectron spectroscopy (XPS) showed a decrease in the percentage of functional groups in the CNTs from the aged suspension. The loss of functional groups resulted in less negatively charged CNTs and thus in fewer adsorption sites for the polycation PDADMA. Consequently, the PDADMA monomer/CNT ratio decreased, which lowered the thickness per bilayer by a factor of three, compared to films prepared with a freshly prepared CNT suspension. The lower linear charge density of the aged CNTs also enhanced their hydrophobicity, which is, in combination with the electrostatic forces, another important factor for multilayer cohesion. In contrast to PEMs made from polycations and polyanions, no swelling of the films occurred when immersed in solutions. This can be attributed to the fact that the increased hydrophobicity of the CNTs and the hydrophobic nature of the PDADMA backbone prevent the incorporation of water into the multilayer film. In solution, the films slightly shrink (by ≈ 2 nm), which makes them even more compact. Yet they remain stable. The result is an increased electrical conductivity from 9.6 kS/m, in the dry state, up to 15.3 kS/m immersed in solutions. To summarize, we showed that by tuning the interpolyelectrolyte forces the swelling and the ensuing decrease of the electrical conductivity of the films can be prevented.
Regarding the application in biology and medicine, we must consider that long-term exposure of cells to nano-materials like CNTs could lead to damage and inflammation of adjacent tissue. Therefore, it is necessary to prevent direct contact between the electrically conductive multilayer, i.e., CNT/PDADMA film, and the cells. The solution to this problem is a biocompatible top film that covers the CNT/PDADMA multilayer completely and still provides a lateral surface structure that supports cell adhesion and proliferation. Additional layers consisting solely of PEs could provide such a top film.
In Article 3 we investigated the self-patterning of PEM films as function of deposition steps. After preparation in water, the films were dried, characterized in air, and in vacuum. The films were built with high and low molecular weight PEs. PDADMA was used as polycation and poly(styrene sulfonate) sodium salt (PSS) as polyanion. The observation via Atomic Force Microscopy (AFM) showed that films prepared with high molecular weight PEs are laterally homogeneous and form no patterns, due to the chain immobility. The flat surfaces are ineligible as a substrate for cell adhesion.
In contrast, films built with a short PSS, especially at Mw, PSS = 10.7 kDa, began to self-pattern after seven deposited PDADMA/PSS bilayers. With each additionally deposited bilayer, the surface got more and more structured, from grooves over stripes to circular domains. Increasing film thickness led to an increased lateral mean distance between the surface structures. Scanning Electron Microscopy (SEM) images showed that exposure to a vacuum resulted in a decrease in the film thickness attributed to water removal, while the mean distance between the domains increased. Thus, by using this self-pattering process we are able to prepare PEMs with a highly structured surface. By adding PDADMA/PSS bilayers, not only the CNT/PDADMA film can be covered completely, but also a suitable surface morphology for cells can be created. Controlling the number of deposited bilayers allows the preparation of suitable coatings for cells.
To further improve the interaction of the cell and coated substrate not only the lateral structure but also the interacting electrostatic forces between cells and substrate are important for the nature of cell adhesion, function, and proliferation. In Article 4 we investigated PEMs, consisting of strong PEs with a low (PDADMA) and high (PSS) linear charge density. We performed asymmetric force measurements with the help of the colloidal probe technique (CP). Here, the forces between a PEM-covered surface and a colloidal probe (silica sphere) glued to a cantilever were investigated. The colloidal probe was either bare or covered with polycation poly(ethylenimine) (PEI). The surfaces were immersed in NaCl solutions with different ionic strengths (INaCl), starting with deionized water, then enriched up to 1 mol/L NaCl. The interaction force between a CP and the surface was measured. Thus, insight into the surface potential/charge was obtained.
During film preparation, two growth regimes (parabolic and linear) exist. These regimes and the terminating layer determine the surface force of the PEM. PEMs with a terminating PSS layer are predominantly flat and negatively charged when the ion concentration is low and the film is in the parabolic growth regime (between 1 and ≈ 15 BL). This indicates charge reversal on PSS adsorption. At the transition point between the parabolic and linear growth regimes, the ratio between polyanion and polycation monomers starts to switch and some cationic monomers are neutralized not by anionic monomers but by monovalent ions. Therefore, the surface charge density in diluted NaCl solutions changed from slightly positive near the transition to positive in the linear growth regime. At the lowest ionic strengths (INaCL) the range of the surface potential goes from – 40.5 mV (9 BL, parabolic) up to + 50 mV (19 BL, linear).
In contrast, polycation (PDADMA) terminated films are overall positive in diluted NaCl solutions. At the beginning of the parabolic growth regime, the layers are more compact and flat. However, with each additional layer deposited, the film becomes less compact and the chains begin to loosen. The now more loosely bound chains start to protrude into the solution and form pseudo-brushes. This could already be observed for 10.5 BL.
It intensifies in the linear growth regime (begin at ≈ 15 BL) and results in steric surface forces. Changing the surrounding INaCl affects this behavior and the pseudo-brushes scale as polyelectrolyte brushes.
By controlling the number of bilayers (thus the growth regime), the surrounding ionic strength, and the conformation of PEs at the PEM surface, it is possible to prepare a suitable range of surface properties i.e., for cell adhesion and proliferation. To prove that these multilayers can provide a suitable surface and have a positive effect on cell behavior, we coated in Article 5 titanium-covered samples with PEMs. Investigated was the cell interaction with the surface at different zeta(ζ) - potentials, a parameter for dynamic surface potential. Here the cell activity is measured by the mobilization of calcium (Ca2+) within the cell as a function of the ζ - potential of the substrate and the externally applied electrical potential. The cell activity indicates if the ζ - potential, provided by the sample surface, is suitable or not for the cells. The favorable interaction with the substrate is also reflected in the cell morphology and proliferation. The results showed that highly negative ζ - potentials between - 90 and - 3 mV led to a decreasing/reduced Ca2+ mobilization which correlates with reduced cell activity. Nearly neutral to moderate positive surfaces (ζ - potential + 1 to + 10 mV) i.e., PSS-terminated PEMs are able to promote cell adhesion and growth as demonstrated by an increased Ca2+ mobilization. The access to the intracellular Ca2+ stores, provided by the external stimulus, is now more effective and suggests a higher cell activity. Increasing the ζ - potentials up to ≈ + 50 mV (highly positive), i.e., PDADMA - terminated PEMs with pseudo-brushes, resulted in restricted cell viability and impaired Ca2+ mobilization, which led to a disturbed cell morphology and proliferation. In conclusion, only surfaces, terminated with i.e., PEI, with moderate positive charges (ζ - potential + 1 to + 10 mV) are able to improve the Ca2+ mobilization and thus the cell activity and proliferation. PEMs with a PSS termination provide negative 𝜁−potentials, onto which cells adhere, and proliferate. Therefore, they are a good alternative for surface functionalization for implant surfaces. In summary, the objective set at the beginning of the thesis is addressed within articles written as part of this thesis. It is possible to fabricate PEMs with modified CNTs to produce coatings that are electrically conductive with tunable sheet resistance, whether dry in air or immersed in an aqueous solution (Articles 1 and 2). Also, for pure PEMs, it is shown that with the right molecular weight of PEs and a certain number of bilayers, a suitable surface structure for cell adhesion can be produced (Article 3). Additional surface properties such as a suitable surface charge density can be provided by PEMs which can improve the cell activity as monitored with Ca2+ mobilization (Articles 4 and 5). The next step is to combine the knowledge gained from Articles 1 – 5 and link it to the application of external electrical fields to cells.
In course of the recent results from Wendelstein 7-X, stellarators are on the brink for assessing their maturity as a fusion reactor. To this end, stellarator specific transport regimes need detailed exploration both with appropriate systematic experimental investigations and models. A way to enhance the efficiency of this process is seen in an systematic evaluation of existing experimental data. We propose appropriate tools developed in information theory for examining large datasets. Information entropy calculations, that have proven to assist the systematic assessment of datasets in many other scientific fields, are used for novelty detection.
Potentially, as a first use-case of this holistic process, this thesis attempts to link and to develop approaches to examine the stellarator specific core-electron-root-confinement (CERC) regime. The specific interest for CERC emerges from the behavior of the radial electric field. While ion-root conditions exhibit negative radial electric fields, CERC’s positive field in the very core of fusion grade plasmas adds an outward thermodynamic force to high-Z impurities and could add to potential actuators to control impurity influx as to be examined for full-metal wall operation in large stellarators. Recently, this feature received revived intent for reactor scale stellarators.
Also, in this work, parameter regions close to the transition from ion-root to CERC are
examined. At lower rotational transform (a characteristic feature of the magnetic field confining fusion grade plasmas), transitions were detected when the plasma current evolved. As in smaller stellarators, it is concluded that low-order rationals and magnetic islands are related to the transitions. This is widely supported by extensive MHD simulations which finally provide indications for the role of zonal flow oscillations. As one of the outcomes, gyrokinetic instabilities are seen interacting for the first time with the neoclassical mechanisms in experiments.
In order to cope with the vast number of highly sampled spatio-temporal plasma data, new
techniques for novelty detection are required. Fundamental prerequisites for the detailed
physics investigations were the feasibility study of entropy-based data analysis techniques, and their adaptation to detect previously unrevealed transition mechanisms. These tools were applied to multivariate bulk plasma emissivity data, which allowed the exploration of large parameter spaces and provided insights in the spatio-temporal dynamics of CERC transitions.
In this manner, this research highlights the feasibility of information flow measure analysis in fusion studies. Applications of different entropy-based complexity measures are explored and this work sheds light on the capabilities, added value and limitations of these techniques. This investigation presents the integration of information flow measures to gain deeper understanding of plasma transport phenomena, by providing an approach to fast systematic data mining suited for real-time analysis. This work paves the way for further development and implementation of information-theoretic methods for plasma data analysis.
In summary, this research highlights the gained insight on CERC transitions, while showcasing the feasibility, added values and limitations of information flow measure analysis for fusion studies, to induce theory based analysis revealing new insights in fundamental, stellarator-specific transport mechanisms.
Research into nuclear physics has enjoyed a long and rich history since the earliest experiments began investigating atomic constituents. The discovery of the atomic nucleus in the early 20th century started a complex field of research that has undergone many transformations with the advancements of modern technology. Today, atomic nuclei are not only studied to advance our understanding of the strong force but also to gain more information on the synthesis of elements in the universe, to exploit nuclear decay to investigate the weak interaction, and to search for physics beyond the standard model.
In this work, we will study the strong force in atomic nuclei, i.e. the way nucleons (protons and neutrons) arrange themselves in a many-body system governed by the repulsive Coulomb interaction and the attractive strong interaction. In particular, we will focus on nuclear structure near nuclei with a "magic number" of Z protons and N neutrons, so-called doubly-magic nuclei, exhibiting a particularly stable configuration with respect to neighboring nuclei.
Within the nuclear shell model, similar to the atomic shells, the magic numbers indicate shell closures accompanied by energy gaps. Nuclei at double-shell closures and their direct vicinity provide an important playground to benchmark nuclear theories and models that aim to predict the intricate interplay of the nucleons that lead to enhanced nuclear binding energies, significant changes in charge radii and transition strengths, etc.
Of particular interest are nuclear isomers, long-lived excited states, in which the nucleon configuration with respect to its ground state is altered, resulting in a modification of their properties despite having the same number of protons and neutrons.
The main part of this work consists of three publications, which report on nuclear structure investigations through mass measurements and laser spectroscopy near the doubly magic nuclei nickel-78, tin-100, and lead-208.
The nuclides investigated in this work include neutron-deficient indium isotopes, neutron-rich zinc isotopes, and neutron-rich mercury isotopes.
This thesis presents the production of polyanionic clusters within two ion storage devices:
Considering a Penning trap, the accessible range of polyanionic aluminium clusters has been expanded up to the 10th charge state. In particular, abundance curves for clusters with 5 to 9 excess electrons have been measured for the first time and analysed with respect to their lifetime-dependent appearance sizes. These sizes reveal a nearly quadratic dependency on the charge state for experimentally accessible lifetimes.
Additionally, the production of polyanionic clusters has been enabled in a radiofrequency ion trap. Therefore, the transition from a harmonic to a digital 2- and 3-state guiding signal has been investigated with respect to the ion storage. The passing of electrons through the trap during field-free periods of the guiding signal led to the first production of polyanionic clusters within a radiofrequency ion trap.
In this thesis, I was able to provide answers to transport processes in lipid monolayers, which are ultimately, all of biological relevance. In particular, I was interested in lipid oxidation and dynamic compression/expansion processes of surfactant monolayers at the air-water interface:
Lipid oxidation was shown to be a consequence of the formation of a high concentration of reactive oxygen species (ROS) during cell respiration, which finally can lead to severe cell damage. It is not yet understood clearly, which part of the lipid molecules is especially prone to a ROS attack. I was particularly interested in the role of the double bonds of the acyl chains of the lipid molecules during oxidation. Further, I wanted to know the time scales of lipid interaction with the ROS.
Compared to lipid vesicles, lipid monolayers have the advantage that many parameters of the system can be adjusted easily. In our system, I made use of this by setting the lateral pressure to low values during H2O2 treatment, which facilitated the ROS to reach the double bonds in the acyl chains.
A prime example of biological systems out of thermal equilibrium was given in the alveolus surface, which is covered with a surfactant monolayer. During breathing, these monolayers undergo such a highly dynamic compression and expansion. Arising flows from breathing could disrupt a film and consequently, it would lose its protective role. One of my goals was to understand flows and their influence on domain shape. Dependent on the strength of the flows, I expected different growth regimes, with differing prevailing transport processes. Once understanding the underlying mechanisms in domain shaping would allow me to draw conclusions on biological systems.
In order to address these questions, I established two systems, both based on the compression of lipid monolayers. I used isotherms to study the phase behavior of the lipids:9 During compression, the lipids can undergo phase transitions from the gaseous phase to the liquid expanded phase (LE-phase) and further from the LE-phase to the liquid condensed phase (LC-phase). A coexistence regime is observed in between the LE-phase and the LC-phase, characterized by a flat increase of lateral pressure with decreasing molecular area. Some lipids exhibited LC-phase domains. These were further investigated with Brewster angle microscopy (BAM). The used BAM was equipped with an integrated Scheimpflug optics, enabling an overall focused image plane. Furthermore, time-resolved observation of the growth of the domains was possible by recording videos (20 frames per seconds).
The first system enabled the investigation of lipid peroxidation, when the lipids were exposed to ROS. I chose DMPC, POPC, DOPC and PLPC, since these are phospholipids differing in the number and position of double bonds in acyl chains, but not in the head group. I used a H2O2 enriched phosphate buffered saline (PBS) solution, which served as a precursor for more reactive ROS, like hydroxyls (.OH). PBS was chosen, since it resembles the cell environment best. During defined waiting times of H2O2 treatment, the ROS diffused vertically from the subphase towards the monolayer. The lipid molecules were in the LE-phase, which facilitated the ROS molecules to reach also the double bonds of the acyl chains. The oxidized monolayers were then compressed at constant compression speed. Since the corresponding isotherms could be measured with high precision, the relative area increase δA/A between oxidized and non-oxidized monolayer along the isotherm proved to be a good measure for lipid peroxidation. The area increase δA in the molecular area of the oxidized molecules was explained by the eventually added, more hydrophilic −OOH group at the position of a carbon atom adjacent to a double bond in the unsaturated acyl chain. The −OOH group is drawn to the hydrophilic head group of the lipid. This leads to a kink in the acyl chain, which increases the molecular area A by δA. A model, which explained this peroxidation process in lipid vesicles, could be adopted to monolayers.
I compared the oxidation of phospholipids, differing in the number and position of the double bonds of their acyl chains. I found that δA/A increased with the growing number of double bonds in one acyl chain. However, a comparison of DOPC with POPC also showed the importance of the position of the acyl chain. I determined a slow reaction kinetic. It could be estimated by a √t dependence of the number density N_surface, which denominates the ROS sticking on the monolayer. The transport of ROS towards the monolayer was found to be diffusive, because it was the slowest process in the reaction. This interpretation was reinforced by a comparison of the temperature dependence of the relative area increase δA/A with the Stokes-Einstein diffusion coefficient of water molecules. The initial ROS concentration c_0 in the trough could be traced back (c_0~ 50 nM), which is indeed a realistic value found in human cells.
Concluding, our results can be understood as a feasibility study. The complexity of the monolayer can be arbitrarily increased, for example by the addition of proteins, allowing the investigation of other oxidative processes occurring in the cell membrane.
The second system allowed the investigation of growth of LC domains during fast compression processes of monolayers. I chose erucic acid monolayers, due to its low line tension and a continuous nucleation phase, enabling the formation of fractal domains. The monolayers were investigated with isotherms and BAM videos. Since v_C (compression speed of the monolayer) was continuous over the whole compression time, I had a system with well-defined hydrodynamic conditions. This allowed me a complete analysis of the system, starting with descriptive features of the observed domains to a classification of the observed growth regimes by means of hydrodynamic theory, through to the distinction and quantification of different kind of flows and supersaturations, involving Ivantsov theory:
Dependent on the compression speed v_C, I observed seaweed or dendritic domains. The LE/LC phase transition pressure pi_t was slightly increased compared to pi_inf of the equilibrium isotherm. A high compression speed v_C induced a supersaturation Δc. I introduced the excess lateral pressure Δpi=pi-pi_inf as an appropriate quantity to describe the supersaturation Δc. I showed a linear behavior of Δc on Δpi. Δc is a macroscopic quantity since it is averaged over the whole monolayer area. I characterized the domains of the seaweed and dendritic regime with respect to tip radii, branch lengths, side branch separations and fractal dimensions. I calculated the growth speed of the main branches. A roughly doubling of the growth speed of dendritic domains, compared to seaweed domains was observed. This was an evidence of adjunctive (Marangoni) flow in the subphase.
For each monolayer, I observed drifts during domain growth, which I explained by an anisotropy in the LE-phase, caused by the continuous nucleation of the domains. These kind of surface flows were superimposed to bulk flows in the subphase. Since I had a well established system, I could analyze the influence of these surface flows on domain shape, in terms of magnitude, direction and duration of the surface flows. I therefore used FFT spectra and directionality histograms. At low flows, the FFT showed six-fold symmetry. Higher drifts exhibited incisions in the FFT, eventually leading to dumbbell shaped FFTs at very high drifts. The domains grew preferentially in the direction parallel to the incision.
I used directionality histograms to analyze the angular distribution of the growing domains. They showed that the drift direction always correlated with a minimum in the histogram. In order to analyze drift duration, I split the domain in downstream and upstream side. I could show that for small drift durations, downstream growth was preferred. However, for longer drift durations, the flows got more isotropic and consequently growth was more balanced then.
I could observe only a weak correlation between drift velocity v_D and compression speed v_C. However, dendrites were formed when the compression speed v_C was high, while seaweed domains were formed when v_C was small. Domain distortion occurred in the same way, independent if seaweed or dendritic domains were considered. I further showed that hydrodynamic flows in the subphase and surface flows are superimposed and scale differently. Consequently, they have different impact on domain shape: hydrodynamic flows act on μm scale and influence the domain morphology (distance between side branches, and tip radius) and the growth speed of the main branches. Surface flows act on the mm to cm scale, cause an anisotropic flow in the LE phase surrounding the domain, and thus affect the overall domain shape.
The anisotropy in the LE-phase led to a locally different degree of supersaturation. To take this into account, I introduced a local normalized supersaturation Δ, based on the Ivantsov solution. Therefore, I calculated Péclet numbers p of measured quantities of the system. I obtained values of 0.88 ≤Δ≤0.90 for the seaweed regime (p<5) and 0.93 ≤Δ≤0.96 for the dendritic regime (p>6). Since the Ivantsov solution can only be applied for purely diffusive processes, I applied a modified Ivantsov solution Δ_mod, which calculates Δ at a distance 𝛿 ahead of the dendrite tip. I was able to determine the progression of the diffusive layer 𝛿, however a quantitative determination failed.
Applying hydrodynamic theory allowed me to classify the two growth regimes with respect to the Boussinesq number Bq. Since for both growth regimes, I achieved values of Bq<1, bulk viscous losses dominated over surface viscous losses. Further, a cross-over length 𝜉 was calculated, from which one can distinguish, whether advective transport dominates over diffusion.
I further connected the two defined supersaturations Δ and Δc via the excess lateral pressure Δpi. From this, I saw differences in the seaweed and dendritic growth regimes: The local normalized supersaturation Δ of seaweed growth seemed to be quite stable for a further increase of the lateral excess pressure Δpi, whereas it reacted quite sensitive in the dendritic regime. This was found to be an indication of a non-equilibrium regime, caused by the strong coupling of the monolayer to the subphase. It reinforces therefore the theory of Marangoni-flow.
The findings of this thesis emphasize the importance of understanding highly dynamic compression/expansion processes arising in surfactant monolayers. Using the example of the compression of the alveolus surface, it can be seen that a more realistic model of the pulmonary alveolus is not only enabled by increasing the complexity of the surfactant monolayer (e.g. by adding specific proteins or lipid mixtures to the monolayer). Equally important is the understanding in transport processes and the consequences for the monolayer structure. By the analysis of domain shapes, I presented a method, which is suitable for such a study.
Graphene is a strictly two-dimensional honeycomb lattice of carbon atoms whose low-energy charge-carrier dynamics obey the massless pseudospin-1/2 Dirac-Weyl equation (or chiral Weyl equation) where the chiral centers (or valleys) are the corners K and K‘ of the Brillouin zone. The linear spectrum near the Dirac nodal points lends graphene its exotic and ultra-relativistic properties.
However, condensed matter systems can possess fermionic excitations with linear dispersions that have no analog in high-energy physics since the crystal space group - instead of the Poincare group - constrains the energy dispersions. Perhaps the first example in this regard is the T_3 lattice (Dice Gitter), a honeycomb-like lattice with an extra atom placed at the center of each hexagon and coupled to only one of the sublattices. The spectrum features a strictly flat band that crosses the two conical intersections of the Dirac cones at K and K' inherited from graphene. The enlarged pseudospin-1 Dirac-Weyl equation describes the low-energy dynamics. By rescaling the transfer amplitude of the additional atoms in the T_3 lattice with a parameter 0<α<1, the resulting α-T_3 lattice continously interpolates between graphene and the T_3 lattice.
In this work, we explore the behavior of generalized Dirac-Weyl quasiparticles in external magnetic and valley-dependent pseudoelectromagnetic fields induced by out-of-plane strain. First, we studied Dirac-Weyl quasiparticles in external fields confined to circular quantum dots by generalizing the infinite-mass boundary condition to the α-T_3 lattices. We verified the analytically derived valley-anisotropic eigenstates of the quantum dot by numerically solving the tight-binding lattice-model in closed (isolated) and open (contacted) systems.
Second, we considered strain fields in the α-T_3 lattices to modify the low-energy transport properties by an effective pseudo-gauge field with opposite signs at the K and K‘ valley. In particular, we showed that the inhomogeneous pseudomagnetic field generated by Gaussian out-of-plane strain at the center of a four-terminal Hall bar setup acts as a valley filter. Most interestingly, the valley polarization is most dominant when incoming electrons are excited to pseudo-Landau level subbands. These bands are linked to different iso-field orbits encircling the lobes of the pseudomagnetic field. Addittionaly, any intermediate α breaks the inversion symmetry of the α-T_3 lattice and thus splits the pseudo-Landau levels into sublattice-polarized bands.
Third, we equipped the out-of-plane strain with a time-periodic drive to induce a valley-dependent pseudoelectric field perpendicular to the pseudomagnetic field. We assessed the steady-state transport properties and found – besides the static regime for small energies – two α-dependent valley-filtering regimes due to the periodic drive. Firstly, we found an additional valley-polarization plateau at the Floquet-zone boundary between the central and first Floquet copy that also displayed a “flower”-like pattern in the local density of states. Secondly, we detected a series of transmission gaps at the center of every Floquet sideband 2mΩ related to the Floquet coupling of the flat band with the central Floquet copy. Under certain strain parameters, a novel valley-filtering regime appears near the transmission gaps where the incoming K electrons are focused through the bump by the pseudoelectric field, instead of encircling the lobes of the pseudomagnetic field. A stability analysis demonstrated that the polarization regimes are tunable by the driving frequency.
Lastly, we demonstrated that the flat band in the Haldane-dice lattice modified by a uniaxial strain along the zigzag orientation remains singular at all band crossings where the model undergoes a topological phase transition between C=+-2 and C=0. To show this, we computed the compact localized eigenstates and the quantum distance of the Bloch wave function around the band-touching points. We derived the resulting non-contractible loop states and an extended state whose components are tunabe by the system parameters.
Ion traps such as Paul traps and MR-ToF (multi-reflection time-of-flight) devices are indispensable tools at radioactive ion beam facilities for the preparation of high-quality radioactive ion beams for subsequent experiments or for precise measurements of the properties of radioactive ions, such as nuclear binding energies or nuclear charge radii.
Within the work of this thesis, Doppler- and sympathetic cooling is implemented in a linear Paul-trap cooler-buncher enabling a reduction of the longitudinal emittance of radioactive ion beams resulting in a significant improvement of the ion beam quality. Moreover, a next-generation MR-ToF device is conceptualized in order to achieve isobaric pure beams with a higher ion intensity than state-of-the-art MR-ToF devices can provide. Once fully constructed and commissioned, it will operate at an unprecedented ion beam energy of 30 keV. Both of these advances are expected to become important for a wide range of experimental programs pursued at low-energy branches of RIB facilities ranging from fundamental symmetry studies, nuclear structure, rare isotope studies with antimatter, searches of physics beyond the standard model to material science and the production of medical isotopes.
The next-generation MR-ToF mass separator is based on MIRACLS’ 30-keV MR-ToF device for highly sensitive and high-resolution collinear laser spectroscopy. By storing the ions in the Multi Ion Reflection Apparatus for Collinear Laser Spectroscopy (MIRACLS), the same ion bunch is probed by a spectroscopic laser for thousands of times compared to a single passage in traditional collinear laser spectroscopy (CLS). Dedicated simulation studies show that the accuracy and resolution will be close to traditional single-passage CLS while the sensitivity is significantly enhanced. Hence, measurements of nuclear properties via fluorescence-based CLS of very rare radionuclides as well as highly sensitive and high-precision measurements of electron affinities via laser-photodetachment-threshold spectroscopy of negatively-charged (radioactive) ions will become possible.
First measurement campaigns employing MIRACLS’ 1.5-keV MR-ToF device confirm the outstanding boost in signal sensitivity and provide confidence in the application of the MIRACLS technique for the measurement of scarcely produced radioactive ions that have been so far beyond the reach of conventional techniques. Furthermore, the electron affinity of 35Cl was measured, which is in perfect agreement with the literature value. These measurements will serve as important benchmarks for modern atomic and nuclear theory, especially in its description of nuclear charge radii.
In summary, the implementation of Doppler and sympathetic cooling at RIB facilities, the conceptualization of a 30-keV MR-ToF apparatus for highly selective and high-flux mass separation as well as for highly sensitive and high-resolution fluorescence-based laser spectroscopy and the expansion of the MIRACLS technique for the study of negatively-charged ions will enable unprecedented new measurement opportunities at RIB facilities.
Polyelektrolyt-Multischichtfilme (PEMs) werden durch schichtweise (eng. Layer by Layer, LbL)
sequentielle Ablagerung von entgegengesetzt geladenen Polyelektrolyten auf einer
geladenen Oberfläche hergestellt. Die LbL Methode kann auf verschiedene Weise zur
Herstellung von PEM eingesetzt werden, z.B. durch Tauchen, Rotation, Sprühen oder
Beschichten mit elektromagnetischen und fluidischen Methoden. In allen Artikeln dieser
Dissertation wurde die Tauchmethode verwendet. Durch zyklische Wiederholung der
Abscheidungsschritte kann die Dicke der PEM leicht gesteuert werden. Die Oberflächen und
Grenzflächen des Films können mit der LbL Technik auch durch die elektrostatische
Wechselwirkung zwischen positiv und negativ geladenen Polyelektrolyten modifiziert werden.
Auf diese Weise lassen sich einige Eigenschaften des Films optimieren, beispielsweise
Oberflächenadhäsion und Biokompatibilität, z. B. in der Gewebezüchtung oder es kann
eine Monoschicht als Barriere an der Grenzfläche des Films adsorbiert werden, um die
Diffusion von Molekülen im Film zu begrenzen z.B. bei Aufnahme oder Freisetzen von
Medikamenten.
Daher wurde die Rolle einiger Faktoren, wie die molare Masse der Polyelektrolyte und das
Vorhandensein von Salzionen in der Präparationslösung auf die interne Struktur sowie die
Oberfläche der PEMs untersucht.
Für alle Untersuchungen dieser Dissertation wurde das häufig verwendete Modell-System aus
dem positiv geladenen Polyelektrolyten Polydimethyldiallylammonium (PDADMA), und dem
negativ geladenen Polyelektrolyten Polystyrolsulfonat (PSS), verwendet. Die Dicke der Filme
wurde mit Röntgenreflektometrie, Ellipsometrie, UV-Vis-NIR-Spektrometrie bestimmt die
interne Struktur mit Neutronenreflektometrie und die Oberflächentopografie mit Rasterkraftmikroskopie
(eng. AFM) und Rasterelektronenmikroskopie (eng. SEM).
In Artikel 1 wurde mit Hilfe der Neutronenreflektometrie die Struktur des Filmes und die
Diffusion des Polyanions PSS (DPSS) senkrecht zur PEM Oberfläche untersucht. Variiert wurde
die molare Masse des Polykations PDADMA und die Salzkonzentration der
Präparationslösung. PEMs wurden aus drei verschiedenen NaCl-Konzentrationen in der
Abscheidelösung hergestellt: 10 mmol/L, 100 mmol/L und 200 mmol/L. Die Salzkonzentration
in der Polyelektrolytlösung bestimmt die Konformation der Polyelektrolyte während der
Adsorption. Die Ketten werden weniger flach adsorbiert, wenn mehr Salzionen in der
Adsorptionslösung vorhanden sind und die Filme werden dicker.
Die Diffusion nahm mit zunehmender molarer Masse von PDADMA in Filmen, die aus 10
mmol/L, 100 mmol/L und 200 mmol/L hergestellt wurden, um mindestens drei Größenordnungen
ab, denn die Zunahme der Kettenlänge, erhöht den Vernetzungsgrad im Film. Dabei zeigten Filme aus 10 mmol/L (NaCl) mit einer niedrigen molaren Masse von PDADMA
die größte Diffusion (DPSS = 4.9 × 10−20 m2/s). Der Diffusionskoeffizient DPSS als Funktion des
Polymerisationsgrades folgt zwei Potenzgesetzen mit einem Übergang bei einem
Polymerisationsgrad von 288. Bei kürzeren Ketten stimmt der Exponent des Potenzgesetzes
gut mit dem Modell der Sticky Reptation überein. Bei längeren Ketten war der Exponent viel
größer, was vermuten lässt, dass die PSS-Ketten in einem zunehmend komplexen
Polymernetzwerk gefangen sind. Wir verstehen den Übergang als Verschränkungsgrenze für
das untersuchte System.
Bei PEMs, die aus 100 mmol/L hergestellt wurden, konnte kein Potenzgesetz festgestellt
werden. DPSS nahm sprunghaft um drei Größenordnungen ab, wenn die molare Masse von
PDADMA von 45 kDa auf 72 kDa erhöht wurde.
In Artikel 2 wurden die Oberfläche von PEMs aus Polyelektrolyten unterschiedlicher molarer
Massen untersucht. Die Oberflächenrauhigkeit und die Dicke des Films wurden mit
Röntgenreflektometrie und Ellipsometrie bestimmt. Die Oberflächentopografie wurde mit AFM
und SEM aufgenommen. Alle PEMs wurden aus PE-Lösungen mit 0,1 mol/L NaCl hergestellt.
Die Oberfläche der PEM, präpariert aus langem PSS und kurzem PDADMA oder langem PSS
und langem PDADMA, war immer flach. Bei einer Filmzusammensetzung aus langen
Polykationen (Mw (PDADMAlang) = 322 kDa) und kurzen PSS Molekülen (Mw (PSSkurz) = 10,7
kDa) wurden drei Wachstumsregime identifiziert: exponentiell, parabolisch und linear. Im
exponentiellen Wachstumsregime bildet sich nach etwa sieben Beschichtungsschritten von
PDADMA/PSS (eng. bilayers, bl) eine granulare Oberflächenstruktur aus mit einer
Oberflächenrauigkeit von 1,6 nm und einer lateralen Periodizität von 70 nm. Mit zunehmender
Schichtzahl nimmt die Oberflächenrauhigkeit sowie die laterale Periodizität zu. Im
parabolischen Wachstumsbereich aggregieren die Strukturen zu Säulen, mit einer
Oberflächenrauigkeit bis zu 23 nm und einer lateralen Periodizität bis zu 210 nm. Im linearen
Wachstumsregime sind die säulenförmigen Domänen vollständig ausgebildet und die
Oberflächenstruktur ändert sich nicht mehr. Diese Strukturen wurden schon während der
Präparation, bereits vor dem Trocknen beobachtet. Dies zeigt, dass sich die Strukturen
während der Abscheidung von PDADMA/PSS bilden.
Bei Beobachtungen im Vakuum (SEM) war im linearen Bereich die Säulenstruktur bei der
PDADMA terminierten PEM ausgeprägter als bei der PSS terminierten.
Diese Strukturen bilden sich nur im Film mit anfänglichem exponentiellem Wachstum, d.h.
wenn kurzen Ketten durch den ganzen Film diffundieren können. Das legt nahe, dass es für
die Strukturbildung nicht ausreicht, dass der Polyelektrolyt kurz ist, sondern dass es auch
beweglich sein muss. Um dies näher zu untersuchen wurde in Manuskript 1 die molare Masse des PSS variiert. Es
wurden PEMs aus langem 322 kDa PDADMA und kurzem 6,5 kDa und 3,9 kDa PSS
hergestellt und mit den Messungen von PEMs aus 10,7 kDa PSS verglichen.
Die Verkürzung von PSS hat subtile Auswirkungen auf den Filmaufbau und die
Selbststrukturierung. Für PEM aus PSS mit einer molaren Masse von 6,5 kDa konnten nur
zwei Wachstumsregime ermittelt werden: ein exponentielles und ein lineares Wachstumsregime.
Der Übergang vom exponentiellen zum linearen Wachstum erfolgte bei 28
Doppelschichten. Bei PEMs, die aus 3,9 kDa PSS hergestellt wurden, wurde bis zu 29 bl nur
ein exponentielles Wachstum beobachtet. Dies zeigt, dass eine Verringerung der molaren
Masse von PSS das exponentielle Wachstum auf eine größere Anzahl von abgeschiedenen
Doppelschichten ausdehnt. Dies ist auf die zunehmende PSS-Diffusion zurückzuführen.
In allen Filmen wurden Selbststrukturierungen beobachtet. Der Abstand und die Höhe der
säulenartigen Domänen nehmen mit jeder abgeschiedenen PDADMA/PSS-Doppelschicht
deutlich zu. Der durchschnittliche Domänenabstand ändert sich weniger und korreliert mit den
vertikalen Wachstumsregimen. Der Domänenabstand schwankt zwischen 70 nm und 750 nm.
Die größten lateralen Abstände und ein längeres exponentielles Wachstumsregime wurden
mit dem kürzesten PSS (3,9 kDa) erreicht, was auf die hohe Mobilität des PSS zurückgeführt
wird. Die Domänenhöhe ist immer kleiner als der Domänenabstand. Wenn die PEM mit
PDADMA terminiert ist, sind die Oberflächenrauhigkeit und der durchschnittliche Abstand
größer als bei PSS terminierten Filme in Wasser und nach dem Trocknen.
Darüber hinaus wurden zwischen den Domänen Filamente beobachtet. Die Filamente
bestehen aus PDADMA/PSS-Komplexen. Eine mögliche Vermutung ist, dass diese Komplexe
zwischen den Domänen diffundieren und ihren Abstand anpassen.
Die Oberflächenstruktur des Films aus PSS 10,7 kDa zeigt eine symmetrische gaußförmige
Höhenverteilung in allen drei Wachstumsregimen von 5 bis 40 bl. Für die kurze PSS war eine
solche Verteilung nur bis 15 bl (6,5 kDa) bzw. 20 bl (3,9 kDa) zu beobachten. Danach wurde
für 6,5 kDa schiefe Verteilung mit Ausläufern zu größeren Höhen beobachtet. 3,9 kDa PSS
zeigte dann sogar eine bimodale Höhenverteilung.
Die lineare Ladungsdichte von PDADMA ist etwa halb so groß wie die von PSS. Folglich
adsorbiert PDADMA in einer bürstenartigen Konformation. Wenn die oberste Schicht
PDADMA ist, dann ist das PDADMA-Molekül nicht fest an die Oberfläche gebunden. Daher ist
die durch die Oberflächenspannung erzeugte Kraft für PDADMA groß genug, um zu einer
Veränderung der Oberflächenmorphologie und folglich zu einer kleineren Gesamtoberfläche
zu führen.Außerdem sind die Domänen in 1 M NaCl-Lösung stabil, schrumpfen aber in 2 M NaCl enorm,
während ihr Abstand leicht zunimmt.
Diese Untersuchungen zeigten, dass die Mobilität des Polyelektrolyten PSS die
Voraussetzung für den Aufbau einer strukturierten Oberfläche in einem PEM-System aus
PDADMA/PSS ist. Diese Ergebnisse zeigten auch, dass die Verkürzung der Kette der PSS Moleküle
die Herstellung von Filmen erleichtert, deren Dicke und Selbststrukturierung je nach
dem gewünschten Zweck angepasst werden kann. Solche Filme können in der Medizin und
Biologie als geeignetes Substrat zur Optimierung der Adsorption von Zellen und anderen
Molekülen oder als Nanofilter effektiv eingesetzt werden.
In dieser Dissertation konnte ich zeigen, wie die Verkürzung der Kette der PSS-Moleküle zur
Bildung einer lateralen selbststrukturierten Oberfläche führt und wie die zunehmende Mobilität
der PSS-Moleküle die Oberflächenmorphologie signifikant beeinflusst.
Interplay of reactive oxygen species with the mechanical properties of cells and mitochondria
(2023)
Cell mechanical properties are a popular label-free method for understanding basic cellular processes. In this thesis, I used Real-time deformability cytometry (RT-DC), a high-throughput microfluidic technology, to investigate the mechanical properties of cells and mitochondria under various conditions such as increased reactive oxygen species (ROS) levels and the application of different ligand coated gold nano-particles (Au-Nps) effect on cells. Initially, we showed the possibility to measure organelles, cells, and tissue-like structures (spheroids) in a single system by constructing a virtual fluidic channel. We investigated a potential application using cytochalasin D (cyto D) treatment, which revealed increased deformation and decreased stiffness in both the normal and virtual channels. Using mechanics as a marker, I investigated the effect of excessive ROS on the mechanical properties of human myeloid precursor cells (HL60). My findings suggest that the mechanical response of HL60 cells to increased ROS levels is mediated by re-localization of microtubules toward the cell center and F-actin to the cell periphery. Interestingly, I also observed intracellular acidification, which is a largely unexplored mechanism that may have contributed to our findings. I then extended our ROS and mechanics assay to investigate cell-AuNP interactions, demonstrating that cell properties vary depending on the cell culture media and ligand coating. The results showed that dextran coated gold nano-particels (Au-Nps) had low cytotoxicity, lower ROS release, and no change in cell mechanics, indicating a potential application for dextran Au NPs. Finally, I expanded our assays to include high-throughput microfluidic characterization of isolated mitochondria. Using both exogenously and endogenously induced ROS, we found an increase in mitochondrial deformation and a decrease in their size, which could have implications on mitochondrial function, i.e., fission and fusion. We believe that advanced applications of RT-DC technology will improve the comparability of results across different sample sizes while also promoting it as a disease detection technique.
In this work, 2-dimensional measurements in the THz frequency range with self-made spintronic THz emitters were presented. The STE were used to optimize the spatial resolution and determine the magnetization in geometric shapes. At the beginning, various combinations of FM and NM layers were produced and measured to achieve an optimal composition of the STE. The layer thickness of the ferromagnetic CoFeB layer and the nonmagnetic PT layer was also varied. The investigations have shown that a layer combination of 2 nm thick CoFeB and 2 nm thick Pt, applied to a fused silica glass substrate and covered with a 300 nm thick SiO2 layer, emits the highest THz amplitude. Based on these, a structured sample, consisting of an STE and an additional layer system of 5 nm Cr and 100 nm Au, was produced. Further, three wedge-shaped structures were removed from the gold layer by an etching process so that the THz radiation generated by the STE can pass through these areas. This enables the optimization of the resolution of the system. For this purpose, the sample was moved perpendicular to the laser beam by two stepping motors with a step size of 5 μm and imaged 2-dimensionally. By reducing the step size to 0.2 μm, the beam diameter could be measured at the edge of the structure using the knife-edge method. Based on this measurement, the resolution of the system could be determined as 5.1 ± 0.5 μm at 0.5 THz, 4.9 ± 0.4 μm at 1 THz, and 5.0 ± 0.5 μm at 1.5 THz. These results are confirmed by simulations considering the propagation of THz wave packets through the SiO2. The expansion of the FWHM of the waves, passing through the 300 nm thick layer, is about 1%. Only a SiO2 layer with a thickness in the μm range occurs an expansion of around 10%. This shows that it is possible to perform 2-dimensional THz spectroscopy with a resolution in the dimension of the exciting laser beam by using near-field optics. Afterward, the achieved spatial resolution was used to investigate the influence of external magnetic fields on the STE and the emitted THz radiation. By implementing a pair of coils above the sample, an external magnetic field could be applied parallel to the pattern. The used sample was designed in such a way that only certain geometric areas on the fused silica glass substrate were coated with an STE so that THz radiation is emitted only in those areas. The 2-dimensional images show the geometric structures for f = 1.0 THz and f = 1.5 THz clearly. By applying a permanent, positive magnetic field (+M), a positive course of the THz amplitude can be seen. A rotation of the magnetic field by 180° (-M) leads to a reversal of the orientation of the emitted THz radiation, whereby the magnetic field does not influence the corresponding frequency spectrum. By using minor loops, the sample was demagnetized by the constant reduction of the magnetic field strength with alternating magnetic field direction. The 2-dimensional representation of the pattern with a step size of 10 μm shows that the sample was demagnetized since both, positively and negatively magnetized structures, could be imaged. In addition, in the 2nd row from the top, a completely demagnetized circle and a rectangle with a division into two domains can be seen. These structures have both positive and negative magnetized areas, which are separated by a domain wall. To investigate this in more detail a 2-dimensional measurement of the divided regions was made with a step size of 2.5 μm. These images confirm the division of the structures into positive and negative domains, separated by a domain wall, which was verified by Kerr-microscope measurements. Both data show a similar course of the domains and the domain wall. However, to be able to examine the domain wall more precisely using 2-dimensional THz spectroscopy, the resolution of the system must be improved to a range of a few nm, because the expected domain wall width is between 𝑙𝑊 = 12.56 nm and 𝑙𝑊 = 125.6 nm. The improved resolution would make it possible to image foreign objects, such as microplastics in biological cells or tissue. For this purpose, different plastics, such as polypropylene, polyethylene, and polystyrene, were investigated in the THz frequency range up to 4 THz. While no specific absorption could be determined for PP, characteristic absorption peaks were found for PE and PS. The energy of the photons with a frequency of about 2.2 THz excites lattice vibrations in the PE. Therefore, this frequency is specifically absorbed, and the intensity in the transmission spectrum is lower than for other frequencies. PS absorbs especially THz radiation with a frequency of 3.2 THz. In addition, all of the investigated plastics are mostly transparent for THz radiation, which makes imaging of these materials feasible. Based on these basic properties, it will be possible to image and identify these types of plastic.
In this work, spatial distributions for reactive stable and transient species that are involved
in the reaction cycle of H2O2, a key species for biomedical applications, were
determined directly in the effluent of a kINPen-sci plasma jet. The small diameter
of cold atmospheric pressure plasma jets and their operation at atmospheric pressure
that causes strong quenching reactions make diagnostics challenging. Here, various diagnostic
techniques have been employed and adapted for the use in the effluent of a
cold atmospheric pressure plasma jet, which were laser atomic absorption spectroscopy
(LAAS) at 811.5 nm for the detection of Ar(3P2), picosecond two-photon absorption
laser-induced fluorescence spectroscopy (ps-TALIF) at 225 nm and 205 nm for the
detection of O and H atoms, respectively, and continuous wave cavity ring-down spectroscopy
(cw-CRDS) at 1.506 µm for the detection of HO2, and cw-CRDS at 8000 µm
for the detection of H2O2. All these methods provide absolute number densities. In
this work, spatial distributions within the small diameter of the effluent of a CAPJ
were obtained, which have not been reported so far literature. In order to overcome the
line-of-sight limitations of CRDS, radial scans were performed and transformed into a
spatial distribution by using Abel inversion.
Based on the determined spatial density distributions for H atoms, O atoms, HO2
radicals, and H2O2 molecules, together with the investigated impact of humidity in the
feed gas on the excitation dynamics and the production of Ar(3P2), and finally on a
comparison of the experimental results to a plasma chemical and reacting flow model,
three different zones with varying reaction kinetics were identified. The densities close
to the nozzle of the kINPen-sci plasma jet were dominated by reactions within the
plasma zone including the dissociation of H2O added to the Ar feed gas and O2 that
was presumably transferred into the plasma zone by counter-propagating ionisation
waves. Notably, also the larger molecules, such as HO2 and H2O2 were mainly formed
within the plasma zone of the plasma jet. Between 1.5 mm and 5 mm below the nozzle,
the atomic species and molecular radicals generated in the plasma zone were consumed
by chemical reactions with the surrounding gas, whose composition was controlled by
applying a gas curtain. At further distances from the nozzle, where typically biological
samples are positioned, only H2O2 and HO2 were observed.
With this work, it is successfully demonstrated that even for the small diameters of
cold atmospheric pressure plasma jets the determination of spatial profiles for reactive
transient and stable species is possible within the effluent. By combining the experimental
results, important insights into the formation and consumption of H2O2 and its
precursors were gained, which are essential for the understanding of use of plasmas in
biomedical applications.
The biomechanical (Young's modulus, adhesion force, deformability) properties of platelets depend on the cytoskeleton and have an undisputed influence on physiological and pathological processes such as hemostasis and thrombosis. The alterations of these biomechanical properties can be used as label-free diagnostic markers in initiation or progressive diseases such as MYH9-inherited disease. Therefore, the focus of my thesis was to investigate the relationship between the changes in platelet cytoskeleton proteins and the resulting biomechanical properties using biophysical methods.
In the first chapter of my thesis I focused on my review of the biophysical methods that are most commonly used to assess and quantify the biomechanical properties of platelets. In this review, I provide an in-depth insight into the governing principles and instrumentation setup and discuss relevant examples applied to platelet mechanics. In addition, my review also summarizes the limitations of these biophysical methods and highlight latest improvements. The review covers the following techniques: micropipette aspiration, atomic force microscopy (AFM), scanning ion conductance microscopy (SICM), tensile force microscopy on hydrogel substrates, microcolumns, and deformable 3D substrates, and real-time deformability cytometry (RT-DC). This review is directed toward clinician scientists who are interested in exploring applications of single-cell based biophysical approaches in unraveling the role of platelet biomechanics in hemostasis and thrombosis research.
In the second chapter of my thesis, I present my research paper on the influence of commonly used ex vivo anticoagulants on the intrinsic biomechanical properties and functional parameters (e.g. activation profils) of human platelets. To comprehensively assess this, platelets obtained in different ex vivo anticoagulants such as ACD-A, Na-Citrate, K2-EDTA, Li-Heparin, and r-Hirudin were used, and their biomechanical properties were determined by real-time fluorescence and deformability cytometry (RT-FDC). Flow cytometry, and confocal laser scanning fluorescence microscopy were used to determine platelet function properties. K2-EDTA and Li-Heparin were found to affect platelet biomechanics by increasing actin polymerization of non-stimulated human platelets. This increased actin polymerization results in decreased platelet deformation. It is recommended that an ex vivo anticoagulant such as ACD-A, Na-Citrate, or r-Hirudin be chosen for the study of the cytoskeleton of human platelets and, if possible, that it not be exchanged, because comparability of results is not assured. Furthermore, I demonstrate the significance of choosing correct ex vivo anticoagulants in RT-FDC by showing that platelets from a healthy donor and a MYH9 patient with the E1841K point mutation differ in their deformation. This paper is the first comprehensive investigation at the single platelet level to establish the relevance of preanalytical standardization in platelet sample preparation for biomechanical studies.
The third chapter of my thesis is focused on the biomechanical analyses of platelets and thrombi from MYH9-related disease. Here I studied three Myh9 mouse lines with a point mutation in the Myh9 gene at positions 702, 1424, or 1841. Furthermore, two MYH9 patients (MYH9 p.D1424N, MYH9 p.E1841K) were examined. MYH9-related disease (MYH9-RD) presents with macrothrombocytopenia with a moderate bleeding tendency. It is caused by mutations in the MYH9 gene that lead to alteration of non-muscle myosin heavy chains type IIA (NMMHC IIA), resulting in disruption of the platelet cytoskeleton. Western blot analysis, flow cytometry, in vitro aggregometry, and transmission electron microscopy demonstrated that Myh9 point mutant mice have comparable primary function compared to the control group. The heterozygous point mutations in the Myh9 gene resulted in decreased platelet deformation (RT-FDC), decreased platelet adhesion to collagen (single platelet force spectroscopy-SPFS), and decreased platelet-platelet interaction forces (SPFS). Decreased platelet force (Micropost Arrays) results in softer thrombi (colloidal probe Spectroscopy), impaired clot retraction, and thus prolonged bleeding time. The R702C, D1424N, and E1841K mutations have a similar effect on platelet biomechanical functions, although the E1841K mutation had less impact on thrombus formation and stiffness. MYH9-RD patients have an increased risk of bleeding, and the antifibrinolytic drug tranexamic acid (TXA) is one way to control bleeding complications in these patients. It was shown that TXA treatment significantly reduced bleeding time in the three Myh9 mouse models, confirming that the enhanced bleeding phenotype due to decreased platelet forces in Myh9 mutant mice can be compensated by the addition of TXA.
With the biophysical methods and research results presented in my thesis, it is clear that it is essential to study the altered response of the platelet cytoskeleton by cytoskeletal mutations, biochemical, physical stimuli, or by pharmacological aspects. This will provide us with an opportunity to better understand the underlying mechanisms and thus contribute to better clinical treatment.
Kinetic modeling and infrared spectroscopy of charge carriers across the plasma-wall interface
(2022)
In this thesis, charge transport at the plasma-wall interface is investigated theoretically, on a semiclassical, microscopic level. Based on the Boltzmann and Poisson equations a set of equations is derived and numerically solved to model charge carriers both within a semiconducting wall and a gaseous plasma in front of it. While the plasma is considered collision-free, within the solid, phonon collisions, as well as recombination processes between conduction band electrons and valence band holes are considered. This results, for the first time, in a self-consistent modeling of both the gaseous electron-ion plasma and the electron-hole plasma in the solid on the same footing. Utilizing specific approximations for different physical scenarios, numerical solutions are presented both for the floating and the electronically contacted (biased) interface. In the latter case, the current voltage characteristic is calculated and shown to heavily depend on the charge kinetics within the wall.
Furthermore, we present optical methods to measure the wall charge noninvasively. These utilize the influence of the deposited surplus charges on the optical reflection coefficient of the surface. By calculating the optical response of these charges, we show that the magnitude of the surface charge can be inferred from the change in the reflectivity of the surface caused by the presence of the plasma. While nonlocal effects are considered, it is shown analytically and numerically that these can be neglected at the scales of the considered physical systems.
This work investigates turbulence in the core plasma of the optimised stellarator
Wendelstein 7-X. It focuses on experimental characterisation and
evaluation of the electrostatic micro-instabilities, which drive turbulent fluctuations,
and the saturation of turbulence by zonal flows. Expectations for
Wendelstein 7-X are formulated by reviewing theoretical work and with
the help of gyrokinetic simulations. The experimental analysis centres on
line-integrated density fluctuation measurements with the phase contrast
imagining diagnostic in electron cyclotron heated hydrogen discharges. An
absolute amplitude calibration was implemented, and a method for reliable
determination of dominant phase velocities in wavenumber-frequency
spectra of density fluctuations has been developed. Line-averaged density
fluctuation levels are observed to vary between magnetic configurations.
The wavenumber spectra exhibit a dual cascade structure, indicating fully
developed turbulence. The dominant instability driving turbulent density
fluctuations on transport relevant scales is identified as ion-temperaturegradient-
driven modes, which are mainly localised in the edge region of the
confined plasma. Despite the line-integrated nature of the measurement, the
localisation of density fluctuations is shown by comparing their dominant
phase velocity with the radial profile of the E × B rotation velocity due to
the ambipolar neoclassical electric field. Nonlinear gyrokinetic simulations
and a simplified plasma rotation model within a synthetic diagnostic confirm
the localisation. Oscillations of the dominant phase velocity indicate
the existence of zonal flows as a saturation mechanism of ion-temperaturegradient-
driven turbulence. A direct effect on turbulent density fluctuation
amplitudes and radial transport is observed.
The layer-by-layer method is a robust way of surface functionalization using a wide range of materials, e.g. synthetic and natural polyelectrolytes (PEs), proteins and nanoparticles. Thus, this method yields films with applications in diverse areas including biology and medicine. Sequential adsorption of different oppositely charged macromolecules can be used to prepare tailored films with controlled molecular organization. In biomedical research, electrically conductive coatings are of interest. In manuscript 1, we investigated films sequentially assembled from the polycation poly (diallyldimethyl-ammonium) (PDADMA) and modified carbon nanotubes (CNTs), with CNTs serving as the electrically conductive material. We assume that charge transport occurs through CNT contacts. We showed that with more than four CNT/PDADMA bilayers, the electrical conductivity is constant and independent of the number of CNT/PDADMA bilayers. A conductivity up to 4∙10^3 S/m was found. It is possible to control the conductivity with the CNT concentration of the CNT deposition suspension. A higher CNT concentration resulted in thicker CNT/PDADMA bilayers, but in a lower conductivity per bilayer. We suspect that an increased CNT concentration leads to a rapid CNT adsorption without the possibility to rearrange themselves. If PDADMA then adsorbs on the disordered CNTs in the next deposition step, the average thickness of the polymer layer is thicker than on the more ordered CNT layer from the dilute solution. This leads to an increased PE monomer/CNT ratio and lower conductivity. More polycations between the CNT layers leads to less CNT contacts. Thus, the controlled composition of films can be used to fulfill specific requirements.
For many applications of polyelectrolyte multilayers (PEMs), cheap PEs with a broad distribution of molecular weights are used. It was unknown whether the distribution of molecular weights of the PE in the adsorption solution is maintained during the adsorption process and hence in the film. To investigate this, the PSS adsorption solution in article 2 consisted of a binary mixture of short and long poly (styrene sulfonate) (PSS). A good model system to study layered films in terms of composition are PDADMA/PSS multilayers. Neutron reflectivity and in-situ ellipsometry measurements were carried out to determine the PSS composition in the film and the growth regimes. At a mole fraction of long PSS of 5 % or more in solution, the exponential growth (which is characteristic of short PSS) is totally suppressed, and only long PSS is deposited in the resulting multilayer. Variation of adsorption time of PSS showed that short PSS first adsorbs to the surface but is displaced by long PSS. Between 0 and 5 % of long PSS in the adsorption solution exponential growth occurs. The fraction of short PSS in the film continuously decreases with the increase of long PSS in the adsorption solution. In the assembly of films prepared from binary PSS mixtures, the short PSS leaves the film through adsorption/desorption steps both during PSS adsorption and during PDADMA adsorption (as PDADMA/PSS complexes). Both techniques show that the composition of the film does not correspond to that of the deposition solution. The composition and thus the properties of the resulting multilayer are influenced by the choice of adsorption time. Moreover, we conclude that a multilayer grown from a polydisperse polyelectrolyte contains fewer mobile low molecular weight polymers than the deposition solution.
In manuscript 1 and article 2, the composition of multilayers was studied. In manuscript 1 adsorption kinetics were important for the arrangement of CNTs on the surface. In article 2, the adsorption kinetics, i.e. the diffusion of the polyelectrolytes to the surface, was also investigated. In article 3, we investigated the influence of the composition of the film as well as the preparation condition on the mobility of PEs in the film. The molecular weight of the polycation PDADMA and the NaCl concentration of the deposition solution were varied. The vertical PSS diffusion constant D_PSS within the PDADMA/PSS multilayers was measured using neutron reflectivity. The salt concentration of the preparation solution defines the polymer conformation during deposition. The molecular weight of the polycation determines the degree of intertwining. Together, both parameters determine the polyanion-polycation coupling and thus the PSS mobility within the network. Log−log display of D_PSS vs the molecular weight of PDADMA and fits to two power laws (D_PSS ∝ X_n(PDADMA)^(-m) ∝ M_w(PDADMA)^(-m)) reveals for films built from 10 or 200 mM NaCl a kink. Below and above the kink, the dependence of D_PSS on M_w(PDADMA) can be described by different power laws. For Χ_n(PDADMA) < X_n,kink(PDADMA) ≈ 288, the exponents are consistent with the predictions of the sticky reptation model. X_n(PDADMA) ≈ 288 is the entanglement limit. For Χ_n(PDADMA) > X_n,kink(PDADMA) ≈ 288, the decrease of D_PSS with M_w(PDADMA) is larger than below the entanglement limit, which is indicative of sticky reptation and entanglement. The PSS diffusion constant of films built from 100 mM NaCl drops three orders of magnitude when increasing the molecular weight of PDADMA from 45 kDa to 72 kDa. To figure out if an immobile PSS fraction exists in the film built from 72 kDa PDADMA (beyond the entanglement limit), the film was annealed at different conditions in article 4: both temperature and salt concentration were varied. For data analysis, the simplest model with two PSS fractions with different diffusion constants was used. These diffusion constants increase as the temperature of the surrounding solution is increased. As assumed in article 3, an immobile PSS fraction exists when annealing at room temperature. At higher annealing temperatures, at least two diffusion processes must be distinguished: the diffusion of the highly mobile PSS fraction through the entire film and a slow PSS fraction, mowing in a limited way. The choice of preparation conditions determines whether a polyelectrolyte multilayer can intermix completely. It is not clear if complete intermixing will ever occur for films built with PDADMA beyond the entanglement limit. It is possible that the diffusion is more complex. Long-term measurements will clarify this question. Calculating scattering length density profiles with subdiffusive behavior would be interesting and is a challenge for the future. Furthermore, immobile fractions are only visible with long annealing times. We hypothesize that an immobile or nearly immobile fraction is present whenever the dependence of D_PSS on the molecular weight of PDADMA cannot be described by the sticky reptation. To verify this hypothesis, further studies are necessary.
All results presented and discussed in the manuscript and articles show that by varying the preparation conditions, tailored films can be built. The composition of the film is also determined by the adsorption kinetics. In addition, the mobility of the PEs within the multilayers can be controlled by varying the conformation, mingling and entanglement of the chains within the film. The influence of the salt concentration in the preparation solution on the growth regimes during film formation is part of our future research. It is planned to investigate films built of different PDADMA molecular weights under varied annealing conditions to better understand the mobile and immobile fractions.
Cell mechanical properties reveal substantial information on cell state and function. Utilizing mechanics as a label-free biomarker allows for investigation of fundamental cellular processes as well as biomedical applications, e.g., disease diagnosis. High-throughput methods for accessing the elastic properties of cells in suspension from hydrodynamic deformation in a microfluidic constriction are available with real-time analysis rates of up to 1000 cells per second. However, accessing elastic as well as viscous properties of cells and multicellular systems in suspension as well as adhered to surfaces at high throughput has not been possible so far. In this thesis, I approached this question and developed as well as applied microfluidic and holographic technologies to analyze the viscoelastic properties of single cells and multicellular aggregates, respectively.
First, I demonstrated that real-time deformability cytometry (RT-DC) can be applied in transfusion medicine, where the highest quality standards have to be maintained while blood product release is time-critical. We showed for platelet and red blood cell concentrates as well as for hematopoietic stem cells that their mechanical properties can be used for label-free quality assessment. The results have been published in Lab on a Chip (Aurich et al. 2020).
For RT-DC and many other methods based on hydrodynamic deformation, the constriction size has to be adapted to the objects of interest to allow for a shear-induced deformation. We introduced virtual fluidic channels, which are established by two co-flowing aqueous polymer solutions. Virtual fluidic channels can be precisely adjusted in their cross section, allowing for mechanical phenotyping of single cells as well as cell clusters or tissue spheroids in one microfluidic system. Importantly, measurements can also be performed in standard microfluidic geometries beyond soft lithography, e.g., in the cuvette of a flow cytometer. For cell spheroids as a model system for multicellular aggregates, we show a 10-fold lower Young's modulus of the tissue compared to single-cell mechanics, suggesting cell-cell and cell-matrix interactions being potential contributors to the mechanics of multicellular aggregates. Our work on virtual fluidic channels has been published in Nature Communications (Panhwar et al. 2020).
Within this thesis, I expanded the high-throughput elastic phenotyping performed by RT-DC towards viscoelastic cell properties by developing dynamic real-time deformability cytometry (dRT-DC). Dynamic tracking of cells while passing the microfluidic constriction allows to access steady-state (elasticity) and time-dependent (viscosity) material properties for a complete viscoelastic characterization of cells in suspension at high throughput. I introduced a shape mode decomposition based on a Fourier transformation, which allows to disentangle the superimposed stress responses to an extensional stress at the channel inlet and a constant shear stress in the channel. These hydrodynamic stress distributions are present in almost every microfluidic channel geometry. From the separated stress responses, viscoelastic material properties can be determined independent of cell shape.
We demonstrated experimentally the sensitivity of dRT-DC to cytoskeletal alterations and confirmed the validity of the method by reference measurements on calibrated hydrogel beads. In our work, we also presented a viscoelastic fingerprint of the major subpopulations of peripheral blood: erythrocytes, granulocytes, and peripheral blood mononuclear cells (PBMCs) (e.g., lymphocytes and monocytes), all characterized by the same method. The technique and the results have been published in Nature Communications (Fregin et al. 2019).
In cell mechanical methods based on hydrodynamic deformation, cell shape is usually monitored while a stress is applied. For extraction of material properties as well as for studying shape dynamics, it is essential to describe cell shape yielding highest strain differences for a given microfluidic system and experimental setting. Using dRT-DC, I compared nine different shape descriptors to analyze cell deformation in an extensional as well as shear flow. A relaxation time analysis was performed on different levels of data aggregation from single cells to an ensemble scale. I demonstrated that the steady-state deformation can be predicted from stress response curves without them reaching the steady-state. This is important for cell mechanical measurements in microfluidic systems as the characteristic times are unknown in general and as the channel length is fixed. In addition, by introducing a cut-off criterion for how much of the response trace needs to be captured within the channel, the analysis time per cell can be reduced while material properties can still be extracted. Performing simulations, I compared the accuracy of relaxation times extracted from ensemble and single-cell studies under experimental conditions. Introducing a scoring system to evaluate which combinations of shape descriptors and analysis strategies provide biggest effect size, we concluded that single-cell analyses in an extensional flow are most sensitive to cytoskeletal modifications independent of shape parametrization. The manuscript was submitted to the Biophysical Journal.
Finally, I translated the fast non-contact cell mechanical probing from suspension to adherent cells. No such technology has been available and with the majority of cells being adherent, a robust label-free method for mechanophenotyping at high-throughput is required. Within this thesis, I have introduced and realized a new concept: holographic vibration spectroscopy (HVS), where adherent cells are mechanically excited on a vibrating surface while their height oscillations are measured optically. Analysis is done in an interferometric heterodyne setup by using frequency multiplexing and time-averaged holography in off-axis configuration. Based on interference images captured by a high-speed complementary metal-oxide-semiconductor (CMOS) camera, I established a mathematical model to reconstruct the vibration amplitude of adherent cells as well as their retardation phase compared to the exciting vibration. From the amplitude and phase response, viscoelastic parameters can be derived, which have to be investigated in subsequent studies.
In summary, I introduced in my work two high-throughput methods for the viscoelastic characterization of suspended as well as adherent cells while highlighting applications in tissue mechanics and transfusion medicine that are relevant not only in basic but also in translational research.
This thesis discusses three publications in the field of dusty plasmas.
In the first section, measurements of the ir absorption of silica nanoparticles confined in an argon radiofrequency plasma discharge using a Fourier transform infrared spectrometer have been performed. By varying the gas pressure of the discharge and duty cycle of the applied radiofrequency voltage, a shift of the absorption peak of silica is observed. This shift is attributed to charge-dependent absorption features of silica. The charge-dependent shift has been calculated for silica particles, and from comparisons with the experiment the particle charge has been retrieved using the infrared phonon resonance shift method. With the two different approaches of changing the gas pressure and altering the duty cycle, one is able to deduce a relative change of the particle charge with pressure variations and an absolute estimate of the charge with the duty cycle.
In the second part, infrared (IR) absorption spectra of melamine-formaldehyde (MF) microparticles confined in an rf plasma are studied at different plasma conditions. Several absorption peaks have been analysed in dependence of plasma power and their temporal evolution. For comparison, the IR absorption spectra of heated MF microparticles without plasma exposition are used to determine the general influence of the temperature on the IR spectra. Measuring the temperature of the particles inside the plasma shows that the temperature is not the only process changing the particles' IR spectra. Chemical changes of the MF particles with increasing plasma power influence the absorption peak structure.
Finally, experiments on dust clusters trapped in the sheath of a radio frequency discharge have been performed for different magnetic field strengths ranging from a few milliteslas to 5.8 T. The dynamics of the dust clusters are analyzed in terms of their normal modes. From that, various dust properties such as the kinetic temperature, the dust charge, and the screening length are derived. It is found that the kinetic temperature of the cluster rises with the magnetic field, whereas the dust charge nearly remains constant. The screening length increases slightly at intermediate magnetic field strengths. Generally, the dust properties seem to correlate with magnetization parameters of the plasma electrons and ions, however only to a small degree.
This thesis describes how the data of the Langmuir probes in the Wendelstein 7-X (W7X) Test Divertor Unit (TDU) were evaluated, checked for consistency with other diagnostics and used to analyse plasma detachment.
Langmuir probes are an electronic diagnostic, and were among the first to be used in plasma physics to determine particle fluxes, potentials, temperatures and densities.
W7X is a large, advanced stellarator, magnetic confinement fusion experiment, operated at the Max-Planck-Institut for Plasma Physics(IPP) in Greifswald, Germany.
Its TDU is an uncooled graphite component, shaped and positioned to intercept the convective heat load of the plasma.
Detachment describes a desirable operation state of strongly reduced loads on this component.
The evaluation of Langmuir probe data relies heavily on models of the sheath, formed at the interface between plasma and a solid surface, to infer plasma parameters from the directly measured quantities.
Multiple such models are analysed, generalised, and adapted to our use case.
A detailed comparison is made to determine the most suitable model, as this choice strongly affects the predicted parameters.
Special attention is paid to uncertainties on the parameters, which are determined using a Bayesian framework.
From the inferred parameters, heat and particle fluxes are calculated.
These are also indirectly measured by two other, camera-based diagnostic systems.
Observations are compared to test the validity of assumptions and calculations in the evaluation of all three diagnostics by checking their results for consistency.
The first comparison, with the infrared emission camera system, shows good agreement with theoretical predictions and reported measurements of the sheath transmission factor, for which we derive and measure a value in W7X.
Parameter dependencies in the quality of this agreement hint at remaining issues.
The second comparison, with the Hydrogen alpha photon flux camera system, shows significant discrepancy with expectations.
These are argued to originate from systematic differences in the measurement locations, which are quantified and related to the magnetic topology.
Langmuir probe observations of individual discharges are analysed to discuss conditions under which detachment occurs, transition into that state and fluctuations observed prior to and during it.
A spatial parametrisation of the data is developed and used to facilitate this.
These observations contribute to the larger aim of understanding particle balance control and fusion plasma edge processes.
Three-dimensionally extended dusty plasmas containing mixtures of two particle species of different size have been investigated on parabolic flights. To distinguish the species even at small size disparities, one of the species is marked with a fluorescent dye, and a two-camera video microscopy setup is used for position determination and tracking. Phase separation is found even when the size disparity is below 5%. Particles are tracked to obtain the diffusion flux, and resulting diffusion coefficients are in the expected range for a phase separation process driven by plasma forces. Additionally, a measure for the strength of the phase separation is presented that allows to quickly characterize measurements. There is a clear correlation between size disparity and phase separation strength.
Molecular dynamics simulations of binary dusty plasmas have been performed and their behavior with respect to the phase separation process has been analyzed. Here as well, it is found that even the smallest size disparities lead to phase separation. The separation is due to the force imbalance on the two species and the separation becomes weaker with increasing mean particle size.
In the second part of the thesis, Experiments on self-excited dust-density waves under various magnetic fields have been performed. For that purpose, different dust clouds of micrometer-sized dust particles were trapped in the sheath of a radio frequency discharge. The self-excited dust-density waves were studied for magnetic field strengths ranging from 0 mT to about 2 T. It was observed that the waves are very coherent at the lowest fields (B < 20 mT). At medium fields (20 mT < B < 300 mT), the waves seem to feature a complex competition between different wave modes before, at even higher fields, the waves become more coherent again. At the highest fields (B > 1 T), the wave activity is diminished. The corresponding wave frequencies and wavenumbers have been derived. From the comparison of the measured wave properties and a model dispersion relation, the ion density and the dust charge are extracted. Both quantities show only little variation with magnetic field strength.
Modern space missions depend more and more on electric propulsion devices for in-space
flights. The superior efficiency by ionizing the feedgas and propelling them using electric
fields with regard to conventional chemical thrusters makes them a great alternative. To
find optimized thruster designs is of high importance for industrial applications. Building
new prototypes is very expensive and takes a lot of time. A cheaper alternative is to rely
on computer simulations to get a deeper understanding of the underlying physics. In order
to gain a realistic simulation the whole system has to be taken into account including the
channel and the plume region. Because numerical models have to resolve the smallest time
and spatial scales, simulations take up an unfeasible amount of time. Usually a self-similarity
scaling scheme is used to greatly speed up these simulations. Until now the limits of this
method have not been thoroughly discussed. Therefore, this thesis investigates the limits
and the influence of the self-similarity scheme on simulations of ion thrusters. The aim
is to validate the self-similarity scaling and to look for application oriented tools to use
for thruster design optimization. As a test system the High-Efficiency-Multistage-Plasma
thruster (HEMP-T) is considered.
To simulate the HEMP-T a fully kinetic method is necessary. For low-temperature plasmas,
as found in the HEMP-T, the Particle-in-Cell (PIC) method has proven to be the best
choice. Unfortunately, PIC requires high spatial and temporal resolution and is hence
computationally costly. This limits the size of the devices PIC is able to simulate as well
as limiting the exploration of a wider design space of different thrusters. The whole system
is physically described using the Boltzmann and Maxwell equations. Using these system
of equations invariants can be derived. In the past, these invariants were used to derive a
self-similarity scaling law, maintaining the exact solution for the plasma volume, which is
applicable to ion thrusters and other plasmas. With the aid of the self-similarity scaling
scheme the computation cost can be reduced drastically. The drawback of the geometrical
scaling of the system is, that the plasma density and therefore the Debye length does not
scale. This expands the length at which charge separation occurs in respect to the system
size. In this thesis the limits of this scaling are investigated and the influence of the scaling
at higher scaling factors is studied. The specific HEMP-T design chosen for these studies is
the DP1.
Because the application of scaling laws is limited by the increasing influence of charge separation with increased scaling, PIC simulations still are computationally costly. Another approach to explore a wider design space is given using Multi-Objective-Design-Optimization
(MDO). MDO uses different tools to generate optimized thruster designs in a comparatively
short amount of time. This new approach is validated using the PIC method. During this
validation the drawback of the MDO surfaces. The MDO calculations are not self-consistent
and are based on empirical values of old thruster designs as input parameters, which not
necessarily match the new optimized thruster design. By simulating the optimized thruster
design with PIC and recalculate the former input parameters, a more realistic thruster design is achieved. This process can be repeated iteratively. The combination of self-consistent
PIC simulations with the performance of MDO is a great way to generate optimized thruster
designs in a comparatively short amount of time. The proof of concept of such a combination
is the pinnacle of this thesis.
Organic molecules are the carbon-based complex of several atoms, is an innovative and essential element to create nano-structural platforms, as a building block in the
field of organic electronics and organic spintronics. Because of its variety and functionality via widely studied synthetic methods, molecules have played an important role in electronics as not only a transport channel in bulk form but also a tuning layer
at the interface of hetero structures. The potential of molecular layers has also stood out in spintronics, owing to its mass-low composition producing long spin life time.
Organic materials can be employed in spintronics applications, benefiting from their low cost, ease of processing, and chemical tunability. Beyond this advantage, the configuration
of molecules on a metal film displays unique phenomena as it can control the molecular spins and interfacial coupling between them, resulting in the emergence
of molecular spinterface.
This thesis work focuses on identifying the interfacial properties between the ferromagnet and the Phenalenyl (PLY) based metal complexes. The growth morphology study of the copper-phenalenyl Cu-PLY based molecules influence the electronic coupling between the molecular layer and the ferromagnet. Zinc- Phenalenyl (ZMP) molecule already have been studied [1] by demonstrate the formation of a spinterface,
resulting interface magneto resistance (IMR) close to room temperature. The
spinterface formation leads to the unique property, that a magnetic tunnel junction
with a ZMP barrier requires only one ferromagnetic metal layer, while the other ferromagnetic layer is formed in the organic barrier directly at the ferromagnet/organic
barrier interface. Here we compare Phenaleny, Copper-Phenaleny Cu-PLY and Zincmethyl- phenaleny molecule based MTJ electrical and magnetic properties which will
be suitable for tunnel barrier and can be used for stable memory devices. We tune the magnetic property of ferromagnet and forma hybrid interface without any oxide layers in between the ferromagnet and molecular layers. The tuning of magnetic properties
via the molecular approach will certainly extend versatile functionalities of organic spinterfaces.
Development of an Electrostatic Ion Beam Trap for Laser Spectroscopy of Short-lived Radionuclides
(2021)
Due to its high accuracy and resolution, collinear laser spectroscopy (CLS) is a powerful tool to measure nuclear ground state properties such as nuclear spins, electromagnetic moments and mean-square charge radii of short-lived radionuclides. Performing CLS with fast beams (>30 keV) provides an excellent spectral resolution approaching the natural linewidth. However, its fluorescence-light detection limits its successful application to nuclides with yields of more than several 100 to 10,000 ions/s, depending on the specific case and spectroscopic transition. To extend its reach to the most exotic nuclides with very low production yields far away from stability, more sensitive methods are needed. For this reason, the novel Multi Ion Reflection Apparatus for CLS (MIRACLS) is currently under development at ISOLDE/CERN. This setup aims to combine the high resolution of conventional fluorescence based CLS with a high experimental sensitivity, enhanced by a factor of 30 to 700 depending on the mass and lifetime of the studied nuclide. By repetitively reflecting the ion beam between the electrostatic mirrors of an electrostatic ion beam trap, often also called Multi-Reflection Time of Flight (MR-ToF) device, the laser beam probes the ion bunch during each revolution. Therefore, the observation time is extended and the experimental sensitivity is enhanced compared to conventional single-passage CLS. As part of this thesis, a MIRACLS proof-of-principle apparatus has been constructed around an MR-ToF system, operating at ~1.5 keV beam energy, which has been upgraded for the purpose of CLS. The goal of this setup is to demonstrate the potential of the MIRACLS concept, to benchmark simulations that are employed to design a future device operating at 30 keV, and to further develop the technique. For this purpose, CLS measurements with ions of stable magnesium and calcium isotopes are performed. This data serves to characterise the performance of the new method, especially in terms of gain in sensitivity and measurement accuracy.
In this thesis, the transport properties of topological insulators are investigated. In contrast to trivial insulators, topological insulators possess conducting boundary states which cross the bulk energy gap that separates the highest occupied electronic band from the lowest unoccupied band. The materials used in this thesis are three-dimensional topological insulators with time-reversal symmetry. Their associated helical surface states are protected against elastic backscattering by Kramers degeneracy. The unique properties of the helical surface states can be utilized to generate spin-polarized currents at the surface of topological insulators and to control their propagation direction. This makes them a promising material class for the field of spintronics.
Here, we perform photocurrent scans of topological insulator Hall bar and nanowire devices. From these measurements, we obtained two-dimensional maps of the polarization-independent and helicity-dependent components of the photocurrents.
We find that the polarization-independent component is dominated by the Seebeck effect and thus driven by thermoelectric currents. On the other hand, the helicity-dependent component is driven by the spin-polarized currents that emerge from the topologically non-trivial helical surface states via the circular photogalvanic effect.
First and foremost, our experiments demonstrate that topological insulator nanowires provide a promising platform for the generation of spin-polarized currents, whose direction can be controlled via the helicity of the excitation light. They also highlight the importance of analysing the spatial distribution of the photocurrent, as we observe a strong enhancement of the spin-polarized current and the thermoelectric current at the interface between the nanowire and the metallic contacts. As our analysis shows, the thermoelectric current is enhanced by the Schottky effect and the spin-polarized current is amplified by the spin Nernst effect. In addition, the spin Nernst effect is also present in Hall bar devices and manifest as an enhancement of the spin-polarized current along the Hall bar sides.
Motiviert durch den Vorschlag einer direkten, optischen Ladungsmessung an Staubteilchen wird die Lichtstreuung an den dielektrischen Kern-Schale-Teilchen tiefgehend untersucht.
Das Streuregime wird durch Analyse des Nah- und Fernfeldes unter Verwendung von Methoden, die für homogene Teilchen entwickelt wurden, eingehend charakterisiert und eine Verallgemeinerung der dazu verwendeten Funktionen auf ein k-fach beschichtetes Teilchen angegeben. Dabei werden die sich im Teilcheninneren manifestierenden Effekte der Hybridisierung der beiden Oberflächenphononen des Kern-Schale-Teilchens herausgearbeitet und visualisiert.
Die vorliegende Untersuchung der unterschiedlichen Kenngrößen ermöglicht ein detailliertes und umfangreiches Verständnis der Lichtstreuung an dielektrischen Kern-Schale-Teilchen und der Art und Weise, wie sich die Hybridisierung der Oberflächenphononen auf diese auswirkt.
Die dabei analysierte Interferenzstruktur des elektromagnetischen Feldes in der Teilchenschale, berechnet mittels der vollen Mie-Rechnung, passt zur Interpretation der optischen Antwort des Kern-Schale-Teilchens mithilfe der Hybridisierungstheorie.
Dieses Hybridisierungsbild und somit die Subsysteme und ihre Wechselwirkung werden in dieser Arbeit aus den analytisch exakten Mie-Koeffizienten heraus präpariert, um die neue Sichtweise mit der alten Mie-Theorie zusammenzubringen.
Die Idee einer spektroskopische Ladungsmessung wird im Hinblick auf die Bestimmung der Wandladung aufgegriffen. Die bisherigen Methoden zur Ladungsmessung sind zwar vielfältig, bieten jedoch nur Zugang zur absoluten Wandladung und liefern keine Informationen über ihre Verteilung senkrecht zur Oberfläche oder über die Dynamik der Aufladung.
Beides wäre jedoch für ein mikroskopisches Verständnis der Plasma-Wand-Wechselwirkung notwendig, sodass die Elektronenenergieverlustspektroskopie zur Ladungsbestimmung vorgeschlagen wird. Die Methode wird zunächst anhand einer lokalen Antworttheorie für verschiedene in die Wand eingesetzte Schichtstrukturen ausgelotet und aufgrund vielversprechender Resultate anschließend mittels der im betrachteten Parameterbereich notwendigen nichtlokalen Antworttheorie eingehend untersucht. Diese Theorie erfasst die Anregung von Resonanzen höherer Moden, die sich als besonders sensitiv auf die zusätzlichen Ladungsträger erweisen. Insgesamt wird ein experimenteller Aufbau mit einer geeigneten, in die Plasmakammerwand einsetzbaren Schichtstruktur vorgeschlagen, mit dem die Wandladung durch Elektronenenergieverlustspektroskopie bestimmt werden könnte.
This work presents the first experimental investigation of the gas balance on the optimized modular stellarator Wendelstein 7-X (W7-X). A balance of all injected and removed particles and a measurement of internal particle reservoirs allows inference of the bound particle reservoir in the wall, which is of interest due to its effects on plasma density control and fuel retention. Different scenarios of the gas balance are presented with data from the operation campaign 1.2 with an inertially cooled graphite divertor. Both net outgassing and net retention scenarios are presented and W7-X is found to operate stable in a wide range of scenarios with varying wall conditions.
Since fusion experiments are conducted in ultra-high vacuum, suitable gauges are required for total and partial pressure measurement. The challenges and opportunities of the operation of pressure gauges in the steady magnetic field extending beyond plasma pulses are discussed. The performance of newly improved neutral pressure gauges, based on crystal cathode emitters is quantified. These provide improved operational robustness since they can be operated for long periods of time in strong magnetic fields. A crystal cathode setup and and its operation performance is presented along with a fast calibration scheme.
Partial pressure measurements provide additional important information complementing the total neutral pressure measurements, and allowing additional physics insights. As part of this thesis work, a new diagnostic of this kind was implemented on W7-X, the so-called diagnostic residual gas analyzer (DRGA). It provides a wealth of information on various neutral gas species, with a relatively high time resolution - of order a few seconds. The diagnostic setup and its first results are presented in this thesis.
In this thesis, I present work motivated, in part, by a series of upcoming laboratory experiments (APEX), which seeks to uncover some of the inner workings of turbulence and stability in electron- positron plasmas in closed field-line systems. I present the results of several distinct, but connected, problems addressing the theory of electron-positron plasmas.
This work is partitioned into several parts, which loosely correspond to different particulars of the APEX experiment and the different theoretical physics problems which reside within.
I begin with the derivation of a kinetic theory for plasmas which are optically thin to cyclotron emission, as indeed, experimental pair plasmas are expected to be. The results of this section include: (1) the derivation of a general kinetic theory of cyclotron radiation in electron-ion plasmas; (2) a calculation showing that cyclotron emission results in strongly anisotropic distribution functions on the radiation timescale; (3) calculation of the evolution of the distribution function under collisional scattering which, in the absence of any radiation terms, acts to drive the plasma towards a Maxwellian; (4) generalisation of this theory to more exotic geometries; (5) specialisation of this theory to pair plasmas of experimental interest; and (6) presentation of the applications and the limitations of this theory.
The second project is primarily concerned with non-neutral plasmas. We begin with gyrokinetic theory and a novel extension of this theoretical framework to plasmas with arbitrary degree of neutrality in straight field-line geometry. I go on to discuss the gyrokinetic stability theory of such plasmas in this simplified geometry. I conclude this project with a discussion of some further
nuances in the theory of singly-charged non-neutral plasmas, this time concerning global features. Namely, I declare an interest in the equilibria such plasmas might be able to attain.
The final project pertains to plasmas which are in state of Maxwellian equilibrium i.e., electron- positron plasmas with sufficiently large number densities of each species to attain a stationary quasineutral plasma. To this end, I present gyrokinetic flux-tube simulations of electron-positron plasmas in complex, and experimentally relevant, magnetic geometries on the road towards a study of gyrokinetic turbulence. The results of this work include: (1) the first simulations of electron- positron plasmas in a stellarator and ring-dipole geometry; (2) an analytic theory of trapped particle modes in electron-positron plasmas, a result which can also be verified numerically; and (3) extension of several important theoretical results in electron-positron plasmas to experimentally relevant geometries. The culmination of this project is the roadmap ahead towards demonstration of the so-called “inward pinch” effect in electron-positron plasmas in a magnetic Z-pinch.
In this work, studies with respect to the exhaust problem were performed
in the stellarator experiment Wendelstein 7-X with different target concepts and different magnetic field geometries. Different infrared cameras were used to study the heat flux from the plasma onto the PFC. In the first publication, the limiter set-up was used with a simpler magnetic topology in the plasma edge. The radial fall-off of the parallel heat flux for inboard limiters in W7-X shows, similar to inboard limiters in tokamaks, two different radial fall-off lengths, a short (narrow) one, characterizing the near-SOL, and a long (broad) characterizing the far-SOL. For the far-SOL, the heating power and connection length have been identified as the main scaling parameters, while for the near-SOL, the electron temperature close to the LCFS has been identified as the main scaling parameter. The two fall-off lengths differ by a factor 10, and the found scalings for both regimes differ from known models and experimental scalings in tokamaks. A turbulent-driven feature was discussed in the publication as a possible explanation for the behavior of the fall-off length in W7-X.
The gained information and data have been further used to support many
other publications, covering the symmetry of the heat loads, the
energy balance of the machine, and seeding experiments.
The heat exhaust in W7-X with an island divertor was studied in the second
and third publication. Definitions of parameters such as peaking factor and
wetted area were applied for the heterogeneous heat flux pattern on the
W7-X divertor. It was shown that the island divertor concept is capable
of spreading out the heat efficiently, resulting in large wetted areas of up to 1.5 m2. The reached values for the wetted area are comparable to the ones of the larger tokamak JET but with a much smaller ratio of wetted
area to the area of the last closed flux surface. Furthermore, a positive
scaling of the wetted area with the power in the SOL was observed. This
scaling is beneficial for future reactors but needs further investigation of the involved transport processes. The peaking factor (discussed in the second publication) describes how concentrated the heat load is within the region of the strike line. It was shown that this factor is decreasing for increasing densities without affecting the wetted area. The present work paves the way for further analysis of the transport processes of the heat flux towards the island divertor of Wendelstein 7-X.
The active screen plasma nitrocarburizing (ASPNC) technology is a state-of-the-art plasma-assisted heat treatment for improving surface hardness and wear resistance of metallic workpieces based on thermochemical diffusion. In comparison to conventional plasma nitrocarburizing, the use of an active screen (AS) improves thermal homogeinity at the workload and reduces soot formation. Further it can serve as a chemical source for the plasma processes, e.g. by use of an AS made of carbon-fibre reinforced carbon. This compilation of studies investigates the plasma-chemical composition of industrial- and laboratory-scale ASPNC plasmas, predominantly using in-situ laser absorption spectroscopy with lead-salt tuneable diode lasers, external-cavity quantum cascade lasers, and a frequency comb. In this way the temperatures and concentrations of the dominant stable molecular species HCN, NH3, CH4, C2H2, and CO, as well as of less prevelant species, were recorded as functions of e.g. the pressure, the applied plasma power, the total feed gas flow and its composition. Additionally, the diagnostics were applied to a chemically similar plasma-assisted process for diamond deposition.
Resulting from this thesis are new insights into the practical application of an AS made of CFC, the plasma-chemistry involving hydrogen, nitrogen, and carbon, and the particular role of CO as an indicator for reactor contamination. The effect of the feed gas composition on the resulting nitrogen- and carbon-expanded austenite layers was proven by combination of in-situ laser absorption spectroscopy with post-treatment surface diagnostics. Furthermore this work marks the first use of frequency comb spectroscopy with sub-nominally resolved Michelson interferometry for investigation of a low-pressure molecular discharge. This way the rotational bands of multiple species were simultaneously measured, resulting in temperature information at a precision hitherto not reached in the field of nitrocarburizing plasmas.
Es wurde eine Methode zur Herstellung ultradünner Filme aus Metall bzw. metallischen Verbindungen (Legierungen) etabliert. Die Struktur und die physikalischen Eigenschaften der Filme wurden untersucht. Die entwickelte Präparationsmethode beruht auf induzierter Filmkontraktion nach erzwungener Benetzung (iFCaFW). Die Filme bestehen aus ultradünnen vertikal heterostrukturierten Multischichten (2D-VHML), sie entstehen durch den Beschichtungsvorgang und bestehen aus jeweils einer nm-dicken metallischen Schicht (M) eingebettet zwischen zwei Metall(hydr)oxidschichten (MOxHy) im nm- bis sub-nm Bereich. Dieser vertikal heterostrukturierte Aufbau wurde bei allen untersuchten Filmmaterialien beobachtet. Alle in dieser Arbeit vorgestellten Schichtsysteme wurden unter atmosphärischem Druck hergestellt. Es konnten Substrate aus Silicium und Muskovit sowie aus Borosilikat- und Kalk-Natron-Glas (Objektträger) beschichtet werden. Jede, aus flüssigem Metall bzw. flüssiger Legierung hergestellte Schicht verfügt über eine feste (Hydr)oxidschicht an der Luftgrenzfläche. Diese feste (Hydr)oxidschicht fungiert als Substrat für die nächste darüber aufgebrachte Schicht aus flüssigem Metall bzw. flüssiger Legierung. Somit entstehen vertikal heterostrukturierte Multischichten durch identische Wiederholung des Beschichtungsvorgangs. Dies ist eine innovative und vergleichsweise umweltfreundliche Methode, um transparente, elektrisch leitfähige und lateral homogene nm-dünne ein- oder mehrschichtige Metallfilme herzustellen. Verwendet wurden Metalle mit sehr niedriger Schmelztemperatur (kleiner als 300 °C), wie Bismut, Gallium, Indium, Zinn und ihre Legierungen. Die hohe Oberflächenspannung der geschmolzenen Metalle und Legierungen sowie die Adhäsion mit der die (Hydr)oxidhaut dieser Metalle und Legierungen auf verschiedenen Substraten haftet ermöglicht die Beschichtungsmethode.
This thesis contains studies on a special class of topological insulators, so called anomalous Floquet topological insulators, which exclusively occur in periodically driven systems. At the boundary of an anomalous Floquet topological insulator, topologically protected transport occurs even though all of the Floquet bands are topologically trivial. This is in stark contrast to ordinary topological insulators of both static and Floquet type, where the topological invariants of the bulk bands completely determine the chiral boundary states via the bulk-boundary correspondence. In anomalous Floquet topological insulators, the boundary states are instead characterized by bulk invariants that account for the full dynamical evolution of the Floquet system.
Here, we explore the interplay between topology, symmetry, and non-Hermiticity in two-dimensional anomalous Floquet topological insulators. The central results of this exploration are (i) new expressions for the topological invariants of symmetry-protected anomalous Floquet topological phases which can be efficiently computed numerically, (ii) the construction of a universal driving protocol for symmetry-protected anomalous Floquet topological phases and its experimental implementation in photonic waveguide lattices, (iii) the discovery of non-Hermitian boundary state engineering which provides unprecedented possibilities to control and manipulate the topological transport of anomalous Floquet topological insulators.
Experience in the construction of optimized stellarators shows the coil system is a significant challenge. The precision necessary allow the generation of accurate flux surfaces in recent experiments affected both cost and schedule negatively. Moreover, recent experiments at Wendelstein 7-X have shown that small field corrections were necessary for the operation of specific desired magnetic configurations. Therefore, robust magnetic configurations in terms of coil geometry and assembly tolerances have a high potential to facilitate swifter and less expensive construction of future, optimized stellarators. We present a new coil optimization technique that is designed to seek out coil configurations that are resilient against 3D coil displacements. This stochastic version of stellarator coil optimization uses the sampling average approach to incorporate an iterative perturbation analysis into the optimization routine. The result is a robust magnetic configuration that simultaneously reproduces the target magnetic field more accurately and leads to a better fusion performing coil configuration.
The non-renewable energy sources coal, oil and natural gas that contribute the major share of the world's energy, will be running out in the next 40-80 years. With the growing energy demands especially in developing countries, which is likely to surpass that of the developed countries in next 50 years, an alternate energy source is the need to the hour. The nuclear fusion energy is foreseen as one of the potential candidates to solve the current global energy crisis. One of the major challenges faced by the fusion community is the problem of power exhaust. With the larger fusion devices to be built in the future, the heat load on the plasma facing components are expected to grow higher. The present work explores two numerical studies performed on the Wendelstein 7-X, the world's largest stellarator type fusion device, to cope with this problem.
The first project on `'Numerical Studies on the impact of Connection Length in Wendelstein 7-X'' identifies magnetic configuration with long connection lengths, which could bring down the peak heat fluxes onto the divertor to manageable levels, by greater role of cross-field transport which may assist to get a wider heat deposition profile. The second project on `'Development of Heating Scenario to Reduce the Impact of Bootstrap Currents in Wendelstein 7-X'' advocates a novel self-consistent approach to reach high plasma density at full heating power without overloading the divertor during the transient phase of the evolution of the toroidal plasma current, by controlling two parameters; density and power. The aim of both the projects is to contribute to tackling the challenge of the tremendous power exhaust from fusion plasma which, if solved, will be a large step closer to a fusion power plant.
An experimental investigation of particle parallel flows has been carried out at Wendelstein 7-X (W7-X), one of the most advanced stellarators in the world. The studies are restricted to the outermost plasma region, the scrape-off layer (SOL), which is shaped to tackle the exhaust problem in vision of future fusion reactors based on plasma magnetic confinement. The aim of the measurements is to set the basis for a physics analysis of the SOL dynamics by obtaining direct information on convective heat transport, together with the assessment of the predominant flow directions of the main plasma ions and of fusion-products or wall-released impurities. In this way, a better comprehension of the interplay between the transport parallel and perpendicular to the SOL field lines can be achieved, contributing to the understanding of the effectiveness of the island divertor configuration.
The chosen instrument for the experimental studies is the Coherence Imaging Spectroscopy (CIS) diagnostic, a camera-based interferometer capable of measuring 2D Doppler particle flows associated with a selected visible line from the plasma. The diagnostic is distinguished by its high time resolution and spatial coverage, allowing the visualisation and measurements of flow velocities for a full module of W7-X simultaneously. A CIS diagnostic has been fully designed for W7-X with an improved level of accuracy achieved thanks to the implementation of a new calibration source, a continuous-wave-emission tunable laser. The laser allowed a full characterization of the diagnostic and a frequent precise calibration, making the CIS system reliable for parallel flow investigations during the operational campaign OP1.2. The validity and importance of the CIS measurements have been further confirmed with dedicated simulation of the SOL plasma parameters by the EMC3-EIRENE code, and by comparisons with other edge diagnostics. The CIS results show the effects related to dynamical changes in the SOL due to impurity gas puffs or the development of a plasma current. Moreover, CIS can be used as a powerful tool to test the limits of the current theoretical models, for example in the case of forward and reversed field experiments.
Barrier corona (BC) arrangements are employed in different plasma-based applications such as material surface and exhaust gas treatments. However, a comprehensive study about the discharge behavior and properties in such strongly asymmetric arrangements is still missing. This dissertation is devoted to the detailed investigation of single microdischarges (MDs) in a sinusoidally driven BC discharge in air at atmospheric pressure. The discharge arrangement consist of a sharp metal pin and a dielectric-covered hemispherical electrode. It is the first study of volume BC discharges, in which phasially-resolved spatio-temporal development of the MDs are recorded using a multi-dimensional time-correlated single photon counting (TC-SPC) technique. The morphology of the MDs is recorded using an ICCD camera. A voltage probe and a current probe are employed to measure the applied voltage and current pulses. Furthermore, phase-resolved current measurements and statistical studies of current pulse amplitudes are realized using an oscilloscope.
Due to the asymmetric geometry and material of the electrodes, discharge behavior in the two polarities of the applied sinusoidal voltage is significantly different. For the voltage amplitude being applied, mostly two MDs appear in the anodic pin half-cycles. It is observed that the breakdown mechanism in both MDs is a positive streamer starting near the anode, similar to the single MDs in symmetric dielectric barrier discharges (DBDs). However, the second MDs have different properties, such as longer duration of the bulk plasma and broader current pulses. It is considered that the differences are mainly due to the positive surface charges deposited by the first MDs on the dielectric. It is proposed, for the first time, that the current pulse derivative maximum corresponds to the arrival of the streamer head at the cathode surface. This is used to synchronize the spatio-temporal development of the MDs with their current pulses. The accuracy of the synchronization is limited to the rise-time of the current probe (350 ps). In each cathodic pin half-cycle, only one major MD appears. The appearance and amplitude of the MDs are more erratic compared to the anodic pin polarity. The TC-SPC recordings show that the MDs appearing at low applied voltages have a similar spatio-temporal development to the MDs of the anodic pin polarity. On the other hand, at high applied voltages a development similar to transient sparks, i.e. a double-streamer starting near the tip of the pin (cathode), is observed. The statistical study shows that in DBD-like MDs the current pulse amplitude is not dependent on the appearance phase (or applied voltage), but this is not the case for the transient sparks.
Since BC reactors are also used for air cleaning, a set of experiments is done with 35 ppm toluene additive. It is observed that adding toluene results in 500~V lower breakdown voltage. Hence, the discharge in the presence of toluene is operated under over-voltage condition, resulting in stronger MDs in the anodic pin, and earlier-appearing as well as weaker MDs in the cathodic pin half-cycles.
The results of this dissertation about the spatio-temporal development and statistical behavior of the single MDs are foreseen to be employed in the study and optimization of plasma reactors, such as "Stacked DBD Reactor," which are developed for exhaust gas and material surface treatment. Furthermore, the results are a benchmark for the study of a novel discharge arrangement with a rotating dielectric electrode.
With this thesis, studies which form the bedrock for the long term goal of first wall heat load control and optimization for the advanced stellarator Wendelstein 7-X are developed, described and put into context. It is laid out how reconstruction of features of the edge magnetic field from plasma facing component heat loads is an important first step and can successfully be achieved by artificial neural networks. A detailed study of plasma facing component heat load distribution, potential overloads and overload mitigation possibilities is made in first order approximation of the impact of the main plasma dynamic effects.
This thesis describes recent developments in multi-reflection time-of-flight mass spectrometry (MR-ToF MS) with ions exhibiting large masses and mass differences at an MR-ToF setup at the University of Greifswald. A series of in-trap manipulation techniques to selectively retain or eject ion bunches of multiple species with disparate mass-to-charge ratios is investigated. These highlight the possibility to correct long-term flight-time drifts using a reference ion species far away in mass from the species of interest and also the ability to use such a pair to perform single-reference precision mass determinations. In both cases, the results obtained with disparate-mass ion pairs are comparable to those known from operation with isobaric species.
In addition, an in-trap photoexcitation technique is developed and applied to study the dissociation behavior of atomic bismuth clusters (systems of some number of bismuth atoms). Compared to previous works by other groups, the probed cluster-size range is expanded for both ion polarities, resulting in a more comprehensive picture of the underlying dissociation pathways. The known significance of neutral-tetramer breakoff is confirmed, however, evidence is also found for the loss of larger neutral fragments.
Lastly, the principle of tandem high-resolution MR-ToF MS is introduced. This new method allows the study of the change in dissociation behavior of the cationic bismuth octamer resulting from substituting one of its atoms for lead. It is found that the lead-doping opens new preferential fragmentation pathways that outstrip the dominant tetramer breakoff for this specific precursor cluster size. As a first proof-of-principle experiment, the case of the cationic octamer shows that tandem MR-ToF MS is well-suited for the investigation of compound clusters.
The importance of ion propulsion devices as an option for in-space propulsion of space
crafts and satellites continues to grow. They are more efficient than conventional chemi-
cal thrusters, which rely on burning their propellant, by ionizing the propellant gas in a
discharge channel and emitting the heavy ions at very high velocities. The ion emission
region of a thruster is called the plume and extends several meters axially and radially
downstream from the exit of a thruster. This region is particularly important for the effi-
ciency of a thruster, because it determines energy and angular distribution of the emitted
ions. It also determines the interaction with the carrier space craft by defining the electric
potential shape and the fluxes and energies of the emitted high energy ions, which are the
key parameters for sputter erosion of satellite components such as solar panels. Developing
new ion thrusters is expensive because of the high number of prototypes and testing cycles
required. Numerical modeling can help to reduce the costs in thruster development, but
the vastly differing length and time scales of the system, particularly the large differences of
scales between the discharge chamber and the plume, make a simulation challenging. Often
both regions are considered to be decoupled and are treated with different models to make
their simulation technically feasible. The coupling between channel and plume plasmas and
its influence on each other is disregarded, because there is no interaction between the two
regions. Therefore, this thesis investigates the physical effects which arise from this cou-
pling as well as models suitable for an integrated simulation of the whole coupled problem
of channel and plume plasmas. For this purpose the High Efficiency Multistage Plasma
Thruster (HEMP-T) ion thruster is considered.
For the discharge channel plasma, a fully kinetic model is required and the Particle-in-Cell
(PIC) method is applied. The PIC method requires very high spatial and temporal resolu-
tions which makes it computationally costly. As a result, only the discharge channel and the
near-field plume close to the channel exit can be simulated. In the channel, the results show
that electrons are magnetized and follow the magnetic field lines. The orientation of the
magnetic field there is mostly parallel to the symmetry axis and the channel walls which re-
sults in a high parallel electron transport and leads to a flat electric potential and a reduced
plasma-wall sheath. Only at the magnetic cusps, which are characteristic of HEMP-Ts the
electrons are guided towards the wall, with ions following due to quasineutrality, where a
classical plasma-wall sheath develops. The ion-wall contact is thus limited to the cusp re-
gion. The small radial drop of the potential towards the wall gives rather low energies of
ions impinging at the wall and minimizes erosion in the HEMP-T.
In the near-field plume, which extends from the thruster exit plane to some centimeters
downstream, the ion emission characteristics is defined. The ratio of radial and axial elec-
tric field components in this region determines the ion emission angle which should be
minimized for maximum thruster efficiency. The plasma discharge in the channel produces
high plasma densities and the subsequent drop from plasma to vacuum potential occurs
further downstream for higher densities. This increases the ratio of radial and axial electric
field components because the plasma expands radially outside of the confinement from the
dielectric discharge channel walls. The potential structure in the near-field plume impacts
also the supply of electrons for the channel discharge because the electrons enter the channel
from the plume. An effect which arises from this coupling is the breathing mode oscilla-
tion. It is an oscillation which is observed in all plasma quantities and is located near the
thruster exit. The oscillation frequency measured in the simulation is in good agreement
with a predator-prey estimate which validates this ansatz. However, the electron tempera-
ture, assumed constant in the predator-prey model, correlates inversely with the oscillation,
i.e. it is minimal at the current maximum and vice versa, which contributes to the observed
oscillations. Because of the oscillation of the plasma number density, the potential drop also
oscillates in the exit region and thus the ratio of radial to axial electric field components,
which results in the oscillation of the mean ion emission angle.
Regarding suitable models for a combined simulation of channel and plume plasmas, the
PIC model for channel and near-field plume is explicitly coupled to a hybrid fluid-PIC
model for the plume. The latter treats the electrons as a fluid, hence increasing the effective
spatial and temporal resolutions which can be applied in the plume simulations at the cost
of reduced accuracy of the electron model. Plasma densities decrease by two orders of
magnitude two meters downstream from the channel exit. The explicitly coupled kinetic
and hybrid PIC models are well suited for the computation of a HEMP-T and its plume
expansion, but they disregard the coupling of channel and plume plasmas for which other
methods are necessary. For this purpose a new approach is presented with a proof-of-
principle validation. The limited spatial resolution in the plume can be overcome with the
mesh-coarsening method, which increases the resolution in regions of low plasma density
without numerical artifacts. Sub-cycling for the electrons in the plume can then be used
to increase the temporal resolution in the plume. The combination of both methods, called
the sub-cycling mesh-coarsening (SMC) algorithm in the scope of this work, promises high
savings in computational cost which can make a combined simulation of plume and channel
plasmas feasible.
In this doctoral thesis, algorithms are presented that are designed for the investigation in the mesopause region between the upper Mesosphere and Lower Thermosphere (MLT). The photochemical models are proposed and applied to represent the oxygen airglow and the oxygen photochemistry in the MLT. Atomic oxygen, O, in the ground state, O(3P), is of special interest because it is a reactive trace gas actively contributing to the Earth’s airglow. The retrievals of O(3P) concentrations, [O(3P)], are based on the nightglow time series of the green line emission measured remotely as in the first part of this thesis and the individual profiles of multiple nightglow emissions of O and molecular oxygen (O2) measured in situ as in the second part of this thesis. To process the complete spectral time series measured by using the satellite-borne instrument SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY), an intricate set of algorithms is developed and applied with the regularized total least squares minimization approach to estimate a set of the optimal regularization parameters and to retrieve a corresponding set of vertical Volume Emission Rate (VER) profiles. Furthermore, these algorithms take emissions of another origin and the Earth's shape into account. Considering not identified states of O2, the established photochemical models are adjusted resulting in two model modifications. Both model modifications are employed to retrieve the [O(3P)] time series on the basis of the VER time series in the MLT. The model input parameters vary in the atmosphere that motivated to propose these two model modifications and to employ available sources of the input parameters. One semi-empirical model, one general circulation model and the satellite-borne instrument SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) are employed as sources of the reference [O(3P)] and input parameters time series. The SABER instrument employed as a source of the input parameters is preferred according to the comparison of the retrieved and reference [O(3P)] time series. Studying the impact of the 11-year solar cycle on O(3P) in the MLT, an algorithm is developed and applied with the Levenberg-Marquardt algorithm to estimate the optimal fit parameters step-wise. The results of the O(3P) sensitivity analysis obtained with respect to the solar activity forcing at the 11 year and 27 day time scales and the lunar gravitational forcing agree with the reference model simulations. The hypothesis regarding vertical shifts between different of Meinel bands at least partly caused by the hydroxyl radical (OH*) quenching with O(3P) is confirmed experimentally. Based on the conclusion drawn in the first part of this thesis that the data sets’ self-consistency is high as for the averaged SABER and SCIAMACHY measurements, a comprehensive set of available data with a higher level of the data sets’ self-consistency is employed in the second part of this thesis. Multiple airglow emissions measured in situ during four campaigns are employed to propose the Multiple Airglow Chemistry (MAC) model. Processed emissions are the Herzberg I, Chamberlain, Atmospheric and Infrared Atmospheric band emissions of O2 and the green line emission of O. Considering all widely known and additionally complemented reactions, the MAC model is proposed to represent the oxygen airglow and the oxygen photochemistry in the MLT. The presented MAC model is based on the hypothesis of Slanger et al. (2004) stating that higher excited states of O2 are coupled with each other through vibronic de-excitation caused by collisions among molecules of this group of O2 states in the MLT. This hypothesis is modified excluding the singlet Herzberg state of O2 from the group of O2 states considered by Slanger et al. (2004). The MAC calculations are carried out sequentially starting with higher excited O2 states to provide the retrieved output concentrations of these O2 states as the input concentrations to the next calculation steps. The final step is only based on concentrations of all species, whereas each of the earlier steps is based on a corresponding VER profile besides of the input concentrations. The oxygen photochemistry in the MLT is represented by all species considered at the final step that makes it possible to adopt the MAC reactions in a general circulation model. Four modifications of the MAC model, i.e. including or excluding the triplet Herzberg states of O2 and including or excluding ozone and odd hydrogen (hydrogen, OH* and hydroperoxy radical), lead to negligible differences in the retrieved [O(3P)] profiles. Based on the MAC calculations verified and validated on the basis of the four rocket campaigns, one of the effective modifications of the MAC model (excluding the triplet Herzberg states of O2, ozone and odd hydrogen) is further reduced to the most effective modification. This implies that for the [O(3P)] retrieval only the O2 Atmospheric band emission, temperature and concentrations of molecular nitrogen (N2) and O2 are sufficient to apply. Calculations carried out by using the most effective modification of the MAC model are verified and validated on the basis of self-consistent in situ measurements obtained simultaneously. The MAC model enables identifying precursors of (1) the three lowest O2 valence states and (2) the second excited O state responsible for (1) the Atmospheric and Infrared Atmospheric band emissions of O2 and (2) the green line emission of O, respectively. Particularly, the singlet Herzberg state of O2 is identified as the major precursor of the second excited O state resulting in the green line emission. In focus of potential further research is an extension of the MAC model with vibrationally excited states of O2 and ionized species.
In this work, we theoretically investigate both aspects of charge-transferring atom-surface collisions: local-moment-type correlations and emission of secondary electrons from surfaces. Ideally, one chooses an approach that keeps as many electronic and lattice degrees of freedom at an ab-initio level as possible. In practice, however, this sophistication is hard to maintain. In this work, we do not aim to perform a description from first principles which could utilize density functional theory or quantum-chemical techniques. Instead, we keep only the most important degrees of freedom of the scattering process and use effective models for them. These are basically the Anderson-impurity model leading to time-dependent Anderson-Newns Hamiltonians and Gadzuk’s semiempirical approach to describe the projectile-target interaction from classical image shifts. In direct comparison with the description from first principles, the semiempirical approach offers a flexible basis for the modeling of a great variety of projectile-target combinations. The addition of further effective models to increase the general quality of the results is possible since the approach is very modular. The clear physical interpretation of each effective model, as well as the requirement for only a few and generally available parameters are further advantages of this approach. Rewritten in terms of Coleman’s pseudo-particle operators, the model is then numerically analyzed. This is done within a non-crossing approximation for the hybridization self-energies which are utilized by contour-ordered Green functions for each relevant electronic state of the projectile.
Matrix-product-state based methods, in particular the density-matrix renormalization group, are used to numerically investigate several one-dimensional systems, focusing on models with symmetry-protected topological phases that generalize the spin-1 Haldane chain. In the first part, ground state properties such as topological order parameters and the criticality at quantum phase transitions are studied.
The second part deals with dynamic properties of spin chains. Using time-dependent matrix-product-state calculations, the dynamic structure factor, and the transport properties of contacted spin chains are analyzed.
This thesis describes experiments with clusters stored in an electrostatic ion trap called Multi-reflection time-of-flight (MR-ToF) analyzer. These devices are established as mass separators and analyzers with high resolving powers and fast processing times. The objective was to characterize an experiment that utilizes such analyzer for cluster research, to this end a laser-ablation ion source was combined with an MR-ToF analyzer.
In the first part, an experiment scheme that combines two operating modes, namely in-trap lift operation and mirror operation, is presented and characterized for the present setup. For ion capture in-trap lift switching was employed and exit-side mirror switching for ejection with higher information content. Measurements were performed with small lead clusters to illustrate individual advantages of both techniques and the gain of combining them with focus on the ions’ ToF ejection window.
In the second part, a recently introduced method of ion separation by transversal ejection of unwanted species inside the trap was studied for the present setup. The ejection is performed by appropriate pulses of the potentials of deflector electrodes located in the trap. The various parameters affecting the selection effectivity and resolving power are illustrated with tin-cluster measurements, with resolving powers of up to several tens of thousands.
The third part presents the experiment in detail, with the construction of each component and measurements for its various performance parameters. Because the heart of the setup is the MR-ToF analyzer the characterization focuses on the trap. In addition, cluster ions were mass selected in the MR-ToF device and photodissociated. The charged fragments were stored and mass analyzed in a proof-of principle MS/MS experiment where both MS steps were performed in the MR-ToF operation mode.
This thesis describes mass measurements at ISOLTRAP/ISOLDE/CERN in the region of the neutron-rich calcium isotopes. For the less exotic and more abundantly produced isotopes 51Ca and 52Ca the Penning trap based ToF-ICR technique could be used to validate the available mass data and to improve their precision. For the isotopes 53Ca and 54Ca, a Multi-Reflection Time-of-Flight Mass Spectrometer (MR-ToF MS) was used to determine the mass of these exotic isotopes for the first time experimentally. This also represents the first time an MR-ToF MS was applied to derive the masses of previously unknown radioactive ions from the high precision time-of-flight data that can be gathered with the device. The mass data was then used to benchmark the strength of the N=32 neutron subshell closure and at the same time to compare to state-of-the-art shell-model calculations.
Furthermore, the capability of the MR-ToF device to deliver isobarically pure beams to a subsequent experiment was developed further and studied in detail. The new technique is based on the in-trap lift, which is normally used to in- and eject ions into and from the device. With this new selective ejection technique after separation of the ion ensemble in the MR-ToF trap, no external components are required.
Additionally, a new stabilization system for voltages supplies, based on a PI-algorithm, was developed and thoroughly tested. The stabilized voltage supply was then used to supply the most sensitive mirror voltage of the MR-ToF MS to significantly increase the short term and long-term mass resolving power of the apparatus.
In the present work high density helicon plasma discharges are created and characterized as a promising concept towards the realization of plasma wakefield accelerators to build up electric fields in the order of GV/m to accelerate electrons to energies in the TeV range with proton driving bunches. For such a concept plasma sources are needed that are able to maintain discharges with plasma densities of n_e = 7E20 m^-3 over long distances with a low variation in plasma density. Measurements at the PROMETHEUS-A device are performed for variable parameters, like magnetic induction, RF heating power and filling gas pressure. A CO2 laser interferometer, a laser induced fluorescence (LIF) diagnostic and a reaction rate model are combined to give a full picture. It is shown that in most cases the plasma density is centrally peaked with a high density region +- 5 mm from the center. The peak plasma density increases with increasing filling gas pressure, RF heating power and magnetic induction, limited by the number of neutral particles in low pressure discharges, by the transferred heating power and the increasing recombination and electron quenching rates of argon ions in high filling pressure cases. The increase in plasma density with increasing magnetic induction correlates to the direct proportionality in the helicon dispersion relation. For all investigated operational parameters the time evolution of the helicon discharge shows the same characteristics and is reliably reproducable inside the error bars. The electron temperature is determined by combining the collisional radiative model with line ratio measurements of two spontaneously emitted LIF lines. The low electron temperature regime of 1.2 eV < T_e < 1.4 eV and the electron temperature profiles are consistent with helicon wave heating via collisional power dissipation. The maximum plasma density of n_e = (6 +- 1)E20 m^-3 is measured at high RF power of P_RF = 24 kW, p_0 = 9 Pa filling gas pressure and a magnetic induction of B = 105 mT with a maximum electron temperature at 1.4 eV. At these operational parameters the plasma density peaking time and width are determined to be 270E-6 s and 50E-6 s, respectively. This shows that specific plasma density requirements for the use of a wakefield accelerator are reachable and the duration of the peak plasma density is more than sufficient for a relativistic particle to pass a 1 km long plasma cell. Additionally time-resolved LIF profile measurements for neutral and singly ionized argon were conducted to complement the previously evaluated measurements. The time resolution of the LIF diagnostic was chosen in a way to adequately represent the evolution of densities and to allow full profile measurements over one day. A resolution of 200E-6 s was chosen. The time-resolved neutral and ion metastable densities show hollow profiles with high densities at the edges over the first ms indicating higher ionization levels and increasing electron quenching rates. The metastable densities are highly determined by electron temperature, RF heating power and filling neutral gas pressure and do not reflect the neutral argon evolution. To investigate the influence of neutral depletion on the density evolution and maximum plasma density, the argon neutral and ion ground state densities are determined. Both time-resolved density profiles show a hollow profile with highest densities at the edges over a longer time interval of 3-4 ms. The penetration depths (ionization mean-free paths) indicate increased ionization of neutral argon while dissipating inwards, corresponding well to the theoretical value of lambda = 20 mm. This results in a depletion of neutrals in the center of the discharge, leading to a limitation and a fast decrease of plasma density after the neutrals are partially ionized. The shown refilling effect of neutral argon is too slow to have an important impact. At operation parameters for highest plasma density, the calculated ground states also show a fast increase in density at the end of the discharge after the RF-heating is switched off. This indicates recombination effects to these atomic states and higher ionization levels than ArII in the helicon discharge.
This dissertation focusses on the numerical modelling of resonant destabilization of Alfvén eigenmodes by fast ions in fusion plasmas. It especially addresses non-linear simulations of stellarator plasmas in which particle collisions are retained. It is shown that collisions are required for a realistic description of Alfvén waves in plasmas relevant to nuclear fusion.
We start by carefully verifying the implementation of the collision operators into the electromagnetic version of the gyro-kinetic delta-f particle-in-cell code EUTERPE. After these initial benchmarks are completed successfully, the code is in a position to be applied to realistic tokamak and stellarator scenarios.
Since every collision operator needs to fulfil conservation laws, a momentum-conserving version of the pitch-angle scattering operator is implemented. This is in particular important for neoclassical transport simulations aimed at computing flux-surface variations of the electrostatic potential in stellarators.
Using the simplified CKA-EUTERPE model (employing a fixed-mode-structure approximation), we perform non-linear simulations in tokamaks and stellarators. We show that the non-linear dynamics of fast-ion-driven Alfvén eigenmodes is significantly influenced by collisions. They have the potential to enhance the saturation level and to affect the frequency chirping of the modes.
It is thus concluded that collisions play an essential role in determining Alfvén-eigenmode-induced fast-ion transport - an important issue for future fusion devices. In order to address this issue the CKA-EUTERPE model is extended to evolve multiple modes at the same time. First results of this multi-mode version (which enhances the level of realism of the simulations) are shown in the Appendix of the thesis.
The goal of this thesis was to characterize the properties of tetramyristoyl cardiolipin (TMCL) and several environmental influences on it. This included investigating the pH and temperature dependency of TMCL as well as the influences of ROS on TMCL and exam-ining the lipid-protein interactions between TMCL and cytc. Furthermore, I extended the research to the analysis of binary mixtures composed of TMCL and dimyristoyl phosphati-dylcholine (DMPC). To this end, I investigated the samples with the aid of the Langmuir monolayer technique. This method allowed me to mimic interactions occurring at the membrane surface as it represents one membrane layer. The recording of π-A isotherms was also coupled with further other techniques like Brewster angle microscopy (BAM), Infrared Reflection-Absorption Spectroscopy (IRRAS), Grazing Incidence X-Ray Diffraction (GIXD) and Total Reflection X-Ray Fluorescence (TRXF) to enable a more comprehensive monolayer study. In addition, some systems were analyzed using Thin-layer Chromatography (TLC) and/or Differential Scanning Calorimetry (DSC) to be able to draw conclusions about sample composition or characteristic temperatures, respectively.
Optomechanical (om) systems are characterized by their nonlinear light-matter interaction. This is responsible for unique dynamic properties and allows the detection of a variety of classical and quantum mechanical phenomena on a microscopic as well as on a macroscopic scale. In this work we have studied the dynamic behavior of two laser-driven om systems, the single om cell ("cavity optomechanics / membrane-in-the-middle setup") and a two-dimensional hexagonal array of these cells ("om graphene"). The first case was motivated by the possibility to detect the transition from quantum mechanics to classical mechanics directly on the basis of the dynamic behavior. For this we focus on multistability effects of the optical and mechanical degrees of freedom, that are modeled by harmonic oscillators. Our description is based on the quantum optical master equation, which takes into account the environmental interaction assuming a vanishing temperature. As a consequence of decoherence, the dynamics occur near the semiclassical limit, i.e. it is characterized by quantum fluctuations. The quantum-to-classical transition is realized formally by rescaling the equations of motion. In the classical limit, quantum fluctuations disappear and the mean field equations were evaluated by analytical and numerical methods. We found that classical multistability is characterized by stationary signatures on the route to chaos, as well as by the coexistence of single-periodic orbits for the mechanical degree of freedom. The latter point was extensively evaluated by means of a self-consistent approach. For the dynamics in the quantum regime quantum fluctuations cannot be neglected. For this purpose, the master equation was solved by means of a numerical implementation of the Quantum State Diffusion (QSD) method. Based on Wigner and autocorrelation functions, we were able to show that quantum multistability is a dynamic effect: chaotic dynamics is suppressed and there is a time-dependent distribution of the phase space volume on classical simple-periodic orbits. The results can be interpreted within a semiclassical picture, which makes use of the single QSD quantum trajectory. Accordingly, the quantum-classical transition is explained as a time-scale effect, which is determined by tunneling probabilities in an effective mean-field potential. The subject of the second part of the work is the transport of low-energy Dirac quasiparticles in om graphene, propagating as light and sound waves. For this purpose, we investigated the scattering of a plane light wave by laser-induced photon-phonon coupling planar and circular barriers. The starting point is the om Dirac equation, which results from the continuum approximation of the Hamiltonian description of the two-dimensional array near the semiclassical limit. This work was motivated by the rich and interesting relativistic transport and tunneling phenomena found for electrons in graphene, which now appear in a new way. The reason is the presence of the new spin degree of freedom, which distinguishes the optical and mechanical excitations. In this spin space, the om interaction can be understood as a potential, which in our analysis consists of a time-independent and a time-dependent sinusoidal part. For the first case of a static barrier, the transport is elastic and is characterized by stationary scattering signatures. After solving the scattering problem via continuity conditions we were able to identify different scattering regimes depending on scattering parameters. In addition to relativistic phenomena such as Klein tunneling, simple parameter variation allows to use the barrier as a resonant light-sound interconverter and angle-dependent emitter. For the oscillating barrier, the transport is inelastic and is characterized by dynamic scattering signatures. To solve the time-periodic scattering problem, we have applied the Floquet theory for an effective two-level system. As a result of the barrier oscillation, photons and phonons can get and give away energy portions in the form of integer multiples of the oscillation frequency. The interference of short (classical) and long-wave (quantum) components leads to mixing of the scattering regimes. This allows to use the barrier as a time-periodic light-sound interconverter with interesting radiation characteristics. In addition, we have argued that the oscillating barrier provides the necessary energetic conditions for detecting zitterbewegung.
Ion thrusters are Electric Propulsion systems used for satellites and space missions. Within
this work, the High Efficient Multistage Plasma Thruster (HEMP-T), patented by the
THALES group, is investigated. It relies on plasma production by magnetised electrons.
Since the confined plasma in the thruster channel is non-Maxwellian, the near-field plume
plasma is as well. Therefore, the Particle-In-Cell method combined with a Monte-Carlo
Collision model (PIC-MCC) is used to model both regions. In order to increase the sim-
ulated near-field plume region, a non-equidistant grid is utilised, motivated by the lower
plasma density in the plume. To minimise artificial self-forces at grid points bordered by
cells of different size a modified method for the electric field calculation was developed in
this thesis. In order to investigate the outer plume region, where electric field and collisions
are negligible, a ray-tracing Monte-Carlo model is used. With these simulation methods,
two main questions are addressed in this work.
What are the basic mechanisms for plasma confinement, plasma-wall-interaction
and thrust generation?
For the HEMP-T the plasma is confined by magnetic fields in the thruster channel, generated
by cylindrical permanent magnets with opposite polarity. Due to different Hall parameters,
electrons are magnetised, while ions are not. Therefore, the dominating electron transport
is parallel to the magnetic field lines. In the narrow cusp regions, the magnetic mirror effect
reduces the electron flux towards the wall and confines the electrons like in a magnetic
bottle. At the anode, propellant gas streams into the thruster channel, which gets ionised
by the electrons creating the plasma. As a result of the electron oscillation between the two
cusp regions, ionisation of the propellant gas is efficient.
The magnetic field configuration of the HEMP-T also influences the plasma potential inside
the thruster channel. Close to the symmetry axis, the mainly axial magnetic field results in
a flat potential. At the inner wall, the field configuration reduces the plasma wall interaction
to only the narrow cusp regions. Here, the floating potential of the dielectric channel wall
and its plasma sheath result in a rather low radial potential drop compared to the applied
anode potential. As a result, the electric potential is rather flat and impinging ions at the
thruster channel wall have energies below the sputter threshold energy of the wall material.
Therefore, no sputtering appears at the dielectric wall. At the thruster exit the confinement
by the magnetic field is weakened and the potential drops with nearly the full anode voltage.
The resulting electric field accelerates the generated ions into the plume and generate the
thrust, but they are also able to sputter surfaces. During terrestrial testing, sputteringat vacuum vessel walls leads to the production of impurities. The amount of back-flux
towards the channel exit is determined by the sputter yield of the vacuum chamber wall. A
large distance between thruster exit and vessel wall reduces the back-flux and smooths the
pattern of deposition inside the thruster channel. Dependent on their material, the evolving
deposited layers can get conductive, modify by this the potential distribution and reduce
the thrust.
For the HEMP-T, ions are mainly generated at high potential close to the applied anode
potential. Therefore, the accelerated ions producing the thrust gain the maximum energy
as observed in experiment. Ions emitted from the thruster into different angles in the
plume contribute mainly to the ion current at angles between 30 ◦ and 90 ◦ . They mainly
originate from ionisation at the thruster exit. The resulting angular distribution of the
ejected ion current is close to the one of the experiment, slightly shifted by about ten
degrees to higher emission angles. In front of the thruster exit, electrons are trapped by
electrostatics forces. This enhanced density allows ionisation and an additional electron
density structure establishes.
What are possible physics based ideas for optimisation of an ion thruster?
An optimised thruster should have a high ionisation rate inside the thruster channel, low
erosion and an ion angular distribution with small contributions at high angles for min-
imised thruster satellite interactions. In experiments, the HEMP-T satisfies already quite
nicely these requests. In the simulations, low erosion inside the thruster channel and angular
ion distributions close to the experimental data are demonstrated. However, the ionisation
efficiency is lower and radial ion losses are larger than in experiment. A possible explanation
of these differences is an underestimated transport perpendicular to the magnetic field lines,
well known for magnetised plasmas.
A successful example for an optimisation using numerical simulations is the reduction of
back-flux of sputtered impurities during terrestrial experiments by an improved set-up of
the vacuum vessel. The implementation of baffles reduces the back-flux towards the thruster
exit and therefore deposition inside the channel. These improvements were successfully im-
plemented in the experiment and showed a reduction of artefacts during long time measure-
ments. This leads to a stable performance, as it is expected in space.
Manipulating and utilizing plasmas becomes a more and more important task in various research fields of physics and in industrial developments. Especially in nowadays spacerelevant applications there are different ideas to modify plasmas concerning particular tasks.
One major point of interest is the ability to influence plasmas using magnetic fields. To study the underlying physical effects that were achieved by these magnetic fields for both scenarios Particle-in-Cell simulations were done. Two examples are discussed in this thesis.
The first example originates from an experiment performed by the European Space Agency ESA in collaboration with the German Space Agency DLR. To verify the possibility of heat-flux reduction by magnetic fields onto the thermal protection system of a space vehicle a simplified experiment on earth was developed. Most of the heat that is created during re-entry comes from compression of the air ahead of the hypersonic vehicle, as a result of the basic thermodynamic relation between temperature and pressure. The shock front, which builds up in front of the vehicle deflects most of the heat and prohibits the surface of the space vehicle from direct contact with the maximum flux. State of the art spacecrafts use highly developed materials like ceramics to handle the enormous heat. An attractive approach to reduce costs is to use magnetic fields for heat-flux reduction. This would allow the use of cheaper materials and thus reduce costs for the whole space mission. A partially-ionized Argon beam was used to create a certain heat-flux onto a target. The main finding of the experimental campaign was a large mitigation of heat-flux by applying a dipole-like magnetic field. The Particle-in-Cell method was able to reproduce experimental observations like the heat-flux reduction. An additionally implemented optical diagnostics module allowed to confirm the results of the spectroscopy done during the experiment. The underlying effect that is responsible for the heat-flux reduction was identified as a coupling between the modified plasma and the dominating neutral flux component. The plasma, that is guided towards the target, act as a shield in front of the target surface for arriving neutrals. These neutrals are slowed down by charge-exchange collisions. Furthermore the magnetic field induces an increased turbulent transport that is also needed to reach a reduction in heat-ux. The turbulent transport was also obtained by three-dimensional Direct Simulation Monte Carlo simulations. Unfortunately, such source driven turbulence can not be expected in space, so that a heat flux reduction in real space applications is questionable. Nevertheless, other effects like the induced turbulence by the rotating vehicle can compensate the missing source driven effect.
The second scenario in which a magnetic field is used to modify the heat flux of a plasma is the operation of the pulsed cathodic arc thruster. The same Particle-in-Cell code was used to simulate a typical pulse of this newly developed thruster of Neumann Space Pty Ltd. The typical behavior of the thruster could be reproduced numerically. The thrust is mainly produced by fast electrons. These electrons are accelerated by electric fields as a result of a plasma-beam instability. This plasma-beam instability was verified by a phase space diagnostics for the electrons. To demonstrate the influence of the magnetic field a simulation of the cathodic arc thruster without magnetic field and one with magnetic field were compared. It was shown that the use of a magnetic field leads to a ten times larger thrust by directing the heat ux. The resulting narrow plume is an additional Advantage of the particle guiding magnetic field. This narrowness of the plume reduces the danger of interaction with other components of the space vehicle.
Both scenarios demonstrate the different capabilities for electromagnetic fields to manipulate plasmas and especially the corresponding heat-flux with respect to certain tasks. The possibilities range from reducing the heat-flux onto a target to maximizing the thrust by directing the heat-ux. This thesis demonstrates that simulations are a great tool to support experiments and to deliver an improved physics understanding. They help to identify the basic physics principles in the different systems, because they can deliver information not accessible to experiments.
In particular, a better understanding of the influence of electromagnetic fields on the heat-flux distribution in space-relevant applications was obtained. This can be the basis for further simulation-guided optimization, e.g. for the design of more effective cathodic arc thrusters. Here, the goal is to minimize costs for prototypes by replacing the hardware by virtual prototypes in the simulations. This allows to test basic design ideas in advance and get more highly-optimized designs at a fraction of time and costs.
This work study a monolayer of branched poly(ethyleneimine (PEI) adsorbed onto oppositely charged surfaces with iron chelates or iron ions in the absorption solution. The conformation of adsorbed PEI is explored in the dependence of the composition of the adsorption solution by measuring the surface forces using atomic force microscopy (AFM) with the colloidal probe (CP) at different ionic strengths (INaCl) in surrounding aqueous solution. The surface coverage of these layers is investigated using X-ray reflectivity.
PEI solutions show different pH values with iron chelates (pH = 3), iron ions (pH = 4.67) or pure water (pH = 9.3) at room temperature. Low surface coverage of PEI at pH = 3 adjusted by monovalent ions was also observed. However, adsorbing PEI with iron ions or iron chelates and washing with pure water shifts the pH, leading to an adsorbed PEI layer with high coverage. In our observation, the influence of iron ions and iron chelates on the surface coverage of PEI film is stronger than the pH effect. PEI adsorbed from a pure water solution shows flat conformation. Surface force measurements with CP show that PEI adsorbed from solutions containing iron chelates or iron ions cause almost identical steric forces. The thickness of the brush L is determined as a function of the ionic INaCl in the measuring solution. It scales as a polyelectrolyte brush.
The maximum number density of gold nanoparticles (AuNPs) adsorbed onto the PEI brushes was identical and larger than on flatly adsorbed PEI. On the PEI layer with the larger surface coverage, the AuNPs aggregate; on the PEI layer with the lower surface coverage they do not aggregate. Taken together, these results contribute to understanding the mechanisms determining surface coverage and conformation of PEI and demonstrate the possibility of controlling surface properties, which is highly desirable for potential future applications.
In this thesis, we also investigate the top layer (PSS and PDADMA) of polyelectrolyte multilayer (PEM) films. PEM films were prepared by sequential adsorption of oppositely charged PEs on solid substrates. PEM films consist of polydiallyldimethylammonium (PDADMA) as polycation and the polystyrene sulfonate (PSS) as polyanion. PDADMA has a smaller linear charge density than PSS. For this system, two different growth regimes are known: parabolic and linear. I studied the top layer (PSS and PDADMA) conformation of PEM films and how the structure of this top layer is affected by increasing the number of PDADMA/PSS layer pairs N and the addition of salt to the surrounding solution.
The INaCl was changed during the force-distance measurements. PSS terminated films always show electrostatic forces at INaCl < 0.1 M and flat conformation. The surface charge density is always negative at INaCl < 0.1 M. The surface charge of the PSS top layer starts to turn from negative to positive at N ≥ 14. At N between 13 and 15, adsorbed PSS cannot compensate all the excess PDADMA charge. This leads to an accumulation of the positive extrinsic sites within the PSS terminated film beyond a specific N. At INaCl ≈ 0.1 M, an exponential decaying force was measured. This is an indication of unusual long-ranged hydration force (decay length λ-1 ≈ 0.2-0.5 nm), and PSS terminated film shows zwitterionic or neutral surface. At INaCl > 0.1 M, a non-electrostatic action occurs and the PSS terminated film reswells in solution.
PDADMA terminated surface consisting of few layers show a flat conformation and the electrostatic forces were measured. For N ≥ 9 and INaCl ≤ 0.1 M, steric forces were measured. The force-distance profiles are well-explained by Alexander and de Gennes theory. PDADMA chains show a maximum L that is around 40-45 % of the contour length. For INaCl ≈ 0.1 M, and N > 9, a flat, neutral or zwitterionic surface is found (λ-1 ≈ 0.3-0.9 nm). For N = 9 and INaCl > 0.1 M, a strong screening of electrostatic interaction and attractive forces are observed. For N > 9 and INaCl > 0.1 M, the ion adsorption into the PE chains leads to an increase in the monomer size and as a result, the L increases and PDADMA brushes reswell again into the solution.
These data show that by varying N and INaCl, different surface forces can be obtained: Electrostatic forces (flat chains) both positive and negative, steric forces (brush), hydration force (flat, neutral or zwitterionic surface), and effects not yet explained (reswelling brush).
In this thesis, size-sensitive phenomena of three-dimensional dust crystals emerged in a low temperature plasma are presented. Depending on the number of particles in the system phase transitions, collective vortex motions and large-scaled expansions can be observed. To investigate these fascinating effects an advanced experimental setup as well as new evaluation methods have been developed. This thesis will present these new techniques and the gained insights.
Lead-cluster investigations
(2017)
In this thesis, investigations on lead clusters stored in a Penning trap are presented. The measurements are performed at the ClusterTrap setup at the Institute of Physics of the University of Greifswald. A Penning trap with a superconducting magnet (B=12 Tesla) makes up the central part of the experiment. In this trap, singly positively or negatively charged lead clusters (a group of lead atoms) are stored, their amplitudes of motion are cooled, and a specific cluster size is selected. Thus, clusters of only a single size are prepared for experimental investigation. After interactions with electrons and/or photons, the trap content is extracted and analyzed by time-of-flight mass spectrometry.
In the first experiment, the size-selected clusters are excited by a frequency-doubled Nd:YAG laser, which leads to fragmentation processes. The preferred fragmentation pathway, which is observed to be break-off of a seven-atom neutral cluster is unusual for metal clusters, which typically evaporate monomers. Furthermore, the already known magic cluster sizes are observed.
In a subsequent experiment, positively charged lead clusters with 31 atoms are irradiated with laser light and fragmentation processes are time resolved investigated. The assumption that lead clusters fragment by break-off of neutral heptamers is confirmed.
In the following experiment, an electron beam is guided through the Penning trap to ionize pulsed-in argon atoms. While the positive argon ions leave the trap, the secondary electrons are trapped together with the selected lead clusters. This allows the electrons to attach to the singly charged lead clusters, which leads to multiply negatively charged lead clusters. The relative abundance of multiply-charged clusters is measured with respect to the cluster size, from which the appearance sizes of di- and trianions can be calculated. In addition to the attachment of electrons, fragmentation products similar to those of the photoexcitation measurements are observed. Furthermore, the cluster sizes 10 and 12 are observed regardless of the investigated precursor size, together with clusters of the precursor size reduced by 10 and 12. This is a first hint for a fission process of doubly negatively charged lead clusters into two singly charged products. In a following measurement, doubly charged lead clusters are produced and photoexcited. The observed abundance spectra confirm this assumption.
This work examines the influence of monovalent and divalent cations on tetramyristoyl cardiolipin (TMCL) monolayers. A lipid monolayer can undergo an ordering transition of the lipid alkyl chains from a disordered fluid phase (liquid-expanded (LE)) to an ordered gel phase (liquid-condensed (LC)). Compression of the lipid monolayer in a Pockels-Langmuir trough was monitored with a Wilhelmy plate tensiometer, yielding the surface pressure π in dependence of the area a molecule can occupy on average A, as a π-A-isotherm. The onset of the first order LE/LC phase transition is marked by an abrupt change in the isotherm at surface pressure πc.
These associated lipid membrane changes were characterized by variation of the compression speed, kind and concentration of the monovalent and divalent salt, pH, and temperature. The CL monolayer phase transition was found to depend on the compression speed, yielding only a small variation in the compression isotherms.
For monovalent cations on the cardiolipin monolayer, the dependence on salt concentration of the lipid liquid gel phase transition surface pressure πc was determined and a non-monotonic behavior was found, with a maximum in πc for a salt concentration of 0.1 mol/l. The maximum in πc can be shifted with pH (e.g. pH = 4.2). This behavior extended to potassium, sodium and cesium cations in the subphase. No ion specific effects were observed, which pointed to the prevalence of electrostatic interactions in the system.
Different divalent salt subphases, of either magnesium, calcium, strontium, manganese, iron or zinc salts, with fixed sodium chloride concentration of 0.15 mol/l at pH of 5.8 and 25 °C were investigated. πc decreases upon addition of divalent salts to the subphase. This points to increased screening and binding effects. Strongest binding effects were observed for calcium and manganese cations.
The electrostatic interactions of the system were modeled with a mean-field theory: Grahame’s equation, and a simple law of mass action. CL is modeled at half its molecular area and half its charge, with a proton dissociation constant of the phosphate group Ka,intrinsic(PO4) = 0.1 mol/l. The agreement with the experiment was satisfactory.
A linear dependence of πc on the temperature was found for cardiolipin monolayers on all subphases. The isothermal area compressibility modulus KA is calculated from selected isotherms. It was found that the flexibility of the monolayer decreases with temperature and the area per molecule for the cardiolipin fluid phase.
The compression speed, monovalent salt concentration, pH, and selected divalent cations were investigated with the BAM. For all a sigmoidal growth of xgel with compression was observed. For high salt concentrations non-circular and dendritic domains were observed.
A simple model for the nucleation process was introduced, yielding an estimate of 20 nm for the critical domain radius, which is below the resolution of the BAM, but a common length scale in biological systems.
This thesis highlights the impact of surface charges and negative ions on the pre-ionization, breakdown mechanism, and lateral structure of dielectric barrier discharges operated in binary mixtures of helium with nitrogen or electronegative oxygen. Sophisticated diagnostic methods, e.g., non-invasive optical emission spectroscopy and the electro-optic Pockels effect as well as invasive laser photodetachment and laser photodesorption, were applied at one plane-parallel discharge configuration to investigate both relevant volume and surface processes. Moreover, the experimental findings were supported by numerical fluid simulations of the discharge. For the first time, the memory effect of the measured surface charge distribution was quantified and its impact on the local self-stabilization of discharge filaments was pointed out. As well, it turned out that a few additional seed electrons, either desorbed from the charged dielectric surface or detached from negative ions in the volume, significantly contribute to the pre-ionization resulting in a reduced voltage necessary for discharge breakdown. Finally, effective secondary electron emission coefficients of different dielectrics were estimated from the measured breakdown voltage using an analytical model.
The content of this thesis can be summarized as follows: (i) The deposition processes of SiOx and SiOxCyHz coatings were investigated in a low-pressure, low temperature HMDSO-O2-N2 plasmas. Infrared laser absorption spectroscopy (IRLAS) and optical emission spectroscopy (OES) were combined to measure the gas temperatures in the hot and colder zones of the plasma as well as to monitor the concentration of the methyl radical, CH3, and of seven stable molecules, HMDSO, CH4, C2H2, C2H4, C2H6, CO and CO2. Tunable lead salt diode lasers (TDLs) and an external-cavity quantum cascade laser (EC-QCL) were simultaneously employed as radiation sources to perform the IRLAS measurements. They were found to be in the range between 10^{11} to 10^{15} cm^{−3}. The influence of the discharge parameters of power, pressure and gas mixture on the molecular concentrations was studied. The plasma generation is characterized by a certain degree of inhomogeneity with different temperature zones, i.e., hottest, hot and colder zones depending on the construction of the reactor. This complexity is characterized by the multiple molecular species including the HMDSO precursor and products in ground and excited states existing in the plasma. (ii) Employing similarly IRLAS and OES techniques, the deposition of nanocrystalline diamond at relatively low temperature in low-pressure MW H2 plasmas with small ad-mixtures of methane and carbon dioxide was investigated. Five methods were applied for an extensive temperature analysis, providing new insights into energetic aspects of the multi-component non-equilibrium plasma. The OES method provided information about the gas temperature of H2 inside the MW plasma. Using lead salt diode lasers, the rotational temperature of the methyl radical, CH3 , and gas temperature of methane molecule, CH4 , was measured. A variety of CO lines in the ground and in three excited states have been analysed using an EC-QCL with a relatively wide spectral range. These methods have shown that based on the construction of the DAA reactor using 16 single plasma sources the plasma generation is characterized by a variety of hottest, hot and colder zones. Extensive measurement of these various species temperatures in the complex plasma enabled the concentration determination of the various stable and unstable plasma species, which were found to be in the range between 10 11 to 10 15 cm −3 . The influence of the discharge parameters, power and pressure, on the molecular concentrations has been studied. To achieve insight into general plasma chemical aspects, the dissociation of the carbon precursor gases including their fragmentation and conversion to the reaction products was analysed in detail. The evolution of the concentration of the methyl radical, CH 3 , of five stable molecules, CH4, CO2, CO, C2H2 and C2H4, and of vibrationally excited CO in the first and second hot band was monitored in the plasma processes by in situ infrared laser absorption spectroscopy using lead salt diode lasers (TDL) and an external-cavity quantum cascade laser (EC-QCL) as radiation sources. OES was applied simultaneously to obtain complementary information about the degree of dissociation of the H2 precursor gas. The analysis of the carbon and oxygen mass balances shows clearly, that the deposition on the reactor walls and the production of other hydrocarbons species may act as sinks for carbon and oxygen. (iii) The absolute line strengths of many P-branch transitions of the ν3 fundamental of {28}^SiH4 were determined using the wide tuning range and the narrow line width of a cw EC-QCL between 2096 and 2178 cm^{−1}. The line positions and line strengths of transitions of the stretching dyad within the P-branch of {28}^SiH4 were determined with an estimated experimental measurement accuracy of 10%. The high spectral resolution available has enabled us to resolve and measure representative examples of the tetrahedral splittings associated with each component of the P-branch. The positions of these components are in excellent agreement with spherical top data system (STDS) predictions and theoretical transitions from the TDS spectroscopic database for spherical top molecules. To our knowledge, this is the first reported measurement of these line strengths in this band and is an example of the applicability of high-powered, widely tunable EC-QCLs to high resolution spectroscopy in the MIR. (iv) Similarly, the determination of the silyl radicals, ν3 band, line strengths is ongoing using the same cw EC-QCL. This effort was impaired by silane and other unknown species lines overlap; however, the silyl radicals was successfully detected in a SiH4/H2 plasma. A method to determine the silyl line strengths has been presented through its iterative decay measurements which relied on the value of the silyl radical self reaction constant. There was a consensus of its value in the literature.
Polyelektrolyt-Multischichten (PEMs) werden durch sequentielle Adsorption von entgegengesetzt geladenen Polyelektrolyten (PE) auf festen Substraten adsorbiert. Die Layer-by-Layer Präparation ermöglicht es cm2 große Flächen zu beschichten und außerdem die Möglichkeit die Molekülanordnung senkrecht zur Substratoberfläche im nm-Bereich zu kontrollieren. Der Schwerpunkt dieser Arbeit ist die Untersuchung der Adsorption hinsichtlich des Molekulargewichts der beteiligten PEs von PEMs bestehend aus dem Polykation Polydiallyldimethylammonium (PDADMA) und dem Polyanion Polystyrolsulfonat (PSS). Zu diesem Zweck wird das Schichtwachstum unter in-situ Bedingungen mittels der Ellipsometrie untersucht. Das Schichtwachstum im Fall von PDADMA/PSS Multischichten verläuft nichtlinear mit der Anzahl an deponierten Schichtpaaren. Dabei wird das nichtlineare Wachstumsregime durch die unterschiedliche Linienladungsdichte zwischen einer PDADMA und PSS Kette in Verbindung gebracht. Die quantitative Analyse der Messungen zeigt, dass alle untersuchten PDADMA/PSS Multischichten präpariert aus 0,1 M NaCl bei Raumtemperatur mindestens zwei verschiedene Wachstumsregimes aufweisen. Zunächst wächst die Schicht parabolisch bis sie nach Nlin Schichtpaaren in ein lineares Wachstumsregime übergeht. Dieses wird durch einen konstanten Schichtdickenzuwachs pro adsorbiertes Schichtpaar Δdlin charakterisiert. Das Adsorptionsverhalten der PDADMA und PSS Ketten wird analysiert indem das Molekulargewicht Mw der Polyelektrolyte systematisch variiert wird (zwischen Mw(PDADMA)=24 kDa…322 kDa, sowie Mw(PSS)=8,6 kDa…168 kDa). Eine Analyse der Schichtparameter Nlin und Δdlin ergibt bei hohen Molekulargewichten von PDADMA und PSS, dass Nlin und Δdlin unabhängig von den jeweiligen Molekulargewichten sind (Nlin=15 und Δdlin=12,3 ± 1,3 nm). Reduziert man das Molekulargewicht von PDADMA auf einen Wert unterhalb eines Schwellwertes von Mw (PDADMA)=80 kDa, so nehmen Nlin und Δdlin linear ab. Unterschreitet das PSS-Molekulargewicht den Schwellwert Mw (PSS)=25 kDa, beobachtet man den gegenteiligen Effekt: beide Wachstumsparameter Nlin und Δdlin nehmen zu und ein zusätzliches exponentielles Wachstumsregime tritt auf. Damit wächst die Multischicht zunächst exponentiell, geht nach Nexp Schichtpaaren ins parabolische Wachstum bis dieses nach Nlin Schichtpaaren ins lineare Wachstumsregime übergeht. Neutronenreflexionsmessungen mit selektiv deuterierten PSS Schichten zeigen eine Diffusion der leichten PSS Ketten innerhalb der Multischicht. Ein solches Diffusionsverhalten ist typisch für exponentiell wachsende Schichten und wurde bereits theoretisch vorhergesagt. Um den molekularen Mechanismus der Adsorptionsprozesse besser zu verstehen, werden PEMs aus binären Mischungen präpariert. Diese setzen sich aus einem Molekulargewicht oberhalb und unterhalb des jeweiligen Schwellwertes (Mw(PDADMA)=80 kDa bzw. Mw(PSS)=25 kDa) zusammen. Dabei wird der Molenbruch des schweren Polyelektrolyts (ΦPDADMA(Mw(PDADMA)>80 kDa) bzw. ΦPSS(Mw(PSS)>25 kDa)) variiert. Im Falle der binären PDADMA Mischung beinhaltet die Adsorptionslösung Moleküle mit den Molekulargewichten Mw(PDADMA)= 35 kDa und 322 kDa. Ellipsometrische Messungen zeigen einen linearen Anstieg der Schichtparameter Nlin und Δdlin mit Erhöhung des Molenbruchs ΦPDADMA(322 kDa). Daraus wird gefolgert, dass die Zusammensetzung in der Adsorptionslösung derjenigen in der Multischicht entspricht. Es wird eine Formel zur Bestimmung der Schichparameter Nlin und Δdlin entwickelt, die zumindest auch auf ternäre Mischungen anwendbar ist. Damit lassen sich die Schichtparameter Nlin und Δdlin bei bekannten Molenbrüchen ΦPDADMA(Mw(PDADMA)) vorhersagen. Der Einfluss der Zusammensetzung der PSS-Adsorptionslösung zeigt ein anderes Verhalten: Die Zusammensetzung des Films entspricht hier nicht derjenigen der Adsorptionslösung. Bereits bei einem Anteil von ΦPSS (76 kDa) = 5% des schweren PSSd Moleküls (95% der Moleküle in der Adsorptionslösung sind leichte PSS Moleküle), findet einerseits kein exponentielles Wachstum statt und die Wachstumsparameter Nlin und Δdlin entsprechen denen solcher PEMs, welche ausschließlich aus schweren PSS Molekülen präpariert wurden. Neutronenreflexionsmessungen bei binären PSS-Mischungen mit schwerem deuteriertem PSSd und leichtem protonierten PSS zeigen, dass bei einer Adsorptionszeit von 30 min, ab ΦPSSd (80,8 kDa)=5% (ΦPSS(10,6 kDa)=95%) lediglich das schwere PSSd in die Multischicht eingebaut wurde. Durch die Streulängendichte wird die genaue Anzahl der PSS bzw. PSSd Moleküle in den PEM quantifiziert und damit die Menge an deponiertem Material bestimmt. Eine Hypothese ist, dass die leichten Moleküle die Oberfläche zwar schneller erreichen, in die Multischicht gelangen und durch den Film diffundieren. Dabei können diese gemäß der IN und OUT Diffusion den Film auch wieder verlassen. Um dies zu verifizieren wird die Adsorptionszeit der PSS Moleküle bei einer binären PSS Mischung mit ΦPSSd(80,8 kDa)=5% reduziert.
The thesis deals with ions stored in an electrostatic ion beam trap. In the first part of the thesis the so-called self-synchronization effect is discussed. It is demonstrated that the time a bunch of injected ions is conserved by the self-synchronization effect depends on the number of injected ions. In the second part of the thesis the cooling of small anionic cobalt and copper clusters is addressed. Measurements on anionic copper clusters consisting of four to seven atoms are presented and the decay of hot clusters is observed in order to draw conclusions on the internal temperature and the cooling process itself. Afterwards measurements on Co4- are discussed and a measurement scheme based on laser induced delayed electron emission is presented enabling to monitor the internal energy distribution of the clusters over storage time in a temperature-controlled environment. The cooling of initially hot clusters as well as the heating of initially cold clusters were observed.
In this Ph.D. project a method is developed to measure the magnetic field and to derive variations in the total plasma pressure due to (dia-) magnetic effects. For this purpose a plasma diagnostic has been set up at the fusion experiment ASDEX Upgrade to measure spectroscopically polarized light. The light is emitted from fast beam-particles excited by the plasma. Since the fast atoms travel through a magnetic field at high velocity, a strong Lorentz field is seen in the moving frame. This electric field gives rise to the so-called motional Stark-effect (MSE) and it is possible to conclude from the Stark-spectrum on the magnetic field.
Comprehensive study of the discharge mode transition in inductively coupled radio frequency plasmas
(2016)
In this contribution, the mode transition of an inductively coupled radio frequency plasma at low pressure is investigated. Therefore, a comprehensive set of plasma diagnostics were applied to determine plasma and processing parameters. Therewith, the plasma kinetics and especially the important elementary processes were studied. Hence, the reason for the mode transition was identified.
Nanoengineering and laser optics allow for the fabrication of a wide range of systems that subject fermionic particles to geometric restrictions. In addition to strong correlations, the fermions may couple to internal or external bosonic fields, such as quantized lattice vibrations or light fields. This thesis considers the theoretical description of two such systems. One is a molecular junction, i.e., a small organic molecule contacted by metallic electrodes or leads. Itinerant electrons induce molecular vibrations and deformations, corresponding to phonon modes of considerable energy. The thesis investigates the effects of this local electron-phonon interaction on the electric and thermoelectric transport through the junction. Starting with an Anderson-Holstein quantum dot model, our ansatz is based on the application of a variational Lang-Firsov transformation that accounts for the polaronic character of the dot state. We solve the steady-state Kadanoff-Baym equations and derive a self-consistent approximation to the polaronic self-energy that accounts for finite densities and multi-phonon scattering processes. The optimal variational parameter is determined numerically by minimizing the thermodynamical potential. This allows a detailed study of the electronic dot spectral function for all interaction strengths and adiabaticity regimes. For instance, we discuss how a voltage dependent polaronic renormalization of the dot-lead coupling and the dot level causes negative differential conductance and novel conductance features. The investigation of the second system is motivated by recent experiments on the Bose-Einstein condensation of excitons in small semiconducting cuprous oxide crystals. At ultra cold temperatures three species of para- and orthoexcitons are caught in stress induced potential traps. Their decay luminescence is the primary method of detection. This thesis considers the thermodynamics of this system in terms of a multicomponent gas of weakly interacting bosons in external potentials. The coupled equations of motion are solved within a Hartree-Fock-Bogoliubov-Popov approximation. For typical experimental parameters the density distributions of the interacting species are calculated numerically. Based on the luminescence formula by Shi and Verechaka we discuss, e.g., how the spectrum of the direct decay of thermal paraexcitons may reveal the formation of a nonluminescent paraexciton condensate as well as the spatial separation of strongly repulsive orthocondensates. First results for an extended luminescence theory are presented, which takes into account the polariton effect.
Achieving commercial production of electricity by magnetic confinement fusion requires improvements in energy and particle confinement. In order to better understand and optimise confinement, numerical simulations of plasma phenomena are useful. One particularly challenging regime is that in which long wavelength MHD phenomena interact with kinetic phenomena. In such a regime, global electromagnetic gyrokinetic simulations are necessary. In this regime, computational requirements have been excessive for Eulerian methods, while Particle-in-Cell (PIC) methods have been particularly badly affected by the "cancellation problem", a numerical problem resulting from the structure of the electromagnetic gyrokinetic equations. A number of researchers have been working on mitigating this problem with some significant successes. Another alternative to mitigating the problem is to move to a hybrid system of fluid and gyrokinetic equations. At the expense of reducing the physical content of the numerical model, particularly electron kinetic physics, it is possible in this way to perform global electromagnetic PIC simulations retaining ion gyrokinetic effects but eliminating the cancellation problem. The focus of this work has been the implementation of two such hybrid models into the gyrokinetic code EUTERPE. The two models treat electrons and the entire bulk plasma respectively as a fluid. Both models are additionally capable of considering the self-consistent interaction of an energetic ion species, described gyrokinetically, with the perturbed fields. These two models have been successfully benchmarked in linear growth rate and frequency against other codes for a Toroidal Alfvén Eigenmode (TAE) case. The m=1 internal kink mode, which is particularly challenging in terms of the fully gyrokinetic cancellation problem, has also been successfully benchmarked using the hybrid models with the MHD eigenvalue code CKA. Non-linear simulations in this TAE case have been performed confirming the analytical prediction of a quadratic relationship between the linear growth rate of the TAE and the saturated amplitude of the TAE for a range of moderate values of the linear growth rate. At higher linear growth rate, a slower scaling of saturated amplitude with linear growth rate is observed. This analysis has been extended to include the non-linear wave-wave coupling between multiple TAE modes. It has been shown that wave-wave coupling results in a significant reduction in the saturated amplitude. It has been demonstrated that both plasma elongation and ion kinetic effects can exert a stabilising influence on the internal kink mode. A population of energetic particles can also exert a stabilising influence at low normalised pressure. At high normalised fast particle pressure the stabilised kink mode has been shown to give way to the m=1 EPM, which has been simulated both linearly and non-linearly (the "fishbone" mode). The first self-consistent simulations of global modes in the magnetic geometry of the optimised stellarator Wendelstein 7-X have been performed both linearly and non-linearly. Limitations have been encountered in performing simulations in 3D geometry. A hypothesis for the cause of these problems is outlined and ideas for mitigation are briefly described. In addition to the hybrid model simulations, some of the first utilisations of a new scheme for mitigating the cancellation problem in the fully gyrokinetic regime have been carried out in the framework of this thesis. This scheme, which was developed separately, is concisely described in this work. The new scheme has been benchmarked with existing gyrokinetic and hybrid results. The linear Wendelstein 7-X simulations and linear and single mode non-linear TAE simulations have been repeated with the new model. It is shown that bulk plasma kinetics can suppress the growth rate of global modes in Wendelstein 7-X. The results of fully gyrokinetic TAE simulations, the first to have been performed to our knowledge, are shown to be in close agreement with those results obtained using hybrid models. In the TAE case, the hybrid models are an order of magnitude less computationally demanding than the new gyrokinetic scheme, which is in turn at least an order of magnitude less computationally demanding than the previous gyrokinetic scheme.
Polyelektrolyt-Multischichten werden durch die sequentielle Adsorption von entgegengesetzt geladenen Polyelektrolyten auf einem festen Substrat hergestellt. Die Präparation layer-by-layer ermöglicht die Beschichtungen von Flächen im cm-Bereich mit Schichtdicken im µm-Bereich sowie einer Kontrolle der Molekülanordnung senkrecht zur Substratoberfläche im nm-Bereich. Aus diesen Eigenschaften ergeben sich zahlreiche Anwendungsmöglichkeiten. Ein Schwerpunkt dieser Arbeit ist die Analyse der Polyelektrolyt-Adsorption bei der Präparation von Multischichten bestehend aus dem Polykation Polyallylaminhydrochlorid (PAH) und dem Polyanion Polystyrolsulfonat (PSS) bzw. Polydiallyldimethylammonium (PDADMA) und PSS. Die Untersuchung der Multischichten unter in-situ Bedingungen erfolgt mittels Ellipsometrie. Zu diesem Zweck wird ein Formalismus der ellipsometrischen Datenauswertung entwickelt, um die Messgenauigkeit bei der Untersuchung dünner, transparenter Schichten zu optimieren. Im Fall von PDADMA/PSS-Multischichten verläuft das Schichtwachstum nicht-linear mit der Anzahl an adsorbierten Doppelschichten. Der nicht-lineare Verlauf wird mit der unterschiedlichen Linienladungsdichte zwischen einer PDADMA- und einer PSS-Kette in Verbindung gebracht. Die quantitative Analyse der ellipsometrischen in-situ-Messungen ergibt, dass alle untersuchten PDADMA/PSS-Multischichten (präpariert aus 0,1 mol/L NaCl-Lösung bei Raumtemperatur) mindestens zwei verschiedene Wachstumsregimes aufweisen: Erst wächst die Schichtdicke parabolisch mit der Anzahl an deponierten Polyanion/Polykation-Schichtpaaren, nach Nlin Schichtpaaren erfolgt ein Übergang in lineares Schichtwachstum, charakterisiert durch eine konstante Dicke pro Schichtpaar dBL. Das parabolische Wachstumsregime lässt sich mit einer Asymmetrie im Adsorptionsverhalten von PDADMA und PSS erklären: Während die adsorbierenden PSS-Moleküle die Oberflächenladung lediglich neutralisieren, führt ein PDADMA-Beschichtungsschritt zu einer Ladungsüberkompensation und hinterlässt eine effektiv positiv geladene Oberfläche. Die deponierte Stoffmenge nimmt mit jeder PDADMA/PSS-Doppelschicht zu, bis nach Nlin Doppelschichten die adsorbierenden PSS-Ketten nicht mehr imstande sind alle positiven Oberflächenladungen zu neutralisieren. Die beiden Wachstumsparameter Nlin und dBL hängen in einem linearen Zusammenhang voneinander ab, da beide einem gemeinsamen Mechanismus folgen: Je mehr Doppelschichten ein parabolisches Wachstumsverhalten zeigen (Nlin), desto höher ist die Oberflächenbelegungsdichte am Ende des parabolischen Wachstumsregimes und desto größer die Doppelschichtdicke dBL. Das Adsorptionsverhalten von PDADMA- und PSS-Ketten wird analysiert, indem das Molekulargewicht Mw beider Polyelektrolyte systematisch variiert wird (zwischen Mw(PDADMA) = 24 kDa ... 322 kDa sowie Mw(PSS) = 8,6 kDa ... 168 kDa). Die Flächenbelegungsdichte pro Doppelschicht wächst proportional zu Mw(PDADMA) an, bis ab dem Schwellwert Mw(PDADMA) = 80 kDa eine Sättigung eintritt und das Schichtwachstum unabhängig vom Wert des PDADMA-Molekulargewichts wird (Nlin = 15 Doppelschichten und dBL = (12,3 ± 1,3) nm). Die Daten legen nahe, dass unterhalb des PDADMA-Schwellwerts lediglich ein Teil einer PDADMA-Kette auf der PSS-terminierten Multischicht adsorbiert und der restliche Teil der adsorbierten Kette in Lösung ragt. Oberhalb des PDADMA-Schwellwertes adsorbiert mindestens noch ein zweiter Abschnitt der Kette und es ragt mindestens ein loop in Lösung. Dies führt zu einer konstanten Gleichgewichtsdicke der Monoschicht unabhängig vom Molekulargewicht. Unterschreitet das PSS-Molekulargewicht den Schwellwert Mw(PSS) = 25 kDa, so beobachtet man den gegenteiligen Effekt: beide Wachstumsparameter Nlin und dBL nehmen deutlich zu. Die größten gemessene Werte (unter Verwendung von 8,6 kDa PSS) lauten Nlin = 33 und dBL = 28,7 nm. Neutronenreflektionsmessungen zeigen, dass dieser Effekt mit der Diffusion der kurzen PSS-Ketten innerhalb der Multischicht einhergeht. Die Ausdehnung der Diffusionszone von 8,6 kDa PSS beträgt 80 nm und nimmt bis zum Erreichen des PSS-Schwellwertes monoton mit Mw(PSS) ab. Im Gegensatz dazu bilden PSS-Ketten mit einem Molekulargewicht oberhalb des Schwellwertes klar lokalisierte, lateral homogene Schichten (mit einer Grenzflächenunschärfe von 2 ... 4,6 nm). Entgegen der intuitiven Erwartung hat eine höhere Adsorptionszeit keinen Einfluss auf die Diffusionszone. Der limitierende Faktor ist die Diffusionszone selbst. In Übereinstimmung mit der theoretischen Erwartung führt die Diffusion von kurzen PSS-Ketten während der Multischicht-Präparation zu einem exponentiellen Wachstum der PDADMA/PSS-Multischichten, sobald Mw(PSS) < 25 kDa. In diesem Fall durchläuft das Schichtwachstum nacheinander erst ein exponentielles, dann ein parabolisches und schließlich ein lineares Regime.
In this work the mechanisms leading to the generation of the various reactive oxygen and nitrogen species (RONS) in a cold atmospheric plasma (CAP) jet and means to control their composition were studied. The investigated CAP jet kinpen is typically operated with Ar feed gas (pure or with molecular admixtures), driven at a frequency of approximately 1 MHz and features fast ionization waves or guided streamers, traveling at velocities of several km/s. The complex reaction networks were investigated by numerical and experimental techniques. Detailed experimental, analytical and computational investigations on the mass and heat transport in the plasma plume were performed: A novel analytical approach to diffusion in jet flows, the non-dispersive path mapping approximation (NDPM) was developed. The method for the first time allows for an estimation of the ambient species density in the near-field of jets that feature a non-homogeneous flow-field. The NDPM approximation was employed for the evaluation of laser induced fluorescence measurements on OH. Through combining measurements and NDPM approximation, this approach yielded an estimation for the ambient species density at the position of the guided streamers, not only in the laminar, but also in the (standard) turbulent operating regime. Accurate measurements of the temporally averaged ambient species density and temperature in the plasma plume were obtained by quantitative Schlieren measurements. The method yields temperature values with sub-Kelvin accuracy and, through combination with computational fluid dynamics (CFD) simulations, allowed for an estimation of the calorimetric power of the jet. In order to obtain a defined environment for the jet to operate in, a shielding gas device was designed in this work, which creates a gas curtain of defined composition around the plasma plume. The plasma dynamics on the ns timescale was investigated by phase resolved optical measurements. The effect of different shielding compositions ranging from pure N2 to pure O2 on guided streamer propagation was investigated. An electrostatic focusing mechanisms was discovered, which promotes the propagation of guided streamers along the channels formed by a noble gas in the plume of plasma jets operating in electronegative gases (such as air or O2). Two zero-dimensional (volume averaged) models were developed: First, the local processes in the guided streamer were modeled using an electron impact reaction kinetic model, which is closely correlated to densities of metastable argon (Ar*) obtained by laser atom absorption measurements. This first model shows that Ar* is the species which dominantly drives the plasma chemistry in the plasma plume. This is exploited in the second plug-flow reaction kinetics model, which is employed to investigate the formation of long-living RONS and uses an Ar* source term as sole energy input. The model uses the previous experimental data on mass and heat transport and temporal dynamics as input and is in turn verified by quantitative FTIR absorption measurements on O3, NO2, N2O, HNO3 and N2O5 in the far-field of the jet, where large absorption lengths can be achieved using a multi pass cell. For the evaluation of the zero-dimensional model, the time-of-flight of RONS from their generation to reaching the multi pass cell was determined using CFD simulations. The insight gained through this combined experimental-modeling approach on the reaction networks revealed relevant control parameters and enabled adjusting the plasma chemistry towards a desired RONS output. Through choosing appropriate feed-gas admixtures and shielding gas compositions, it is possible to generate an NOx-dominated plasma chemistry, although the jet usually produces a strongly O/O3-dominated chemistry. Understanding and controlling the plasma chemistry of cold atmospheric plasma sources for medical applications is not only essential for research, but is also the key for designing future plasma sources for specific medical applications that yield an optimum efficacy and avoid potential side effects of plasma treatment.
Because of the vital role of the liquid as interface in plasma medicine, this work is focused on the elucidation of the interaction of plasmas with biologically relevant liquids. The results of this thesis are an important step in the direction of the applications to real biological liquids such as blood and wound secretion ex vivo as well as in vivo. In this thesis the following questions are investigated and answered with the special focus on the free radicals as highly reactive and, therefore, hard to detect relevant group of chemical species: What is the impact of the atmospheric-pressure argon plasma jet on biologically relevant solutions? Which species are generated due to the plasma treatment of liquids? What is an appropriate detection procedure for the qualification and quantification of the short-lived species? Does the surrounding conditions influence the formation of liquid-phase reactive species and can this influence be used to tailor a desired liquid composition? What is the influence of the plasma surroundings? What is the influence of feed gas manipulation regarding the reactive species generation? Can these impacts be used for a selected reactive species composition generation? Does the treated liquid medium affect the plasma-generated reactive species output and in what way? Which are the underlying mechanisms and origins of the plasma-caused chemical changes in the solutions? Do reactive species exist, which origin is located in the gaseous phase? What is the impact of the plasma jet radiation?
In dieser Arbeit wird ein einfaches Verfahren zur Herstellung ultradünner (3 nm) Galliumschichten unter Umgebungsbedingungen beschrieben. Die Schichten sind stabil bis zu einem Auflage-Druck im GPa-Bereich und replizieren die zugrundeliegende Substratrauheit sowie größere Strukturen. Weiterhin wird ihre Eignung als Permeationsbarriere gezeigt. Mithilfe von optischen und elektrischen Messungen wird schließlich anhand des Drude-Modells die Alterung (Oxidation) der Schichten unter Umgebungsbedingungen beschrieben.
This thesis is devoted to experiments on three-dimensional dust clouds which are confined in low temperature plasmas. Such ensembles of highly electrically charged micrometer-sized particles reveal fascinating physics, such as self-excited density waves and vortices. At the same time, these systems are challenging for experimental approaches due to their three-dimensional character. In this thesis, new optical diagnostics for dusty plasmas have been developed and, in combination with existing techniques, have been used to study these 3D dusty plasmas on different size and time scales.
With the growing importance of advanced lighting technologies, customers expect additional functionality and higher comfort from fluorescent lamps. However, the ability to regulate light intensity (dimmed operation), in particular, exerts enormous stress on fluorescent lamps’ electrodes, leading to increased electrode erosion and significantly reduced lifetimes. During the operation of a fluorescent lamp, free barium (the main compound of the electrode emitter) is produced at the electrode responsible for lowering the work function in order to enable energy-efficient and durable electrodes with lifetimes of up to 20,000 hours. Despite their relatively long lifetimes, electrodes remain the lifetime-limiting factor of a fluorescent lamp. Therefore, for practical applications (e.g., maintaining quality control, adjusting operational parameters, and evaluating new electrode designs), electrode erosion is of special interest. The actual erosion-measurement methods determine a time-averaged erosion level over several hundred operation hours. Thus, a quasi-instantaneous measuring method (short measurement) is still necessary to determine erosion during operation. Such a method would allow us to compare erosion under different discharge conditions (currents, frequencies, or heating currents) from the same electrode in the same lamp. This work focuses on the determination of absolute electrode erosion during the stationary operation of commonly used fluorescent lamps. Commercial T8 lamps (fluorescent lamps with a diameter of 8/8 inch) are investigated at the operating mode of commonly used electronic ballasts with frequencies of several kHz. Operations under standard and dimmed conditions with an additional heating current to reduce electrode erosion are investigated. Electrode erosion is characterized by the erosion of barium, the main compound of the electrode. Therefore, laser-induced fluorescence (LIF), which is the most sensitive method for this application, is applied to determine the absolute densities of the eroded barium in the electrode region. These densities are affected by the plasma in the electrode region and do not directly represent the absolute barium erosion. To overcome this limitation, a new method based on a special measurement technique in combination with a barium-diffusion-model is developed to determine the absolute barium erosion based on the measured densities. It has been found that the barium densities in the electrode region are lower than the equilibrium pressures produced by the reduction of the barium oxide. This could be caused either by a reduced reaction rate, the reduced diffusion of the reactant (primarily barium oxide) or by reduced barium transport through the porous emitter. However, these results suggest that barium erosion depends on temperature and emitter structure, which vary over an electrode’s lifetime. For currents significantly higher than the nominal lamp current, a drastic increase in emitter evaporation is found. Such, an increase in the lamp current from 300 mA to 500 mA leads to an increase in emitter evaporation by a factor of five. Using the lamp for a long period of time under these conditions therefore reduces the lifetime by a factor of five. Notably, at this dramatically increased erosion level, the hot spot temperature only increases from 1120 K to 1170 K. Investigation of various frequencies from 50 Hz to 5 kHz revealed no significant dependence of emitter evaporation on frequency.
The main issue of this thesis was the investigation of dusty plasmas in magnetic fields. We made use of spherical paramagnetic as well as non-magnetic plastic particles in the micrometer range, so-called dust particles. The particles were then trapped in the sheath region of the driven lower electrode of an rf discharge. The plasma chamber was surrounded by coils to apply a horizontal magnetic field with field strengths of up to B=50mT at the particles’ position. In this configuration the sheath electric field and the external magnetic field were perpendicular to each other. Only the electrons could be magnetized but this leads to several forces acting on the dust particles. In some aspects the dust clusters with the magnetic particles show a behavior that is in complete contrast to those consisting of the standard non-magnetic plastic particles. Both types of particles have in common that the dust clusters were found to move either towards the positive or negative ExB-direction as a reaction to the magnetic field. Whether the positive or negative direction was preferred depended on the experimental conditions. The forces that lead to this transport are plasma-based forces induced by the magnetic field. These investigations were performed on two-dimensional horizontal particle systems. Vertically aligned dust particles due to the ion focus interaction have also been studied to determine the influence of horizontal magnetic fields on the stability of such dust pairs. Under certain conditions the vertical alignment can be broken up by the magnetic field. Some additional experiments on the interaction of non-magnetic dust particles in a plasma with UV irradiation were performed, but a significant decrease of dust charge due to a photoelectric effect was not detected. In summary, even relatively weak horizontal magnetic fields have a strong influence on dust particle systems.
In this thesis, the first on-line mass measurements of the isotopes 52,53K have been performed. These measurements by multi-reflection time-of-flight mass spectrometry with the ISOLTRAP setup at ISOLDE/CERN are linked to previously measured masses of exotic Ca isotopes, which had shown an unexpected large neutron-shell gap at the neutron number N = 32 for the magic proton core Z = 20. The new measurements provide the first exploration of the N = 32 neutron-shell closure below the proton number Z = 20. With a measured empirical two-neutron shell gap of about 3MeV for 51K, the N = 32 gap is smaller as compared to that of 52Ca, which measures about 4MeV, but is still significantly present. This confirms that the nuclear shell effect measured for calcium isotopes is not a phenomenon purely raised by its closed-proton-shell configuration, but is also present in potassium isotopes that possess an open proton shell and an unpaired proton. The second main objective of this thesis was the development of new techniques for efficient mass separation in Penning traps and multi-reflection devices, because the success of nuclear mass measurements with high precision depends crucially on the purity of the ion ensemble. The two main difficulties that have been addressed are, first, when the masses of the ions of interest and the masses of contaminant ions are very similar, and second, when the contaminant ions are predominantly present in the beam from ISOLDE. For the removal of contaminant ions in a high-vacuum Penning trap with high resolving power, a new technique for mass separation has been developed. A simultaneous application of a dipolar radio-frequency field at the magnetron frequency of all ions (mass independent at leading order) and a quadrupolar radio-frequency field at the cyclotron frequency (highly mass dependent) of a chosen ion species provides a new way of ion purification. The result is that the magnetron radius of all ions is increased by the effect of the dipolar excitation, and, at the same time, the quadrupolar excitation leads to a conversion of the radial eigenmotions for the chosen species. The consequence of this simultaneous process is that the wanted ions move back to the trap axes while all other ions are radially ejected from the trap. The advantage of the new method is the simultaneous ejection of all unwanted species in a high vacuum, which otherwise have to be addressed by a dipolar excitation at different frequencies, or by use of complex waveforms if a broadband ejection is required. A comparable (general) broadband ejection as achieved by the new method was previously only achieved in buffer-gas filled Penning traps. Further technical developments were performed with ISOLTRAP’s multi-reflection time-of-flight mass separator. The goal was to improve on situations when dealing with highly contaminated beams from ISOLDE during on-line Penning-trap measurements. In such cases, the number of events obtained in a limited time can be very low for the reason that only a limited number of ions, which predominantly consist of contaminant ions, can be stored and separated in the multi-reflection device at a given time to avoid non-negligible Coulomb interactions between the ions. The situation at ISOLTRAP has been significantly improved by a more efficient use of the separation cycle of the multi-reflection device. The mass-separation cycle is by far shorter (on the order of 10 ms) than a Penning-trap mass measurement (on the order of seconds). Thus, the separation in the multi-reflection device has been decoupled from the Penning-trap mass measurement and is repeated rapidly, while the purified ions are accumulated, stored, and cooled in the preparation Penning trap of ISOLTRAP. The collected ions of interest can then be transferred to the precision-measurement trap. This method increases the possible ratio of the number of contaminant ions to ions of interest by up to two orders of magnitude, i.e. the ratio of the corresponding process durations. Additionally, space-charge problems in multi-reflection devices have been investigated by setting up an off-line apparatus at Greifswald. The dynamical effects of ions in multi-reflection devices under non-negligible Coulomb interactions have been investigated in order to search for possibilities for improvements on such situations. This resulted in a new method of manipulating the ion densities in the device. The ions move in a cloud with large spatial extend for the major part of the trapping time and can later be compressed to small bunches for high-resolution mass separation. Proof-of-principle measurement have been performed with a low number of stored ions, where successful isobar separation has been demonstrated.
In der vorliegenden Arbeit wurden die Wechselwirkungen zwischen den reaktiven Sauerstoffspezies (ROS) und den Lipiden, die das Grundgerüst der Zellmembran bilden, sowie die daraus resultierenden chemischen und physikalischen Veränderungen der Membran untersucht. Außerdem wurde der Schutz einer Modellmembran durch Adsorption eines Polymers untersucht. Da natürliche Zellmembrane hoch komplexe Systeme sind, in und an denen chemische und strukturelle Prozesse häufig gleichzeitig ablaufen, wurden Lipidmonoschichten und Liposomen als Modellmembranen für die Untersuchungen gewählt. Die Radikale wurden mithilfe der Fenton-Reaktion erzeugt. Um ein vollständiges Bild des Radikalangriffs auf Modellmembrane zu erhalten, war es notwendig verschiedene Untersuchungsmethoden zu verwenden. Die Lipidmonoschichten und deren Phasenumwandlungen wurden vor und nach dem Radikalangriff mithilfe des Langmuir-Troges und den damit aufgenommenen Isothermen untersucht. Die Fluoreszenz- und die Brewsterwinkel-Mikroskopie wurde genutzt, um die Veränderungen des Phasenübergangs und somit die Veränderungen der Form und des Wachstums von Lipid-Domänen (flüssig-kondensierten Phase) durch den Radikalangriff zu beobachten. Die laterale periodische Struktur und das vertikale Elektronendichteprofil der Lipidmonoschicht wurden mit der Röntgendiffraktion und Röntgenreflexion vor und nach dem Radikalangriff untersucht. Mit der Infrarot-Reflexion-Absorption Spektroskopie (IRRAS) können Aussagen über die chemische Veränderungen der Lipide nach dem Radikalangriff getroffen werden. Die Liposomen wurden mit Differenzkalorimetrie untersucht, um die Verschiebung der Phasenübergangstemperatur durch den Radikalangriff zu beobachten. Der Radikalangriff auf Liposomen wurde mit Fluorenzmikroskopie verfolgt. Im ersten Teil der Arbeit wurde die Wechselwirkung von ROS mit zwitterionisch geladenen Lipiden untersucht. Die Modellmembranen bestanden aus Phosphatidylcholinen. Fasst man alle Ergebnisse der Untersuchungen zusammen, führt dies zum folgenden Resultat: Die Radikale reagieren bevorzugt mit der Kopfgruppe der zwitterionischen Phosphatidylcholinen. Es entstehen negativ geladene Phospholipide mit einer kleineren Kopfgruppe. Die Alkylketten bleiben nach dem Radikalangriff unverändert. Wie die Isothermen und die Brewsterwinkel-Mikroskopie bei Monoschichten sowie die Thermogramme bei Liposomen zeigen, hat die Reaktion keinen erkennbaren Einfluss auf das thermodynamische Verhalten. Erst durch Zugabe bzw. bei Vorhandensein von „freien“ Eisen-Ionen wird die Veränderung der Phosphocholine durch die Radikale beobachtbar. Die „freien“ Eisen-Ionen binden an die negativ geladenen Phospholipde. Dies führt im Fall von DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) Monoschichten zu einer irreversiblen Verfestigung. Weiterhin kommt es zur Abnahme der molekularen Fläche in der flüssig-kondensierten Phase. Im Fall von DMPC (1,2-dimyristoyl-sn-glycero-3-phosphocholine) Liposomen führt der Radikalangriff, bei Vorhandensein von „freien“ Eisen-Ionen, ebenfalls zu einer Verfestigung. Am Ende des Radikalangriffs sind die Liposomen zerstört. Im zweiten Teil der Arbeit wurde die Wechselwirkung von ROS mit negativ geladenen Phospholipiden untersucht. Dabei wurde herausgefunden, dass die negative Ladung der Phospholipide nach dem Radikalangriff erhalten bleibt. Die Alkylkettenlänge bleibt konstant. Die durchgeführten IRRAS Messungen zeigen ebenfalls keine Veränderungen der Alkylketten. Aufgrund der Beobachtung, dass Eisen-Ionen negativ geladene Modellmembranen verfestigen, sollte im dritten Teil dieser Arbeit die Eisenanbindung an negativ geladene Monoschichten am Beispiel des Cardiolipin TMCL quantifiziert werden. Bei einem physiologischen pH-Wert führt eine Eisen-Ionen Konzentration im mikromolaren Bereich zu einer irreversiblen Verfestigung der Monoschicht. Der pH-Wert ist ein entscheidender Parameter. Eine irreversible Verfestigung der Monoschicht kann durch einen pH-Wert von 1,3 oder niedriger verhindert werden, wenn die Eisenkonzentration kleiner als 1000 µM ist. Bei höheren Eisenkonzentrationen tritt auch bei diesem pH-Wert eine Verfestigung ein. Im vierten Teil dieser Arbeit wurde die Wechselwirkung von ROS mit einer negativ geladenen Lipid-Monoschicht (DMPG) mit adsorbierten Polykationen (Polyethylenimin (PEI)) untersucht. Die adsorbierte Polymerschicht dient zum Schutz der Lipidmonoschicht. Der Radikalangriff wurde mit verschiedenen Fenton-Konzentrationen durchgeführt. Es war eine signifikant höhere Fenton-Konzentration als bei dem Radikalangriff auf eine DPPC Monoschicht notwendig, um Veränderungen zu induzieren.
This thesis delves into some very important scientific challenges for the stellarator concept as a whole and W7-X in particular, namely, how one effectively interfaces the hot plasma with the material walls of the experiment, in special how the plasma heat and particle fluxes are controlled. The fundamental concept that will be used in W7-X for particle and heat exhaust is the island divertor. A number of theoretical and numerical studies have been performed to guide the design of the divertor components. The actual divertor components are in series production at this time, and are largely compatible with the expected heat loads. However, with the sophisticated codes now available, it has become clear that there are some, otherwise very attractive, operational scenarios that could lead to overloading of the W7-X divertors. At least one mitigation strategy was proposed but was until now not analyzed in sufficient detail. In this thesis, state-of-the-art codes are used to analyze this previously proposed mitigation strategy; they are also used to develop several alternative mitigation schemes, which may in the end be advantageous. The work performed here shows not only that it is conceivable to solve this already identified problem in new and arguably better ways but also that the W7-X coil set has enough degrees of freedom that many important long-pulse plasma effects can be effectively mimicked in short-pulse operation. This opens up a rich research program in the early phases of operation and may therefore lead to a significant acceleration of the scientific program to control and optimize the divertor operation in W7-X. The main scientific challenge for the island divertor operation in W7-X is that, since the divertor geometry is now fixed, the magnetic field structure must be adjusted to the divertor geometry, or additional plasma-facing components must be manufactured and installed. Well before this thesis work was done, such additional plasma-facing components were proposed. These are called scraper elements (SEs). As a part of this work, computer simu- lations were performed in order to obtain a better knowledge base regarding the SEs. To analyze the effect of the SE, edge plasma physics simulation code EMC3-Eirene, was used, in combination with state-of-the-art magneto hydrodynamic (MHD) equilibrium codes. This combination was computationally non-trivial and new, and it has led to important insights. One main result of this study is that the SEs significantly reduce the particle exhaust capabilities in steady state operation; this is a concern for W7-X. To test and further quantify this deleterious effect, physics experiments with a prototype SE should be performed as soon as possible, ideally in the first operation campaigns before the approximately two-year break needed to complete W7-X for steady-state operation. In 3 this first operation phase, however, the necessary combination of plasma parameters, heating power, and achievable pulse length is not accessible. This means, on the one hand, that the problem described will not be present in the first operation phase; on the other hand, the physics implications of installing an SE would appear not to be experimentally testable in that phase. One major finding of this thesis is that the coil system of W7-X is flexible enough to allow such an early experimental test. Different stages of high performance long-pulse discharge can be effectively mimicked in the experiment by a targeted use of the available coil sets. Thus, even in the early phases of the W7-X program one can assess both the protection capabilities of the SEs and their effects on particle exhaust and plasma performance in general. These mimic scenarios also have the potential to test other possibilities for divertor pro- tection besides the SE. Such strategies are addressed in this thesis. The two most promising strategies identified here can be classified as plasma shift and iota control. Both adjust the edge magnetic field to better fit the divertor geometry. This is done slowly but dynamically — i.e. during a long plasma discharge.
The current work is focused on the study of two surface modification plasma processes, (i) the active screen plasma nitriding (ASPN) and nitrocarburizing (ASPNC) for the hardening of ferrous surfaces and (ii) the microwave plasma assisted chemical vapor deposition (MW-PACVD) for the synthesis of single crystal and doped diamond. Conventional and active screen plasma nitriding processes have been investigated in a cylindrical, industrial scale ASPN reactor with a volume of about 1 m3, using low-pressure pulsed dc H2-N2 plasmas with admixtures of CH4 or CO2. The experiments were carried out (i) with the plasma at an internal model probe, (ii) with the plasma at the active screen (floated model probe) and (iii) with the plasma at the active screen and an additional plasma at the biased model probe. For deeper insights in ASPN and ASPNC processes, a laboratory scale plasma nitriding monitoring reactor, PLANIMOR, has been constructed. The main feature of this reactor is the linear configuration of the electrode setup combined with a tubular glass vessel, overcoming the experimental disadvantages of cylindrical laboratory scale ASPN reactors. With the help of infrared laser absorption spectroscopy (IRLAS) the rotational temperature of the stable molecules in the gas phase and the concentrations of the precursor, CH4, and the reaction products (NH3, HCN, C2H2, C2H4, CO, CH3) could be determined in both reactors, depending on the plasma power, the gas mixture, the plasma at the model probe and the admixture of CH4. Furthermore, the admixture of CO2 as the carbon containing precursor has been studied in the ASPN reactor leading to an additional reaction product H2O. The concentration of the molecular species has been found being in a range of 1012 to 1016 molecules cm-3. Also optical emission spectroscopy (OES) has been applied during the studies for analyzing the emission of the plasmas in the nitriding and nitrocarburizing processes. A similar behavior of the plasma chemistry in PLANIMOR comparing to that in the ASPN reactor has been found. Beside the plasma chemical investigations, both reactors have been used for the treatment of C15 steel samples. These samples have been analyzed with the help of GDOES resulting in the elements profile of the treated surfaces. It has been found that samples treated in PLANIMOR reach comparable nitriding results as samples treated in the ASPN reactor. Another focus of interest during the investigations about plasma nitrocarburizing has been the application of a carbon containing screen electrode as carbon source. For this purpose the carbon containing precursor and the steel screen have been substituted by a meshed carbon electrode, acting as the active screen. This change of the setup leads to a decrease of the NH3 production by a factor of 2.5 and an increase of the concentrations of HCN by a factor of 30 and of C2H2 by a factor of 70. The investigations of MW-PACVD processes used for diamond layer deposition have been carried out in a jacketed stainless steel reactor (JR), dedicated to the deposition of single crystalline diamond under high pressure and plasma power conditions. Using H2-plasmas with admixtures of CH4 and B2H6, the experiments were carried out in order to analyze the dependence of the plasma chemistry on several parameters, such as plasma power, pressure and gas mixture, in a wide pressure (p = 25…270 mbar) and power range (P = 0.6…4 kW). Using IRLAS the concentrations of six molecular species (B2H6, CH4, C2H2, C2H4, C2H6, CH3) have been monitored. With the help of OES the concentration of atomic boron could be determined. The concentrations of the detected molecular and atomic species were found to be in a range of 1010 to 1017 cm-3. With the help of the line-ratio-method the rotational temperature of the stable molecules has been determined. The temperature increased with pressure and power from 340 to 425 K. Using the Doppler broadening of the absorption line of CH3 at ν = 612,413 cm-1, the gas temperature has found to be Tg = (2000 ± 200) K under lower pressure and power conditions. For the H2-CH4 gas mixture, the experimental obtained molecular densities have been compared to those of a 1D-radial thermochemical model. The calculated radial densities have been integrated axially. For the same range the chemical processes in JR have been compared with those in a bell-jar (BJ) reactor. The hydrocarbon chemistry in JR has found to be similar to that in a BJ reactor.
The aim of this thesis is to concentrate on the investigation of these ROS&RNS composition distribution and their production pathways in the gas phase produced by a plasma jet. By understanding the physical mechanisms behind the generation of the ROS&RNS a precise tuning and design of the composition distribution in the gas phase can be achieved. One crucial physical parameter is the dissipated power inside the plasma. Only if this parameter is known a meaningful comparison of different feed gas settings is possible. Therefore, a concept for measuring the dissipated power inside the plasma for the modified micro-scaled atmospheric pressure plasma jet( µAPPJ) is designed. Additionally, due to achievements within this thesis it is now possible to ignite a homogeneous discharge in argon and helium within the geometry of the µAPPJ. The used feed gas is a determining factor concerning the electron energy distribution function and consequently influencing the production mechanism of the ROS&RNS. First of all, the electrical characterisation of the modified µAPPJ was performed including the alpha-to-gamma transition. It is shown that the alpha-to-gamma transition power is increasing with increasing frequency. For the first time it is now feasible to investigate the influence of the dissipated power on the neutral gas temperature, the metastable atom densities and the ROS&RNS production for the modified µAPPJ with argon and helium as feed gas. Due to the possibility of changing the feed gas and controlling the dissipated power a fundamental insight into the production mechanism of the ROS&RNS generated by the plasma jet is achieved. With rising dissipated power the temperature and the metastable densities as well as the ozone and nitrogen dioxide concentrations are increasing. By adding molecular oxygen and nitrogen to the feed gas of a plasma jet the ROS&RNS composition can be tuned. However, also the dissipated power is changed by the small amount of admixtures. Due to the developed dissipated power measurements within this thesis it was possible to disentangle the influence of the admixture on the power and on the ROS&RNS production. If the dissipated power is fixed for the µAPPJ with argon and helium feed gas, respectively, the highest amount of ozone was measured with oxygen admixture in an argon discharge, the highest amount of dinitrogen pentoxide with nitrogen admixture in an argon discharge and the highest amount of nitrogen dioxide with nitrogen admixture in a helium discharge. Beyond the influence of the dissipated power and the molecular admixture on the ROS&RNS production the feed gas temperature is a crucial parameter for the corresponding chemical reactions. By changing this parameter the distribution of ozone and nitrogen dioxide can be tuned precisely in such a way that with increasing temperature the ozone density goes down and the nitrogen dioxide density rises. Another determinant for the ROS&RNS composition produced by an atmospheric pressure plasma jet is the influence of ambient air. If the ambient air is changing from pure nitrogen to pure oxygen atmosphere the ozone density produced by the plasma jet is increasing. For the same conditions the nitrogen dioxide has a maximum at an oxygen-to-nitrogen ratio of 1:1. To avoid the influence of the ambient air on the reactive species production the afterglow of the µAPPJ was prolonged with a glass tube. By increasing the amount of molecular admixtures to the feed gas with each in equal quantities a totally different ROS&RNS composition can be obtained compared without the glass tube. It figures out that for small molecular admixtures the reactive species composition is nitrogen dominated and for higher admixtures it is oxygen dominated. Consequently, by shielding the ambient air from the active effluent and by admixing molecular oxygen and nitrogen the ROS&RNS composition can be designed.
Im ersten Teil der Arbeit wird der erfolgreiche Aufbau einer Diagnostik zur quantitativen Bestimmung von Oberflächenladungsdichten beschrieben. Das Messprinzip bedient sich des elektro-optischen Pockelseffekts eines BSO-Kristalls, der in der Entladungszelle als Dielektrikum eingesetzt ist. Diese Methode arbeitet zeitlich und lateral aufgelöst, was die Untersuchung der Dynamik von Oberflächenladungen auf drei verschiedenen Zeitskalen ermöglicht. Die erste Zeitskala liegt in der Größenordnung von einigen 100 ns. Damit kann erstmals die Deposition von elektrischer Ladung auf einer dielektrischen Oberfläche während eines Entladungsdurchbruchs beobachtet werden. Die Deposition beginnt im Zentrum eines zuvor deponierten Ladungsspots. Die Polarität der neudeponierten Ladung ist der des ursprünglichen Ladungsspots entgegengesetzt. Die Folge ist, dass die absolute Ladungsdichte im Zentrum im Verlauf einiger hundert Nanosekunden kleiner wird als in den Randbereichen. Der Umladungsprozess wird so lange fortgesetzt, bis das elektrische Feld der neu deponierten Ladungen dem äußeren Feld so stark entgegenwirkt, dass die Spannung zur Aufrechterhaltung der Entladung unterschritten wird und die Entladung erlischt. Die zweite untersuchte Zeitskala liegt in der Größenordnung der Periodendauer der externen Spannung. Im Nulldurchgang der Spannung liegen zeitlich stationäre Ladungsdichteverteilungen auf dem Dielektrikum vor. Die Geometrie eines mittleren Ladungsspots wird in Abhängigkeit der anliegenden Spannungen und des Gasdrucks untersucht. Einerseits ist der Spotradius abhängig von den Ionisationsprozessen im Volumen, weil die Dichte der Raumladungen die Stärke des Elektronenfokus in das Innere der Entladung steuert. Andererseits wird die Spotbildung durch eine laterale Drift von Ladungsträgern kurz vor der Oberfläche aufgrund des elektrischen Feldes deponierter Ladungsträger beeinflusst. Die dritte untersuchte Zeitskala liegt in einer Größenordnung von Sekunden. Im Fall einer initial homogenen Oberflächenladungsverteilung nimmt die mittlere Ladungsdichte in einer Größenordnung von Sekunden monoton ab. Dieser Prozess stellt einen Ladungsabbau dar, dessen zeitliches Verhalten durch zwei überlagerte Exponentialfunktionen beschreiben ließ. Dadurch werden zwei Ladungsträgerpopulationen im BSO angenommen, die verschieden abgebaut werden. Im Fall einer initial inhomogenen Ladungsdichteverteilung wird ein Transport elektrischer Ladung auf der BSO-Oberfläche in einer Größenordnung von Sekunden beobachtet. Es wird weiterhin erstmals die durch einen Atmosphärendruck-Plasmajet deponierten Ladungen auf BSO zeitaufgelöst gemessen. Die zeitliche Entwicklung der Oberflächenladungen kann mit der Messung des elektrischen Stroms an einer der Ringelektroden des Jets korreliert werden. Dadurch wird geschlossen, dass der Ladungsaustauch nicht direkt durch einen Bullet verursacht wird. Er erzeugt stattdessen einen elektrisch leitfähigen Kanal zwischen der Düse des Jets zur BSO-Oberfläche. Infolgedessen kann Ladung, die sich auf der Innenseite der Jetkapillare befindet, auf den BSO-Kristall transportiert werden. Im zweiten Teil der Arbeit werden Kenngrößen entwickelt, die den Ordnungszustand einer aus Einzelobjekten zusammengesetzten Entladungsstruktur quantitativ beschreiben. Die Kenngrößen werten dabei die laterale Leuchtdichteverteilung der Entladungsemisssion, u.a. auf Basis der Tripel-Korrelationsfunktion. Dabei werden zwei separate Bifurkationsspannungen zwischen einer hexagonalen und einer ungeordneten Anordnung beobachtet: Bei der Verringerung der Spannung wird zunächst der Bifurkationspunkt der azimutalen Ordnung durchlaufen und anschließend der Bifurkationspunkt der radialen Ordnung. Die Systeme gehen jeweils in einen Zustand geringerer Ordnung über. Die Ursache des Ordnungsverlusts ist das zunehmende Fehlen von Entladungsspots, was im Mittel zu einer geringeren Wechselwirkung der Spots untereinander führt und das System an Freiheitsgraden gewinnt. Im dritten Teil dieser Arbeit wird erstmals ein Ansatz verfolgt, der die Steuerung lateral strukturierter Entladungen ermöglicht. Dafür wurde ein Aufbau konstruiert, bei dem ein gekühlter Halbleiter als Dielektrikum in der Entladungszelle dient. Dessen externe Beleuchtung führt bei einer anliegenden Spannung zu einer Änderung des Spannungsteilerverhältnisses der kapazitiven Elemente und schließlich zu einer lokalen Erhöhung der Spannung über dem Entladungsraum. Die Größe und Leuchtintensität der durch die Beleuchtung gezündeten Entladung ist stark abhängig von der beleuchteten Fläche, der Leistungsdichte der Beleuchtung und der anliegenden Spannung.
The realistic description of the physical processes in quantum optical systems requires careful investigation of the interplay between quantum dissipation and entanglement generation. In this thesis, we have considered from a microscopical perspective the entanglement generation in semiconductor microcavities at short times, the dissipative evolution of the quantum harmonic oscillator towards a stationary state, and the nonclassical properties of the asymptotic states of different photonic systems. In our description of two-dimensional semiconductor microcavities we showed that two different pump configurations can be used to stimulate parametric scattering processes between polaritons that lead to the generation of internal polariton entanglement. A moving polariton induces an ultrafast electric polarisation as a source of light that serves as a probe of the internal entanglement properties. The identification of the nonclassical correlations of the emitted photons is based on entanglement witnesses that can also be used for the quantification of entanglement, e.g., in terms of the Schmidt number. The simultaneous creation of multiple branch entangled photon pairs renders it possible to generate an arbitrary number of entangled qubit states. By adjusting the number of pump beams and their spectral properties, one can optimize the Bell-type correlations within one ore more of those entangled qubits. Quantum dissipation can be studied in a microscopic setting with the well known model of a central oscillator coupled linearly to a bath of harmonic oscillators. We showed that equilibration of the central oscillator is the generic behaviour, which is prevented only in situations in which the classical oscillator equation of motion possesses undamped oscillatory solutions. Because of its localised spectral function, the infinite linear harmonic chain is an example for this behaviour. Thermalisation of the central oscillator depends on additional conditions. Equipartition of kinetic and potential energies requires the weak damping limit but is independent on the initial condition. The initial bath preparation enters the asymptotic temperature. Essential for the thermalisation of several oscillators is, that the asymptotic temperature is independent of the central oscillator frequency, which is fulfilled if the initial bath energy distribution matches that of a thermal state. Nevertheless, because this condition involves the sum of kinetic and potential energy, full thermalisation is possible in environments with nonthermal individual energy distributions, even in those far from thermal equilibrium. We showed, that even in the absence of full thermalisation the fluctuations of the central oscillator follow a generalised fluctuation dissipation theorem that reduces to the well known thermal result whenever the central oscillator thermalises in the strict sense. Photonic systems such as two-level emitters in a cavity or semiconductor microcavities are employed in quantum optics applications. The realistic theoretical description of the physical processes requires the use of methods from quantum optics as well as fromthe field of quantum dissipation. Our focus was on the correct theoretical description of the emission from systems with strong coupling. The analysis of the light generated by emitters in a cavity reveals a non-trivial dependence of the photon statistics on the light-matter coupling and temperature. Clearly identifiable parameters regimes with sub- and super-Poissonian photon statistics appear at strong and ultrastrong coupling, and lie immediately next to each other. We provided an approximate rule to relate the emission characteristics for a single emitter to those obtained for few emitters under an appropriate scaling of the emitter-cavity coupling. In accordance with this rule, the generation of noncassical light is easier with more emitters. The outright failure of the quantum optical master equation at predicting any of the features observed in the emission statistics shows that using the correct master equation is essential in all situations. Including internal dissipation channels we showed that a continuously driven semiconductor microcavity generates entangled light even at infinitely large times. The entanglement generation is thus robust against decoherence under realistic experimental conditions. Because the pair correlations between polaritons can sustain over long times and distances in these solid-state devices, a microcavity is a highly efficient source of entangled light and therefore well suited for quantum optics applications.
In the last decade a new domain has developed in plasma physics: plasma medicine. Despite the successes that have already been achieved in this exciting new field, the interaction of plasmas with “biological materials” is not yet fully understood. Further investigations in particular with respect to the properties of the applied plasmas sources are therefore essential in order to decode this complex interaction process. Currently, a great variety of different discharge types are used in plasma medical investigation which are generally are operated in noble gases like helium and argon or with dry air. In the present work, the main focuses is on the diagnostics of reactive oxygen and nitrogen species (RONS) resulting from the plasma chemistry of an argon radio-frequency (RF) atmospheric pressure plasma jet (APPJ) and its interaction with the ambient atmosphere. To conduct this study, a commercially available plasma device, so-called kinpen is used due to its technical development maturity and its accessibility on the market. As a method of choice, diagnostic techniques are based on optical spectroscopy known to be a reliable tool to investigate plasmas. Consequently, three complementary optical laser diagnostics, namely quantum cascade laser absorption spectroscopy (QCLAS), laser induced fluorescence (LIF) and planar single shot LIF (PLIF), have been successfully applied to the plasma jet itself or its effluent. All of these diagnostics offer a high species selectivity and an excellent spatial and temporal resolution. They are used in this work for i) the characterization of the plasma chemical dynamics with respect to the generation of biological active RONS – in particular for the case of N2 and O2 admixtures. ii) the measurement of the NO density profile in the plasma effluent iii) the investigation of the flow characteristics of the neutral gas component (laminar vs. turbulent) and its influence on the plasma chemistry. Numerical analysis have been carried out in collaboration with PLASMANT (University of Antwerp) via kinetic simulations of the entire plasma chemistry. Expectingly, atomic oxygen (O) and nitric oxide (NO) turn out to be precursors of ozone (O3) and nitric dioxide (NO2). However, it was intriguing to unveil that atomic oxygen and nitrogen metastable (N2(A)) play together a key part --as intermediate species-- in the generation of more stable RONS, e.g. NO. The absolute density of NO space resolved was measured by LIF and absolutely calibrated molecular beam mass spectrometer. LIF was used to determine relative density of OH radical in the plasma plume. 2D-LIF was used to investigate the gas flow pattern with OH as a flow tracer. The results are discussed in details and show different operating mode of the jet, e.g. laminar or turbulent and that the plasma influences these regimes. The first detection and relative measurement by LIF of nitrogen metastable (N2(A)) produced by an argon APPJ is also shortly reported in this work. The outcome of this thesis will bring new insights in the field of argon APPJs chemistry and its interaction with the ambient atmosphere which can be valuable to support plasma modelling and to consider for the applications in plasma medicine.
Ein System zu Abscheidung intermetallischer Cu-Ti basierter Schichten durch Magnetronsputtern wurde entworfen, aufgebaut und plasmadiagnostisch charakterisiert. Die duale, extern schaltbare Leistungselektronik mit hoher Parallelkapazität erlaubt den Pulsbetrieb beider Magnetrons gegeneinander mit beliebiger Frequenz f, Tastgrad t_a/T und Pulsverzögerung t_d. Auf diese Weise kann neben konventionellen Pulsmodi (dual-MS: f = 4.6 kHz, t_a/T = 50 %) das Hochenergieimpulsmagnetronsputtern (dual-HiPIMS: f = 100 Hz, t_a/T = 1 %) realisiert werden. Außerdem können die unterschiedlichen Sputterausbeuten von Ti und Cu durch individuell einstellbare mittlere Entladungsströme kompensiert werden. Die Entladungscharakteristika zeigen besonders hohe temporäre Ströme (I > 50 A) während HiPIMS. Langmuir-Sondenmessungen bestätigen höhere Elektronendichten (n_e = 10^18 m^-3) und eine breitere Elektronenenergieverteilung im Vergleich zu dual-MS. Als Folge kommt es zur verstärkten Ionisierung und Anregung von Plasmaspezies, nachgewiesen durch optische Emissionsspektroskopie (OES). Spektral integrierte OES wurde zur Beschreibung der räumlich und zeitlichen Entladungsentwicklung herangezogen, während mittels Gegenfeldanalysator die für die Schichtbildung wichtige Ionengeschwindigkeitsverteilung zeitaufgelöst erfasst wurde. Die gewonnenen Schichten wurden röntgenographisch analysiert und deren Eigenschaften in Hinblick auf ihre Bildung unter verschiedenen Entladungsmodi gedeutet. Schichtdicke, Kristallinität und Dichte zeigen eine klare Abhängigkeit vom Entladungsmodus mit vorwiegend höherer Güte durch HiPIMS. Die Variation des mittleren Cu-Entladungsstromes erlaubt Einfluss auf die Schichtzusammensetzung, maßgeblich für praktische Anwendungen. Des Weiteren wurden Kompositschichten auf Basis von Cu-Nanopartikeln (Cluster) eingebettet in einem dielektrischen Matrixmaterial (TiO2) synthetisiert und untersucht. Zunächst wurde das Cu-Clusterwachstum durch ein neues Buffergas-Pulsverfahren zeitaufgelöst untersucht und die damit einhergehende Performance der Partikelquelle auf Basis eines einfachen Modells evaluiert. Als Resultat kann die Größen-/Massenverteilung und damit die Eigenschaften der Cluster auf einfache Weise beeinflusst werden. Schließlich wurden die Cu-TiO2-Kompositschichten in einem eigens angefertigten Co-Depositionssystem erstellt. Dabei werden die aus der Gasaggregationsquelle emittierten Cu-Cluster simultan zu reaktiv (mit O2) gesputtertem Ti abgeschieden. Separat abgeschiedene Cu-Cluster zeigen vorwiegend polykristallines fcc-Cu, dass an Luft oder unter Zugabe von molekularem O2 oberflächlich Cu2O ausbildet. Während auch das separat reaktiv abgeschiedene Ti Titandioxid (TiO2) bildet, weist das Cu im Nanokomposit grundlegende strukturelle Unterschiede auf: Im Komposit liegt kein metallisches Cu mehr vor, stattdessen ist es vollständig zu CuO konvertiert. Dies ist auf die Anwesenheit eines reaktiven Sauerstoffplasmas beim Co-Depositionsprozess zurückzuführen. Es konnte gezeigt werden, dass molekularer O2 lediglich eine diffusionsbegrenzte Menge Cu2O erzeugt, während entladungsaktivierte Sauerstoffspezies zur völligen Durchoxidation der Cu-Cluster zu CuO führt. Dies ist eine wichtige Erkenntnis für die zukünftige Herstellung ähnlicher Komposite.
Magnetic reconnection is a ubiquitous phenomenon observed in a wide range of magnetized plasmas from magnetic confinement fusion devices to space plasmas in the magnetotail. The process enables the release of accumulated magnetic energy by rapid changes in magnetic topology, heating the plasma in the vicinity of the reconnection site, generating fast particles and allowing a wealth of instabilities to grow. This thesis reports on the results from a newly constructed linear, cylindrical and modular guide field reconnection experiment with highly reproducible events, VINETA.II. A detailed analysis of the reconnecting current sheet properties on a macroscopic and microscopic scale in time and space is presented. In the experiment, four parallel axial wires create a figure-eight in-plane magnetic field with an X-line along the central axis, as well as an axial inductive field that drives magnetic reconnection. Particle-in-cell simulations show that the axial current is limited by sheaths at the boundaries and that electrostatic fields along the device axis always set up in response to the induced electric field. Current sheet formation requires an additional electron current source, realized as a plasma gun, which discharges into a homogeneous background plasma created by a rf antenna. The evolution of the plasma current is found to be dominantly set by its electrical circuit. The current response to the applied electric field is mainly inductive, which in turn strongly influences the reconnection rate. The three-dimensional distribution of the current sheet is determined by the magnetic mapping of the plasma gun along the sheared magnetic field lines, as well as by radial cross-field expansion. This expansion is due to a lack of equilibrium in the in-plane force balance. Resistive diffusion of the magnetic field by E=η j is found to be by far insufficient to account for the high reconnection rate E=-dΨ/dt at the X-line, indicating the presence of large electrostatic fields which do not contribute to dissipative reconnection. High-frequency magnetic fluctuations are observed throughout the current sheet which are compared to qualitatively similar observations in the Magnetic Reconnection Experiment (MRX, Princeton). The turbulent fluctuation spectra in both experiments display a spectral kink near the lower hybrid frequency, indicating the presence of lower hybrid type instabilities. In contrast to the expected perpendicular propagation of mainly electrostatic waves, an electromagnetic wave is found in VINETA.II that propagates along the guide field and matches the whistler wave dispersion. Good correlation is observed between the local axial current density and the fluctuation amplitude across the azimuthal plane. Instabilities driven by parallel drifts can be excluded due to the large required drift velocities or low resulting phase velocities that are not observed. It is instead suggested that a perpendicular, electrostatic lower hybrid mode indeed exists that resonantly excites a parallel, electromagnetic whistler wave through linear mode conversion. The resulting fluctuations are found to be intrinsic to the localized current sheet and are independent of the slower reconnection dynamics. Their amplitude is small compared to the in-plane fields, and have a negligible contribution to anomalous resistivity through momentum transport in the present parameter regime.
The collisionless tearing mode is investigated by means of the delta-f PIC code EUTERPE solving the gyrokinetic equation. In this thesis the first simulations of electromagnetic non-ideal MHD modes in a slab geometry with EUTERPE are presented. Linear simulations are carried out in the cases of vanishing and finite temperature gradients. Both cases are benchmarked using a shooting method showing that EUTERPE simulates the linearly unstable tearing mode to a very high accuracy. In the case of finite diamagnetic effects and values of the linear stability parameter Delta of order unity analytic predictions of the linear dispersion relation are compared with simulation results. The comparison validates the analytic results in this parameter range. Nonlinear single-mode simulations are performed in the small- to medium-Delta range measuring the dependency of the saturated island half width on the equilibrium current width. The results are compared with an analytic prediction obtained with a kinetic electromagnetic model. In this thesis the first simulation results in the regime of fast nonlinear reconnection~(medium- to high-Delta range) are presented using the standard gyrokinetic equation. In this regime a nonlinear critical threshold has been found dividing the saturated mode from the super-exponential phase for medium-Delta values. This critical threshold has been proven to occur in two slab equilibria frequently used for reconnection scenarios. Either changing the width of the equilibrium current or the wave number of the most unstable mode makes the threshold apparent. Extensive parameter studies including the variation of the domain extensions as well as the equilibrium current width are dedicated to a comprehensive overview of the critical threshold in a wide range of parameters. Additionally, a second critical threshold for high-Delta equilibria has been observed. A detailed comparison between a compressible gyrofluid code and EUTERPE is carried out. The two models are compared with each other in the linear regime by measuring growth rates over wave numbers of the most unstable mode for two setups of parameters. Analytical scaling predictions of the dispersion relation relevant to the low-Delta regime are discussed. Employing nonlinear simulations of both codes the saturated island half width and oscillation frequency of the magnetic islands are compared in the small-Delta range. Both models agree very well in the limit of marginal instability and differ slightly with decreasing wave vector. Recently, the full polarisation response in the quasi-neutrality equation was implemented in EUTERPE using the Padé approximation of the full gyrokinetic polarisation term. Linear simulation results including finite ratios of ion to electron temperature are benchmarked with the dispersion relation obtained from a hybrid model. Finite temperature effects influence the saturated island width slightly with increasing ion to electron temperature ratio which has been verified by both models.
This thesis investigated dielectric barrier discharges (DBDs) in N2-O2 gas mixtures at atmospheric pressure, with a focus on the gas discharge physics. The main goal was to evaluate whether possible control mechanisms exist that can manipulate the breakdown and the development of DBDs, especially for pulsed operation. To examine the pre-breakdown phase, the actual breakdown and the main DBD development, DBDs in a double-sided, single filament arrangement with a 1 mm discharge gap were investigated by means of electrical and optical diagnostics with high resolutions. Spectrally- and temporally-resolved iCCD pictures (2D in space), spectrally- and spatio-temporally-resolved streak camera and CCS images (1D in space) were simultaneously recorded accompanied by a full electrical characterisation with fast voltage and current probes. Sinusoidal- and pulsed-driven DBDs were found to have a qualitatively similar spatio-temporal development, i.e. a cathode-directed ionisation front (v ~ 10^6 m/s, positive streamer mechanism), followed by a transient glow-like phase in the gap. For sinusoidal operation, the slope of the applied voltage is flat (dU/dt ~ 1 V/ns) compared to pulsed operation (dU/dt ~ 100 V/ns). Thus, during the longer pre-phase of the sine-driven DBD, many more charge carriers were generated, in contrast to the pulsed-driven DBDs, where the pre-phase is limited by the short voltage rise time. Consequently, just before the breakdown occurs, the charge carrier density is higher for sine-driven DBDs, i.e. the positive streamer starts in a highly pre-ionised environment, which leads to a lower propagation velocity. In addition to limiting the pre-phase (lower pre-ionisation), the steep voltage slope of the pulsed DBD amplifies the streamer breakdown because the applied voltage rises significantly during its propagation. Therefore, the transferred electrical charge and the electrical power of a single DBD can be controlled by the applied voltage amplitude, but only in pulsed operation. In addition to the effects of different voltage slope steepness, the pulse width is an excellent parameter in the pulsed operation to set the pre-ionisation, by shifting the DBDs into the after-glow of the previous discharge using asymmetrical HV pulse waveforms. The subsequent DBDs ignite in different pre-ionised conditions, defined by the residual charge carrier densities in the gap that originated from the previous DBD. The breakdown characteristics of these DBDs could be controlled down to the fundamental level. This thesis has described for the first time four different breakdown regimes in single filament DBDs for 0.1 vol% N2 in O2 and connected them to the processes during their pre-phases. The “classic” DBD development (a cathode-directed streamer followed by a transient glow discharge) could be controlled in a certain range, followed by a transition first to a breakdown regime featuring a simultaneous propagation of a cathode- and an anode-directed streamer, and finally to a reignition of the previous DBDs without any propagation, just by reducing the pulse width (time between two subsequent DBDs), i.e. increasing the pre-ionisation level. All differences between the DBDs at rising and falling slopes could be explained by the different pre-conditions in the gap. The O2 concentration in the N2-O2 gas mixtures offers another way of controlling the pre-ionisation. Due to the electron attachment as a consequence of the electronegativity of oxygen, the electron density decreases for higher O2 admixtures. Furthermore, the differences in the first Townsend ionisation coefficient and in the photo-ionisation between N2 and O2 influence the DBD behaviour as well. To some extent, some of the reported effects achieved by varying the pulse width at a fixed O2/N2 ratio were also observed for a fixed pulse width and changing O2 concentration. Hence, the response of the DBD properties to changing pre-ionisation levels seems to be a general principle of DBD control. Additional effects of the O2/N2 ratio, such as an increasing DBD inception jitter or higher streamer velocities, were also reported. Finally, a reverse of the effects induced by the O2 admixture such as DBD emission duration or DBD inception delay, was observed for O2 concentrations below 0.01 vol%, and were especially pronounced at a pressure of 0.5 bar. For 0.1 vol% O2 in N2, a minimal electron recombination rate was found, which can be explained by the different decay and recombination rates of positive nitrogen and oxygen ions. These different rates effect the charge carrier dynamics and consequently, the pre-ionisation in the gap. In conclusion, this investigation has highlighted the importance of volume memory processes on the breakdown and development of single filament DBDs at elevated pressures.
The present thesis deals with dynamic structures that form during the expansion of plasma into an environment of much lower plasma density. The electron expansion, driven by their pressure, occurs on a much faster time scale than the ion expansion, owed to their mobility. The high inertia of the ions causes the generation of an ambipolar electric field that decelerates the escaping electrons while accelerating the ions. The ambipolar boundary propagates outwards and forms a plasma density front. For a small density differences, the propagation of the front can be described with the linear ansatz for ion acoustic waves. For a large density differences, experiments have shown that the propagation velocity of such a front is still related to the ion sound velocity. However, the reported proportionality factors are scattered over a wide range of values, depending on the considered initial and boundary conditions. In this thesis, the dynamics during plasma expansion are studied with the use of experiments and a versatile particle-in-cell simulation. The experimental investigations are performed in the linear helicon device Piglet. The experiment features a fast valve, which is used to shape the neutral gas density profile. During the pulsed rf-discharges, plasma is generated in the source region and expands collisionless into the expansion chamber. The computer simulation is tailored very close to the experiment and provides a deeper insight in the particle kinetics. The experimental results show the existence of a propagating ion front. Its velocity is typically supersonic and depends on the density ratio of the two plasmas. The ion front features a strong electric field. The front can have similar properties to a double layer is not necessarily a double layer by definition. The computer simulation reveals that the propagating electric field repels the downstream ambient ions. These ions form a stream with velocities up to twice as high as the front velocity. The observed ion density peak is due to the accumulation of the repelled ions and is located at their turning point. The ion front formation depends strongly on the initial ion density profile and is part of a wave-breaking phenomenon. The observed front is followed by a plateau of little plasma density variation. This could be confirmed for the expansion experiment by a comparison with virtual diagnostics in the computer simulation. The plateau has a plasma density determined by the ratio between the high and low plasma density. It consists of streaming ions that have been accelerated in the edge of the main plasma. The presented results confirm and extend findings obtained by independent numerical models and simulations.
The central aim of this thesis was the investigation of protein/polyanion interaction using circular dichroism (CD) spectroscopy, enzyme immune assay (EIA), isothermal titration calorimetry (ITC) and flow cytometry (FC). A further aim was to understand why an endogenous protein becomes immuno-genic when forming a complex. The focus was on the protein platelet factor (PF4), which gained wide interest in the clinical field, due to its role in the life-threatening, immune-driven, adverse drug effect heparin-induced thrombocytopenia (HIT). PF4 is a small homotetrameric chemokine with several basic amino acids on its surface, forming a positively charged ring. The antibodies that are formed during HIT recognize an epitope exposed on PF4, when it is in a complex with heparin at a certain molar ratio at which, PF4 tetramers are aligned on the heparin and forced into close approximation. The main results and conclusions of the thesis are summarized below: 5.1 Evolutionary Conservation of PF4 (Paper I – PF4/Evolution) By carrying out an amino acid sequence survey we found that the positively charged amino acids contributing to the heparin binding site on the surface of PF4 and related proteins are highly conserved in all vertebrates, including fish species. PF4 interacts with the phospholipid lipid A, the innermost part of the lipopolysaccharide (LPS) of Gram negative bacteria. We showed that the shorter the sugar chain of the O antigen, outer and inner core of the LPS were the more PF4 was binding. The interaction of PF4 with lipid A is inhibited by heparin, suggesting that the amino acids known to contribute to heparin binding are also involved in binding to lipid A. 5.2 PF4 Interaction with Polyanions (PA) of varying Length and Degree of Sulfation (Paper II – PF4/PA) CD spectroscopy was found to be a powerful technique to monitor structural changes of PF4 caused by binding to various clinically relevant polyanions. Therefore PF4 was titrated with different PA to investigate the dependencies: i. impact of the PF4:PA molar ratio, ii. degree of polymerization of the PA and iii. degree of sulfation of the PA. In all cases, exposure of HIT-relevant epitope(s) was only observed for PA that also induced changes in secondary structure of PF4. A comparison of results of an immune ¬assay with CD spectroscopic data showed that the extent of complex anti¬genicity correlates well with the magnitude of changes in PF4 secondary structure, and that the structural changes of PF4 have to exceed a certain threshold to achieve PF4/PA complex antigenicity. These findings allowed us to calculate expectation intervals for complex antigenicity solely using CD spectroscopic data. To our knowledge, this was the first demonstration that the capability of drugs to induce antigenicity of PF4 can be assessed without the necessity of in vivo studies or the use of antibodies obtained from immunized patients specific for the antigens. The antigenicity of PF4 in complex is not restricted to negative charges originating from sulfate groups, PA with phosphate groups are also capable (binding to phospholipids). We investigated inorganic polyphosphates (polyP) with a chain length of 75 Pi and showed that the induced secondary structural changes are even higher compared to the changes induced by the different heparins and that the PF4/P75 complexes are antigenic as well. 5.3 PF4 Interaction with defined oligomeric Heparins (Paper III – PF4/defined Heparins) We tested highly purified, monodisperse heparins. In contrast to the clinically relevant but relatively undefined (high polydispersity index) glycosamino glycans reported in paper II (PF4/PA). The defined heparins induced higher secondary structural changes. Here we showed for the first time that strong conformational changes during PF4/PA complex formation are necessary but not sufficient for to the expression of the anti-PF4/heparin antibody binding site. Also, the size of the complexes is not the only prerequisite for anti-PF4/heparin antibody binding (tested by atomic force microscopy). By ITC we found that antigenicity is only induced if the PF4/PA complex has a high binding enthalpy and the complex formation leads to a negative change in entropy. 5.4 PF4/Polyphosphates (polyP) Complex Antigenicity and Interaction with Escherichia coli (E. coli, Paper IV – PF4/polyP) PolyP with chain lengths of 45 Pi and 75 Pi induced remarkable secondary structural changes in the PF4 molecule, thereby exposing the epitope recognized by anti-PF4/heparin antibodies. The induced conformational changes were similar to the changes induced by the defined heparins. Again a high binding enthalpy was observed but here in connection with a positive change in entropy. Further we showed that polyP (≥45 Pi) enhance PF4 binding to the surface of Gram negative E. coli at intermediate concentration and disrupt the binding at elevated polyP concentrations. The increased amounts of PF4 on the bacterial surface also improved the binding of anti-PF4/heparin antibodies and thereby the phagocytosis of the bacteria by poly¬morpho¬nuclear leucocytes. 5.5 Nucleic acid based Aptamers induce structural Changes in the PF4 Molecule (Paper V – PF4/Aptamer) Nucleic acids are another class of molecules containing phosphate groups. Especially after cell damage their extra¬cellular concentration can be locally quite high (>2 mg/ml). We found that certain aptamers form complexes with PF4 and thereby inducing anti-PF4/aptamer antibodies which cross-react with PF4/heparin complexes. Moreover by CD spectroscopy we showed that the protein C-aptamer caused similar secondary structural changes of PF4 like heparin, but already at much lower concentration. The maximally induced changes by the protein-C aptamer were even higher and persisted over a broader concentration range. 5.6 Protamine Interaction with Heparin (Paper VI – PS/Heparin) After the intensive investigation of the complex formation between PF4 and many different classes of PA we assessed another protein for structural changes upon complex formation with heparin. Protamine (PS) a protein in routinely used in post-cardiac surgery to reverse the anticoagulant effects of heparin was found to unfold but not to refold with increasing concentration of PA in solution. 5.7 Conclusion and Outlook When starting this thesis, it was believed that repetitive structures formed by PF4 on a heparin chain mold the epitope recognized by antibodies inducing HIT. These repetitive structures might exhibit similarities with viral capsids and are therefore recognized by the immune system of some patients. We found that induced by the close approximation PF4 changes its conformation, thereby exposing a neoepitope. The conserved positively charged amino acids of the heparin binding site and the involvement of these amino acids in the binding to lipid A confirm our hypothesis of PF4 as part of an ancient immune-mediated host defense mechanism. As possible consequence of the “primitive mechanism of defense” the highly variable O-antigens of LPS might have significantly contributed to an efficient escape mechanism by hiding the structures that made the bacteria vulnerable. In turn polyP might be an adaption of the host improve pathogen recognition by PF4 and further by antibodies inducing phagocytosis of the PF4-marked objects. Although shown only for PF4 and PS, our findings might be applicable to other proteins that also express epitopes upon changes in their secondary structure. Our physicochemical methods may further be applied: i. to drug development for the prediction of antigenicity induced by polyanionic drugs, ii. to guide the development of synthetic heparins and other polyanion based drugs, e.g. aptamers, that do not lead to HIT and iii. to provide relevant aspects for other biological functions of heparins.
Die Forschung an mikrowelleninduzierten Atmosphärendruckplasmen am INP führte zu verschiedenen potentiellen Applikationen. Dabei besitzt die mikrobiologische Dekontamination sowohl von thermolabilen Medizinprodukten als auch von Lebensmitteln schon zum jetzigen Zeitpunkt ein hohes industrielles Anwendungspotential. Den aufgeführten Anwendungen gemeinsam ist, dass für eine erfolgreiche Weiterentwicklung der Prozesse, sowie der Plasmaquelle, ein grundlegendes Verständnis der vorliegenden dynamischen Mikrowellenplasmawechselwirkung notwendig ist. Durch den begrenzten diagnostischen Zugang der zu untersuchenden Plasmaquelle wird ein kombinierter Ansatz aus diagnostischen Methoden und Modellierung gewählt. Die Entladung wird in Argon bei reduziertem Druck (ab 10 mbar) zur Vereinfachung des Modells betrieben. Daher musste die Plasmaquelle für diesen Einsatz weiterentwickelt werden. Dieses beinhaltet die Neuauslegung der Prozesswärmeabfuhr, auf Grund der nicht oder nur teilweisen Anwendbarkeit von etablierten Verfahren im Atmosphärendruck (hohe Gasflüsse, Wasserkühlung). Die Plasmamikrowellenwechselwirkung dieser Quelle ist anschließend mit Methoden zur Charakterisierung des Plasmas und des Mikrowellenfeldes für unterschiedliche Arbeitspunkte in Druck und Leistung untersucht worden. Zur Bestimmung der Elektronendichte des Plasmas wurde ein frequenzvariables Mikrowelleninterferometer auf Basis eines Vektornetzwerkanalysators erstmalig etabliert. Dieses neue Messsystem wurde im Vorfeld detailliert untersucht, um das korrekte Zusammenspiel aller Komponenten zu überprüfen. In diesem Zusammenhang wurde ein frequenzaufgelöstes Mikrowelleninterferometer zur Messung der Elektronendichte in einer Fluoreszenzlampe aufgebaut. Durch diesen neuartigen Ansatz konnte der Einfluss der dielektrischen Umhüllung (Glasrohr der Lampe) auf die Mikrowelleninterferometrie untersucht werden. In einer weiteren Untersuchung an einem Induktiv Gekoppelten Plasma wurden die Resultate dieses Messsystems mit denen von Langmuir-Sondenmessungen. Auf Grund der konstruktiven Gegebenheiten des Reaktors ist das Plasma nur über ein Fenster für das Mikrowelleninterferometer zugänglich. Der Vergleich der ermittelten Elektronendichten ergab einen Unterschied von Faktor zwei zwischen Interferometer und Langmuir-Sonde. Die Untersuchungen an der Fluoreszenzlampe und dem Induktiv Gekoppelten Plasma zeigten zum einen die korrekte Funktion des neu etablierten frequenzvariablen Mikrowelleninterferometers mit erreichbaren Phasenauflösungen unterhalb von 0,1 mrad. Zum anderen wurde festgestellt, dass die dielektrische Umhüllung des Plasmas zu einem systematischen Fehler von bis 53 % bei der Elektronendichtebestimmung führen kann. Diese gewonnenen Erkenntnisse hatten bei der Konzipierung des Mikrowelleninterferometers zur Untersuchung der Plasmamikrowellenwechselwirkung einen entscheidenden Einfluss. Neben der Untersuchung des Plasmas ist ebenfalls eine Diagnostik des Mikrowellenfeldes nötig, um die Plasmamikrowellenwechselwirkung dieser Entladung experimentell zu charakterisieren. Auf Grundlage dieser Daten können die Resultate des Modells bewertet werden, die einen Einblick in die Plasmaquelle und ihrer dynamischen Vorgänge erlaubt, was für die Weiterentwicklung der Applikationen essentiell ist. Aus diesem Grund ist ein heterodynes Reflektometer entwickelt worden. Dieses Messsystem wurde umfangreich getestet und kann mit einer maximalen Zeitauflösung von 100 ns den komplexen Reflektionsfaktor mit einer Phasengenauigkeit von 10 mrad bestimmen. Das Reflektometer erlaubt einen experimentellen Zugang zur aktiven Zone schon in der Frühphase der Entladung. Mit Hilfe der Diagnostiken zur Untersuchung des Plasmas und des Mikrowellenfeldes wurde die Entladung von der Zündung bis zur stationären Phase charakterisiert und mit den Ergebnissen des Modells verglichen. Es zeigte sich eine gute Übereinstimmung im Millisekundenzeitbereich, sowie eine starke Dynamik im Reflektionsfaktor in der ersten Millisekunde, hervorgerufen durch die Plasmamikrowellenwechselwirkung. Durch die hohe Zeitauflösung des Reflektometers konnten diese Vorgänge im Mikrosekundenzeitbereich erstmalig experimentell aufgelöst werden, was die Interpretation mittels des Modells möglich macht. Es konnten die Vorgänge während der Zündung des Plasmas detailliert untersucht werden und damit die Richtigkeit von Annahmen, die bei der Entwicklung der Zündtechnologie getroffen wurden, überprüft werden. Dieses erworbene grundlegende Verständnis ermöglicht eine Weiterentwicklung dieser Technologie. Mit Hilfe der erzielten Ergebnisse wurde eine neue Optimierungsstrategie für die Abstimmung der Mikrowellenplasmaquelle entwickelt. Dies führte zu einer wesentlichen Verbesserung der Reproduzierbarkeit der mikrobiologischen Ergebnisse. Darüber hinaus bilden die erzielten Ergebnisse eine solide Grundlage für weitere experimentelle und theoretische Untersuchungen dieser Entladung in beispielsweise anderen Arbeitsgasen.
In der vorliegenden Arbeit wurde die Wechselwirkung reaktiver Sauerstoffspezies (ROS) mit organischen Molekülen anhand zweier unterschiedlicher Systeme analysiert. Während einerseits der Einfluss von ROS auf eine organische Monoschicht am Beispiel des synthetischen, kationischen Polyelektrolyten Polyethylenimin (PEI) untersucht wurde, stand andererseits die Wechselwirkung von ROS mit einem DNS-Molekül, dem Biopolyelektrolyten pBR322 im Fokus des Interesses. Für die Untersuchungen der ROS-PEI-Wechselwirkung wurde zunächst verzweigtes PEI flach (RMS-Rauigkeit < 1 nm) auf einem Silizium-Substrat adsorbiert. Mit Hilfe der Fenton-Reaktion wurde die PEI-Monoschicht dem Einfluss von ROS ausgesetzt. Anhand von Kraft-Abstands-Kurven (KAK) konnte gezeigt werden, dass die flache Konformation der PEI-Monoschicht nach dem ROS-Einfluss erhalten bleibt. Des Weiteren konnte mittels Adsorption negativ geladener Gold-Nanopartikel (AuNP) demonstriert werden, dass die PEI-Oberfläche auch nach der Wechselwirkung mit ROS positiv geladene Gruppen enthält. Darüber hinaus konnte mit Hilfe der KAK gezeigt werden, dass sowohl die Oberflächenladungsdichte (OFL) als auch das Oberflächenpotential (OFP) unter ROS-Einfluss um einen Faktor 0,5 reduziert wurden. Es wurde gezeigt, dass die Reduzierung von OFL bzw. OFP auf die Abspaltung positiv geladener Gruppen der PEI-Monoschicht zurückgeführt werden kann. Mit Hilfe der dreidimensionalen Kraftspektroskopie wurde gezeigt, dass OFL und OFP auf einer Längenskala von 1,8 bis 30 µm lateral homogen sind. Darüber hinaus wurde anhand der AuNP-Belegungsdichte demonstriert, dass die Ladungsträger innerhalb der PEI-Oberfläche auf einer Längenskala oberhalb von 36 nm homogen verteilt sind. Hinsichtlich kleiner Längenskalen (< 36 nm) kann konstatiert werden, dass aufgrund einer verzögerten Adsorptionskinetik der AuNP nach der ROS-PEI-Wechselwirkung mit einer partiell reduzierten Bindungswahrscheinlichkeit zu rechnen ist. Vermutlich bewirkt der ROS-Einfluss eine inhomogene Verteilung der positiven Ladungsträger innerhalb der PEI-Monoschicht auf einer Längenskala von einigen nm. Experimentell ergibt sich darüber hinaus, dass eine 50 %ige Reduzierung des PEI-Oberflächenpotentials einer Abnahme von etwa 10 % der maximalen, anteiligen AuNP-Belegungsdichte entspricht. Diese experimentell bestimmten Parameter konnten unter Einbeziehung eines erweiterten drei-Körper RSA-Modelles erklärt werden. Im zweiten Teil der vorliegenden Arbeit wurde eine neue Methode der Quantifizierung ROS-induzierter DNS-Schäden eingeführt. Dazu wurden die DNS-Moleküle zunächst mittels Fenton-Reaktion dem Einfluss von ROS ausgesetzt, auf Polyallylamin-Hydrochlorid-funktionalisiertem Glimmer adsorbiert und mittels des RKM im intermittierenden Modus (RKM-IM) abgebildet. Die Klassifizierung der DNS-Moleküle erfolgt unter Berücksichtigung des Kettenhöhenunterschiedes von doppelsträngiger- (dsDNS) und einzelsträngiger (esDNS) DNS. Als ROS-induzierter DNS-Schaden wird hierbei der Konformationsübergang von dsDNS (intakt) in esDNS (defekt) identifiziert. Die zentrale Messgröße der vorgestellten Methode ist demnach die DNS-Kettenhöhe, welche sich im Falle immobilisierter DNS-Moleküle mit einer Genauigkeit im Sub-Ångström-Bereich mit Hilfe des RKM-IM bestimmen lässt. Für die automatisierte Quantifizierung der Flächen, welche mit dsDNS respektive esDNS belegt sind, wurde ein Höhengrenzwert-basierter Auswertungs-Algorithmus konstruiert. Neben der Variation der Stärke der ROS-DNS-Wechselwirkung mittels verschiedener H2O2-Konzentrationen innerhalb der Fenton-Reaktion, wurde der Einfluss eines Radikalfängers am Beispiel des Natriumacetats (NaOAc) auf diese Wechselwirkung untersucht. Mit der Quantifizierung der ROS-DNS-NaOAc-Wechselwirkung wurde gezeigt, dass der anteilige DNS-Schaden mit wachsender H2O2-Konzentration zunimmt und mit steigender NaOAc-Konzentration abnimmt. Darüber hinaus wurde die Anwendbarkeit der in dieser Arbeit eingeführten Quantifizierung ROS-induzierter DNS-Schäden mit Hilfe eines reaktionskinetischen Ansatzes unter Verwendung des Modelles der kompetitiven Hemmung analysiert. Auf diese Weise wurden Ratenkonstanten der Wechselwirkung zwischen NaOAc mit Hydroxylradikalen verifiziert und somit die Validität des eingeführten Konzeptes demonstriert. Des Weiteren ermöglicht die automatisierte Datenanalyse einen vergleichsweise hohen Datendurchsatz und eignet sich daher gut für die Untersuchung der komplexen Wechselwirkung zwischen ROS, Radikalfänger und DNS. Anhand eines Vergleiches mit den etablierten Methoden zur Quantifizierung ROS-induzierter DNS-Schäden ergibt sich unter Einbeziehung des, in dieser Arbeit eingeführten Analyseverfahrens, ein komplementäres Verständnis der ROS-DNS-Wechselwirkung über einen großen Längenskalenbereich.
Ausgangspunkt aller Untersuchungen sind Langmuir-Monoschichten an der Wasser/Luftgrenzfläche. Denn mit diesen Monoschichten kann die Oberflächeladungsdichte eingestellt werden. Sie werden durch amphiphile Moleküle gebildet.Die hydrophoben Alkylketten sind zur Luftseite und die hydrophile Kopfgruppe zur Wasserseite orientiert.Die Phasen der Lipid- Monoschicht und die Belegungsdichte werden extern eingestellt. Die Lipid- Monoschicht kann je nach Anordnung der Alkylketten verschiedene unter- schiedliche Phasen zeigen. Um die adsorbierten Polyelekrolyte zu beschreiben zu können, benötigt man experimentelle Methoden, die in der Lage sind, Konformation und Be- legungsdichte von adsorbierten Polyelektrolyten an Oberflächen aufzulösen. Diese Strukturen, auf der Nanometerskala, werden mit Röntgenreflektion und Röntgendiffraktion unter streifendem Einfall untersucht.Die Strukturda- ten werden durch thermodynamische Untersuchungen ergänzt. Die Untersuchungen werden für elektrostatische Kräfte mit hoher Ampli- tude und langer Reichweite durchgeführt. Die Lösungen der Polyelektrolyte (c PSS = 0.01 mmol/L bezogen auf die Monomerkonzentration) sind so einge- stellt, dass eine fast vollständige Ladungskompensation (70-90%) stattfindet. Unter diesen Bedingungen adsorbieren PE entweder flach als 2-dimensionales Knäuel oder geordnet in einer 2-dimensionalen lamellaren Phase. Die Untersuchungen sollen herausfinden, welchen Einfluss die elektrostatische Wechselwirkung auf die Kettensteifigkeit von adsorbierten Polyelektrolyten,die durch die Persistenzlänge LP charakterisiert werden.So kann geklärt werden,ab welcher Konturlänge LK Polyelektrolyte nicht mehr stäbchenförmig sondern als 2-dimensionales Knäuel adsorbieren. Untersucht werden negativ geladene PSS (Polystyron Sulfonat) mit un- terschiedlichen Konturlängen LK.Diese adsorbieren an die positiv geladene DODA Lipid-Monoschicht (Dioctadeyldimethylammonium). Die Persistenzlänge LP für adsorbiertes PSS an DODA ist direkt aus den thermodynamischen Daten ermittelt worden. Als Funktion der Konturlänge LK wurde der Oberflächendruck π c sowie die erste Ableitung dπc /dT unter- sucht. Daraus läßt sich die Persistenzlänge des adsorbierten PSS LP ≈ 210 Å bestimmen. Die Oberflächenladungsdichte der Monoschicht wird durch die Kompression verdoppelt.Simultan wird der Abstand der Polyelektrolytketten dPE halbiert, so dass immer eine 70-90% Ladungskompensation erreicht wird. Es wird mit Röntgendiffraktion immer eine fache 2-dimensionale lamellare Phase der adsorbierten PSS Ketten detektiert.Hierbei bilden Konturlängen LK < 110 Å eine Ausnahme. Es wird keine 2-dimensionale lamellare Phase für geringe Oberflächenladungsdichte (LE Phase der Lipide) gefunden. Die Röntgenreflektion kann aber zeigen, dass PSS Ketten, unabhängig von der Konturlänge LK, immer flach an der Lipidmonoschicht adsorbieren. Wenn steife kurze Ketten (LK≤110Å<LP ) an der LC Phase der Lipid- Monoschicht adsorbieren, liegen sie aber in der 2-dimensionalen lamellare Phase. Der Übergang von der LE in die LC Phase der Lipid-Monoschichten erfolgt gleichzeitig mit dem Übergang von der ungeordneten flachen zur 2- dimensionalen lamellare Phase der adsorbierten kurzen PSS Ketten. Daher zeigt die Enthalpie ∆H des LE/LC Phasenübergangs ein Maximum bei L K =110 Å. Im nächsten Schritt wird die Elektrostatik zwischen der Lipid-Monoschicht und den Polyelektrolyte untersucht.Die maximale Oberflächenladungsdichte der Lipid-Monoschicht wird durch eine Mischung von geladenen DODA und ungeladenen DPPC (Dipalmitoylphosphatidylcholine) eingestellt. Röntgendiffraktionsmessungen zeigen bis zu einem DODA-Anteil von 75% die Ausbildung einer 2-dimensionalen lamellaren Phase an der LC Phase der Lipid-Monoschicht. Nimmt die maximale Oberflächenladungsdichte weiter ab, so wird keine Bildung der 2-dimensionalen lamellaren Phase beobachtet. Die Abnahme der maximalen Oberflächenladung führt zu einer niedrigen Belegungsdichte und so zu einem größerem Kettenabstand dPE. Die Linienladungsdichte wird durch unterschiedliches P−TrisAAx−rand−AMPS1−x auf 90% oder 50% verringert. Röntgendiffraktionsmessungen an PE mit unterschiedlichen Linienladungsdichten zeigen auch hier eine 2-dimensionale lamellare Phase. Die verringerte Linienladungsdichte erzwingt eine höhere Belegungsdichte um eine 70-90% Ladungskompensation zu erreichen. Dieses führt zu einem kleineren Kettenabstand dPE. Ein Grenzfall ist bei einer 50%-igen Linienladungsdichte zu beobachten. Dort konnte an der LC Phase der Lipid-Monoschicht keine 2-dimensionale lamellare Phase der PE beobachtet werden. Der Kettenabstand dPE ist so gering, dass er mit der Röntgendiffraktion nicht mehr aufgelöst werden kann.
This work describes the recent scientific and technical achievements obtained at the high-precision Penning trap mass spectrometer SHIPTRAP. The scientific focus of the SHIPTRAP experiment are mass measurements of short-lived nuclides with proton number larger than 100. The masses of these isotopes are usually determined via extrapolations, systematic trends, predictions based on theoretical models or alpha-decay spectroscopy. In several experiments the masses of the isotopes 252-255No and 255,256Lr have been measured directly. With the obtained results the region of enhanced nuclear stability at the deformed shell closure at the neutron number 152 was investigated. Furthermore, the masses have been used to benchmark theoretical mass models. The measured masses were compared selected mass models which revealed differences between few keV/c² up to several MeV/c² depending on the investigated nuclide and model. In order to perform mass measurements on superheavy nuclei with lower production rates, the efficiency of the SHIPTRAP setup needs to be increased. Currently, the efficiency is 2% and mainly limited by the stopping- and extraction efficiency of the buffer gas cell. The stopping and extraction efficiency of the current buffer gas cell is 12%. To this end, a modified version of the buffer gas cell was developed and characterized with 223Ra ion source. Besides a larger stopping volume and a coaxial injection the new buffer gas cell is operated at a temperature of 40K. The operation at cryogenic temperatures increases the cleanliness of the buffer gas. From extraction measurements and simulations an overall efficiency of 62(3)% was determined which results in an increase by a factor of 5 in comparison to the current buffer gas cell. Aside from high-precision mass measurements of heavy radionuclides the mass differences of metastable isobars was measured to identify candidates for the neutrinoless double-electron capture. Neutrinoless double-electron capture can only occur if the neutrino is its own antiparticle and a physics beyond the standard model exists since the neutrinoless double-electron capture violates the conservation of the lepton number. Due to its expected long half-life this decay has not yet been observed. However, the decay rate is resonantly enhanced if mother and daughter nuclide are degenerate in energy. Suitable candidates for the search of the neutrinoless double-electron capture have been identified with mass difference measurements uncertainties of about 100eV/c². In this work the results of the mass difference measurements of 12 possible candidates are presented.
The Atomic Force Microscope (AFM) has become an important tool for probing the mechanical properties of cells and microparticles by force-indentation experiments. In this thesis optimized AFM approaches for these experiments are developed and applied to three types of living human cells in order to answer biologically relevant questions about their mechanics. These microscopic investigations are then interpreted with respect to nanoscopic and macroscopic biologic parameters, such as the function of cell surface receptors or the size of human heart ventricles. This thesis comprises two physical/technical chapters and three medical/biological chapters. The physical/technical chapters discuss the measurement process itself, aiming for its improvement with respect to a proper data analysis and contact model (for spherical cells). The medical/biological chapters investigate the elasticity of cells by the use of optimized AFM approaches, with respect to the used data analysis.
Magnetic reconnection is a fundamental plasma process where a change in field line connectivity occurs in a current sheet at the boundary between regions of opposing magnetic fields. In this process, energy stored in the magnetic field is converted into kinetic and thermal energy, which provides a source of plasma heating and energetic particles. Magnetic reconnection plays a key role in many space and laboratory plasma phenomena, e.g. solar flares, Earth’s magnetopause dynamics and instabilities in tokamaks. A new linear device (VINETAII) has been designed for the study of the fundamental physical processes involved in magnetic reconnection. The plasma parameters are such that magnetic reconnection occurs in a collision-dominated regime. A plasma gun creates a localized current sheet, and magnetic reconnection is driven by modulating the plasma current and the magnetic field structure. The plasma current is shown to flow in response to a combination of an externally induced electric field and electrostatic fields in the plasma, and is highly affected by axial sheath boundary conditions. Further, the current is changed by an additional axial magnetic field (guide field), and the current sheet geometry was demonstrated to be set by a combination of magnetic mapping and cross-field plasma diffusion. With increasing distance from the plasma gun, magnetic mapping results in an increase of the current sheet length and a decrease of the width. The control parameter is the ratio of the guide field to the reconnection magnetic field strength. Cross-field plasma diffusion leads to a radial expansion of the current sheet at low guide fields. Plasma currents are also observed in the azimuthal plane and were found to originate from a combination of the field-aligned current component and the diamagnetic current generated by steep in-plane pressure gradients in combination with the guide field. The reconnection rate, defined via the inductive electric field, is shown to be directly linked to the time-derivative of the plasma current. The reconnection rate decreases with increasing ratio of the guide field to the reconnection magnetic field strength, which is attributed to the plasma current dependency on axial boundary conditions and the plasma gun discharge. The above outlined results offer insights into the complex interaction between magnetic fields, electric fields, and the localized current flows during reconnection.
Modern cavity QED and cavity optomechanical systems realize the interaction of light with mesoscopic devices, which exhibit discrete (atom-like) energy spectra or perform micromechanical motion. In this thesis we have studied the crossover from the quantum regime to the classical limit of two prototypical models, the Dicke model and the generic optomechanical model. The physical problems considered in this approach range from a ground state phase transition, its dynamical response to general nonequilibrium dynamics including Hamiltonian and driven dissipative chaotic motion. The classical limit of these models follows from the classical limit of at least one of its subsystems. The classical equations of motion result from the respective quantum equations through the application of the semiclassical approximation, i.e., the neglect of quantum correlations. The approach of the results from quantum mechanics to the prediction of the classical equations can be obtained by subsequently decreasing the respective scaling parameter. In order to obtain exact results we have utilized advanced numerical methods, e.g., the Lanczos diagonalization method for ground state calculations, the Kernel Polynomial Method for dynamical response functions, Chebyshev recursion for time propagation, and quantum state diffusion for open system dynamics. We have studied the quantum phase transition of the Dicke model in the classical oscillator limit. Our work shows that in this limit the transition occurs already for finite spin length but with the same critical behavior as in the classical spin limit. We have derived an effective model for the oscillator degrees of freedom and have discussed the differences of both classical limits with respect to quantum fluctuations around the mean-field ground state and spin-oscillator entanglement. In this thesis we have proposed a variational ansatz for the Dicke model which extends the mean-field description through the inclusion of spin-oscillator correlations. The ansatz becomes correct in the limit of large oscillator frequency and in the limit of a large spin. For the latter it captures the leading quantum corrections to the classical limit exactly including the spin-oscillator entanglement entropy. We have studied the dynamics of spin and oscillator coherent states in the nonresonant Dicke model at weak coupling. In this regime periodic collapses and revivals of Rabi oscillations occur, which are accompanied by the buildup and decay of atom-field entanglement. The spin-oscillator wave function evolves into a superposition of multiple field coherent states that are correlated with the spin configuration. In our work we provide a description of the underlying dynamical mechanism based on perturbation theory. Our analysis shows that collapse and revival at nonresonance is distinguished from the resonant case treated within the rotating wave approximation by the appearance of two time scales instead of one. We have extended our study of the Dicke dynamics to the case of increasing spin length, as the system approaches the classical spin limit. We described the emergence of collective excitations above the ground state that converge to the coupled spin-oscillator oscillations observed in the classical limit. With increased spin length the corresponding Green functions thus reveal quantum dynamical signatures of the quantum phase transition. For the dynamics at larger coupling and energy, classical phase space drift and quantum diffusion hinders the direct comparison of quantum and classical observables. As we show in our work, signatures of classical quasiperiodic orbits can be identified in the Husimi phase-space functions of the propagated wave function and individual eigenstates with energies close to that of the quasiperiodic orbits. The analysis of the generic optomechanical system complements our study of cavity QED systems by a quantum dissipative system. In this thesis we have shown for the first time, how the route to chaos in the classical optomechanical system takes place, given as a sequence of consecutive period doubling bifurcations of self-induced cantilever oscillations. In addition to the semiclassical dynamics we have analyzed the possibility of chaotic motion in the quantum regime. Our results showed that quantum mechanics protects the optomechanical system against irregular dynamics. In sufficient distance to the semiclassical limit simple periodic orbits reappear and replace the classically chaotic motion. In this way direct observation of the dynamical properties of an optomechanical system makes it possible to pin down the crossover from quantum to classical mechanics.
The laser-matter interaction is a topic of current research. In this context, the interaction of intensive laser radiation with atomic clusters is of special interest. Du to the small cluster size, the laser field can penetrate the whole cluster volume, which leads to a high absorption of energy in the cluster. As a result, plasmas with high density and high temperature are produced. In the early phase of the laser-cluster interaction, free electrons are initially created in the cluster due to tunnel ionization or photoionization. Via collisions of these electrons with the cluster atoms, the ionization is increased and thus a dense nanoplasma is produced, which is heated by the laser. If free electrons leave the cluster during the laser-cluster interaction (outer ionization), a positive charge buildup is created. The associated charge repulsion finally can lead to the fragmentation of the cluster due to Coulomb explosion. Experimentally, interesting phenomena emerging from laser-excited clusters are observed, e.g., the creation of fast electrons, the production of highly charged ions, and X-ray emission. In this dissertation, the interaction of Gaussian laser pulses in the infrared regime with argon and xenon clusters is simulated by means of a nanoplasma model. Considering laser intensities in the non-relativistic regime, the relevant processes such as ionization, heating and expansion are theoretically described in this model with a set of coupled rate equations and hydrodynamic equations. One focus of the thesis is on the heating of the nanoplasma via inverse bremsstrahlung (IB), which is due to the absorption of laser photons in electron-ion collisions. In particular, the important question is investigated whether the consideration of the ionic structure – that means, the nuclear charge and the bound electrons – modifies the electron-ion collisions and thus the IB heating rate. Starting from a quantum statistical description, effective electron-ion potentials are used which account for both the screening due to the dense plasma and the inner ionic structure. Within the quantum mechanical first Born approximation, the consideration of the ionic structure leads to a drastic increase of the IB heating rate, in particular for high nuclear charges and low ionic charge states. However, for the parameters relevant in experiments, the applicability of the first Born approximation is questionable. Therefore, quantum mechanical calculations going beyond the first-order perturbation theory are performed. In addition, the IB heating rate is investigated with different classical methods. These are based either on transport cross sections for elastic electron-ion scattering or on classical simulations of inelastic scattering processes. Also within the classical approaches, the consideration of the ionic structure leads to an increase of the heating rate. However, this increase is shown to be only moderate. In a further part, the thesis focuses on the question how the dynamics of the laser-cluster interaction is influenced by the consideration of excited states. This is explored exemplarily for argon clusters excited by single or double laser pulses. The consideration of excitation processes in the nanoplasma leads to a decrease of the electron temperature and to an increase of the density of free electrons. Moreover, it is shown that the consideration of excitation processes results in an essential acceleration of the ionization dynamics. As a consequence, the mean ionic charge state in the plasma as well as the number of highly charged ions is significantly increased. For the population of ground states and excited states within an ionic charge state Z, collisional deexcitation processes play an important role. By means of an analytical relation between excitation and deexcitation cross sections, the rates for the respective processes in the presence of the laser field are calculated. The role of deexcitation processes is studied in detail, showing that the inclusion of these processes is essential for the correct theoretical description of the photon emission from laser-excited clusters. Based on these results, the photon yield is calculated for selected radiative transitions resulting from highly charged argon ions in the UV and X-ray regime.
Energetic ions are made to collide with atmospheric molecules. Positively charged ions of argon (Ar^+), helium (He^+), hydrogen (H_2^+ ), and protons (H^+) with energies of 50 keV to 350 keV are used as the bombarding ion. The ion beam of desired energy is produced using a linear ion accelerator at the University of Greifswald. The mass and energy distribution of sputtered particles were analysed using an Electrostatic Quadrupole SIMS (EQS) analyser. The target gases used are oxygen (O_2), sulfur hexafluoride (SF_6), and nitrogen (N_2). The ionized and fragmented particles due to collisions have been investigated. We have discovered a new process for negative ion formation in energetic ion collision with O_2 and SF_6 molecules. The process is a two body reaction between the projectile and the molecule without the need for a third particle (such as an external electron). It requires a direct charge transfer from the projectile to the molecule leaving it intact as O_2^- or SF_6^- . The process is experimentally confirmed by using a proton as projectile which does not have an electron to transfer. In comparison with positive ion fractions (O_2^+ , SF_5^+ ), the negative ions fraction is smaller by 2 orders of magnitude. This shows that the two body charge exchange process is weak due to the larger energy transfer required compared to the positive ion forming mechanisms. The two body charge exchange mechanism is not observed for ion collisions with N_2 molecule. No stable negative ion exist for N_2 molecule. The collision cross section for the ion formation during energetic ion – O_2 collision has been determined within the investigated impact energy. For SF_6 molecule the partial ion fraction of the secondary ions are determined for different projectiles involved. This kind of investigation is of great importance mainly in atmospheric physics. Energetic ions are constantly emitted from mass of the energy sources in the universe (e.g. sun). They interact with planetary objects or atmosphere on their way. A deep knowledge about the interaction processes is necessary to understand the ionospheric physics and space exploration. As second part of my thesis, a GaAs(100) surface is bombarded with 150 keV Ar^+ ion beam. From etching the surface to thin film coating, ion bombardment on solid surface found great role in the fabrication process of modern electronic and optical devices. In order to increase the knowledge on sputtering materials and because of profound importance in modern electronics, we choose GaAs(100) as our target. Among the sputtered atoms and ions, small sized cluster ions having more than 6 atoms have been identified. GaAs is a heteroatomic semiconductor containing gallium and arsenic in equal ratio. A preferential phenomenon of ’abundant sputtering’ of gallium compared to little arsenic (GaAs) has been investigated from their mass intensity. The experimental ion counts are compared with theoretically predicted relative abundance. This phenomenon of preferential sputtering is known for atomic species of sputtered GaAs but not for the sputtered cluster ions. The main reasons for this abundant sputtering of one element is attributed to the difference in ion formation energies and surface compositional change taking place during the sputtering process. Another notable characteristics is the preference in charge state among the sputtered ions. For instance, among sputtered atomic ions the ion counts of Ga^+ is 3 orders larger than As^+ ion and As^- is 2 orders larger than Ga^- ion. To get a clue for this behavior, we have investigated the energy distribution of both negatively and positively charged clusters. Different ion formation mechanisms were discussed. The energy distribution of atomic ion is partially explained by using a modified theory given by M. W. Thompson.
Die vorliegende Arbeit liefert Beiträge zur optischen und elektrischen Charakterisierung des dynamischen Verhaltens von Plasmaspezies in Atmosphärendruck-Plasmen insbesondere mit Hinsicht auf den Einsatz in der Plasmamedizin. Dabei wurde ein breites Spektrum verschiedener Diagnostiken angewandt, um die Zugänglichkeit zur Bestimmung weiterer Plasmaparameter an Atmosphärendruck zu prüfen. Diese Arbeit stellt eine neue Methode zur Bestimmung der Ionendichte bei Atmosphärendruck- Bedingungen vor, bei der elektrische Oszillationen ausgewertet werden, deren Ursprung ionenakustische Wellen im Plasma sind. Weiterhin wurden neben relativen optischen Messungen wie der phasenaufgelösten optischen Fotografie (PROI) und der Kreuz- Korrelations-Spektroskopie (CCS) auch absolute optische Messungen mit der interferometrischen Hakenmethode und dem Pockels-Effekt durchgeführt. Anhand von elektrischen Messungen wurde ferner gezeigt, dass mit einer Strom- und Spannungs-Charakteristik der Einfluss von Aufbauparametern, wie der Kapillarposition oder dem Gasfluss, auf das Plasma untersucht werden kann. Gegenstand der Untersuchungen waren verschiedene Plasmaquellen, die für eine Nutzung in der Plasmamedizin entwickelt wurden. Sowohl die elektrischen Messungen des Parametereinflusses als auch die Bestimmung der Ionendichte erfolgten an der selbstpulsenden transienten Funkenentladung in Argon an offener Atmosphäre. Der geringe Filamentdurchmesser und der dennoch hohe Entladungsstrom ermöglichen die Detektion der ionenakustischen Instabilität. Darüber hinaus wurde diese erratisch zündende Entladung räumlich und zeitlich aufgelöst mit der CCS spektroskopisch untersucht. Dabei wird insbesondere die Selbst-Triggerung der CCS ausgenutzt, um einen Zeitbezug trotz des großen Entladungsjitter zu erhalten. Für die PROI wurden die räumlich und zeitlich stabilen Entladungsanordnungen der Nadel-Platte-Geometrie und des Kapillarjets in Helium gewählt. Die Anordnungen wurden mit einer periodischen Sinusspannung betrieben und wiesen Entladungsspalte von d = 5 - 15 mm auf. Eine besondere Anforderung der Messung mit dem Pockels-Effekt ist zu der räumlichen und zeitlichen Stabilität eine dielektrische Gegenelektrode, welche bei der Anordnung des Kapillarjets möglich war. Bei der Anwendung der interferometrischen Hakenmethode kam neben einem Erdgas-Sauerstoff-Mischgasbrenner sowohl eine Mikrowellen-Entladung (Plexc) als auch ein MHz-Plasmajet (kINPen) zur Anwendung. Die Bedeutung der elektrischen Messungen, besonders der Strom- und Spannungscharakteristik einer Entladung, wurde an dem Parametereinfluss der Kapillarposition einer erratisch zündenden transienten Funkenentladung vorgestellt. Es konnte gezeigt werden, dass der Zeitunterschied zwischen dem Stromsignal eines Vorstreamers und der Hauptentladung durch das Einbringen einer Kapillare in den Entladungsspalt deutlich verringert wird. Insbesondere der Beitrag der lokalen elektrischen Feldstärkeerhöhung an der Kapillarkante und der Diffusionsanteil der Umgebungsluft wurden als Ursachen, durch Vergleich einer Feldsimulation mit der Beobachtung der Vorphase an der Kapillarkante in den CCS-Messungen, diskutiert. Anschließend konnte gezeigt werden, dass der Leistungseintrag in die Vorphase durch die Platzierung der Kapillare deutlich reduziert werden kann. Ein wesentliches Ergebnis dieser Arbeit ist die Beobachtung von ionenakustischen Wellen als Oszillationen im Abklingen des Stromsignals einer erratisch zündenden transienten Funkenentladung. Hierzu war es nötig, elektrische Störungen zu erkennen und zu eliminieren. Es konnte ein Erdschleifen-freier Aufbau realisiert werden. In diesem Aufbau zeigt sich, dass die Signale der ionenakustischen Welle ausschließlich in einem bestimmten Gasflussbereich beobachtet werden. Die gemessene Frequenz der Oszillationen wurde als Ionenplasmafrequenz f_{pl ,i} identifiziert und enthält daher Angaben zu den Ionendichten im Bereich von n_{Ar_2^+} = 3•10^{14} cm^{-3} bis 1•10^{12} cm^{-3}. Nach einer Abschätzung der zu erwartenden Elektronendichte, die der gemessenen Ionendichte sehr nahe kommt, wurde die Dispersionsrelation für die vorhandenen Entladungsbedingungen aufgestellt und gelöst. Dabei zeigt sich eine starke zeitliche Dämpfung über die Ionen-Neutralteilchenstöße sowie eine räumliche Verstärkung für die Ionenplasmafrequenz. Aus der Dämpfung der Oszillationsamplituden konnte die Ionen- Neutralteilchen-Stoßfrequenz nu_i = 3•10^7 Hz ermittelt werden. Weiterhin ergibt sich aus der Lösung der Dispersionsrelation ein Existenzbereich für die ionenakustischen Wellen in Abhängigkeit von der Ionendichte und der elektrischen Feldstärke.
Bei moderaten sinusförmigen Betriebsspannungen tritt in reinem Stickstoff der diffuse Townsend-Modus (APTD) auf. Das elektrische Feld ist hier über den Entladungsspalt annähernd konstant, weshalb ein anodengerichteter exponentieller Anstieg der Intensität der Emission beobachtet wird. Dementsprechend ist das Intensitätsmaximum direkt vor der Anode lokalisiert. Überraschenderweise lässt sich die APTD unter den gegebenen experimentellen Bedingungen (Breite des Entladungsspalts d_Spalt=1 mm und sinusförmige Betriebsspannung) ebenfalls in einer Helium-BE genieren. Für gewöhnlich wird jedoch in einer Helium-Entladung der diffuse Glimmentladungs-Modus (APGD) beobachtet, wobei der Entladungsspalt zwischen 2-5 mm breit ist. Das Emissionsmaximum einer solchen Entladung befindet sich durch die Ausbildung eines Kathodenfallgebiets vor der Kathode. Die geringe Breite des Entladungsspalts verhindert hier jedoch die Ausbildung der APGD. Entsprechend kann sich das Kathodenfallgebiet nicht entwickeln, wodurch die Spaltspannung nur schwach einbricht. Das Intensitätsmaximum der Emissionsentwicklung befindet sich wie bei der diffusen Stickstoff-BE direkt vor der Anode. Die Zünd- und Brennspannung ist in Stickstoff größer als in Helium, da die Vibrationszustände des Stickstoffs effizient durch Elektronen angeregt werden und diesen dabei Energie entzogen wird. Helium hat jedoch keine Vibrationszustände, weshalb die Elektronentemperatur ansteigt und die Zünd- und Brennspannung deutlich geringer ist. Eine Erhöhung des Spannungsanstiegs dU/dt beeinflusst signifikant die Entladungsentwicklung in der diffusen Helium-Entladung. So führt eine Variation von der Sinus- zur Rechteckspannung zu einem Wechsel des Entladungsmodus, nämlich von der APTD zur APGD. Die Ursache hierfür ist der deutlich höhere Energieeintrag, was sich auf die Ionisationsprozesse auswirkt. Die Verwendung einer Sägezahnspannung stellt in Bezug auf den Spannungsanstieg dU/dt eine Kombination aus der Sinus- und der Rechteckspannung dar. Mit dieser Betriebsspannung war es erstmals möglich, in einer Entladungsperiode entsprechend der Spannungsgradienten beide Entladungsformen (APTD und APGD) zu beobachten und zu studieren. Durch die Oberflächenladungsmessung konnte nachgewiesen werden, dass die während eines elektrischen Durchbruchs im Entladungsvolumen transferierte Ladung vollständig auf den Dielektrika akkumuliert wird. Der Vergleich der phasenaufgelösten Oberflächenladungsdichtemessung mit der zeitlichen Integration der Stromdichte zeigt, dass die Akkumulation von Oberflächenladungen instantan mit dem Auftreten eines Strompulses stattfindet. Nach einem Entladungsstrompuls bleiben die Oberflächenladungen unabhängig vom Entladungsmodus auf dem Dielektrikum konstant, bis die Entladung in der nächsten Halbwelle erneut zündet. In der filamentierten Entladung markieren die Oberflächenladungen den Auftreffpunkt der einzelnen Mikroentladungen. Die Oberflächenladungen sind an diesen Stellen stark lokalisiert. Die gemittelten radialen Oberflächenladungsdichteprofile haben gezeigt, dass diese sowohl für die negativen als auch für die positiven Oberflächenladungen einer Gauß-Verteilung folgen. Die volle Halbwertebreite der entsprechenden Oberflächenladungsdichteprofile unterscheidet sich. Die negativen Oberflächenladungen nehmen eine größere Fläche ein als die positiven Oberflächenladungen. Es konnte erstmals gezeigt werden, dass Mikroentladungen über viele Entladungsperioden immer wieder an der gleichen Stelle zünden, wo sich aus einer vorhergehenden Entladung ein lokalisierter Oberflächenladungsfleck entgegengesetzter Polarität befand. Dieses Phänomen wird als Memory-Effekt bezeichnet. Durch zeitlich definiertes Abschalten der Entladung konnten die Lebensdauern von Oberflächenladungen beider Polaritäten auf dem BSO-Kristall gemessen werden. Es konnte gezeigt werden, dass der Abbau der Oberflächenladungen in zwei Zerfallsprozesse k_1 und k_2 unterteilt ist. Während des Prozesses k_1 nimmt die Oberflächenladungsdichte innerhalb einiger weniger Sekunden deutlich ab. Die Zeitkonstante k_1 ist trotz der photoleitenden Eigenschaft des BSO-Kristalls unabhängig von der Beleuchtungsfrequenz des Kristalls ist. Der zweite deutlich langsamer ablaufende Prozess zeigte hingegen eine starke Abhängigkeit von der Beleuchtungsfrequenz der BSO-Kristalls. Wurde der Kristall kontinuierlich beleuchtet, verschwanden die Oberflächenladungen unabhängig von ihrer Polarität nach wenigen Sekunden vollständig. Je kleiner die Beleuchtungsrate des Kristalls ist, desto länger waren die Oberflächenladungen nachweisbar. Der Zerfallsprozess k_2 beruht auf intrinsischen Transportprozessen. Hierbei wird davon ausgegangen, dass die negativen Oberflächenladungen durch Elektronen nahe der Oberfläche gebildet werden. Die positiven Oberflächenladungen sind Löcher im Valenzband, die durch Elektronen-Ionen-Rekombination entstehen.
During the past decade, various physical properties of the Yukawa ball, like structure and energy states, were unraveled using experiments. However, the dynamical features served further attention. Therefore, the main aim of my thesis was to investigate and understand how a finite system-represented by Yukawa clusters-evolves from a solid, crystalline structure to a liquid-like system, how it behaves in this phase and in what manner the reordering back into the solid state can be described. As a method of choice to reach this goal, laser heating has been proven successful. Moreover, the special importance of wakefields for dust clusters confined at low neutral gas pressure was addressed. Melting of finite dust clouds can be induced in two ways, either by altering the properties of the ambient plasma or by laser heating. The latter was shown to be a generic melting scenario, allowing to estimate a critical coupling parameter at the melting point. Moreover, the melting transition of finite 3D dust systems was found to be a two-step process where angular order is lost before the radial order starts to diminish at higher energies. Next, the mode dynamics of finite 3D dust ensembles in the solid and the liquid phase was studied. Crystal and fluid modes revealed the main spectral properties of the system. The normal modes are mainly suited to describe crystalline states. Fluid modes were excited naturally and via laser heating, with excitation frequencies almost independent of the coupling parameter in the solid and the liquid-like regime. Tuning the plasma parameters can be used to vary the particle-particle interaction via the ion focus. Both methods, even though assuming equilibrium situations, allowed to hint at these wakefields. The corresponding peaks in the fluid and normal mode spectra were no eigenmodes, confirming the nonequilibrium character of the ion focusing effect. First steps to extend the normal mode theory to achieve the dynamics of wake-affected nonequilibrium dust clusters were presented. Statistical quantities were obtained evaluating long-run experiments and transport coeffcients for finite dust systems were calculated via the instantaneous normal mode technique. Diffusion was found considerably higher for 3D than for 2D dust clusters. Using the configurational entropy, we have shown that in 2D and 3D disorder increases with increasing size of the system, in agreement with simulations. The temperature dependence of the configurational entropy differs for 2D and 3D dust clouds, with a threshold behavior found for finite 2D ensembles only. Finally, using instantaneous normal modes to reveal the total fraction of unstable modes, the predictive connection of Keyes (Phys Rev E 62, p7905, 2000), between transport and disorder was tested and verified for 2D, but not for 3D clusters. The reason for this has to be left open. Finally, laser-mediated recrystallization processes of finite 3D dust clouds were investigated. First, the temporal evolution of the Coulomb coupling parameter was traced during heating and recrystallization. A cooling rate has been determined from the initial phase of recrystallization. This cooling rate is lower than damping by the neutral gas, in agreement with simulations. We have observed a large fraction of metastable states for the final cluster configurations. Further, we have revealed that the time scale for the correlation buildup in the finite 3D ensemble was on even slower scales than cooling. Thus, different time scales can be attributed to the fast emergence of the shells and to the slower individual ordering within the shells.
The extraction of raw materials in mining, as for example copper, generally requires a separation of the natural resources quarried. In most cases complex ores, mixtures of different minerals and gangue have to be separated in order to enable an economic processing. In particular for the extraction of sulfides, oxides, carbonates, phosphates, but also of coal, froth flotation is mainly used for this purpose, therefore it is considered as the most important separation process in raw material industries. Several billion tons of ores are processed annually. The principle of flotation is based on the surface properties of the mixtures components and the separation efficiency, which decisively determines the required amount of water and various chemicals, if nothing else, is an important criterion in mineral exploration and it also significantly influences the environmental impact of mineral processing. The aim of, this work was to investigate the influence, of, low-temperature plasmas on the mineral surface and, based on the acquired knowledge, to develop and verify strategies that would increase the efficiency of flotation processes through plasma pre-treatment of mineral mixtures. Since these studies are unprecedented, the results presented can be classified as a contribution to application-oriented basic research. Powder of the sulfide minerals, pyrite (FeS2), chalcopyrite (CuFeS2), chalcocite (Cu2S) and molybdenum sulfide (MoS2), were treated with plasmas of a radiofrequency and a microwave discharge and the resulting surface modifications were investigated by structure analysis such as X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). During the plasma process, the argon/oxygen and argon/hydrogen process gas mixtures used were analyzed by mass spectrometry (MS), taking into account the quantity of gaseous reaction products released, in order to estimate the rate at which chemical reactions occur. Furthermore, Langmuir and thermal probes, as well as different methods of optical emission spectrometry (OES) were utilized, which enabled a characterization of the discharges used with regard to different plasma parameters. It has been shown that sulfur dioxide (SO2) in Ar/O2 plasmas and hydrogen sulfide (H2S) in Ar/H2 plasmas are the only reaction products which can be detected by MS during the mineral treatments. Thus, the resulting sulfur rate loss could be time-resolved determined by means of additional calibrations with calibrating gases. Especially at Ar/O2-MW plasma treatments two fundamental mechanisms of mineral modification could be separated by time. Pure plasma-surface interactions at the beginning and, additionally, thermally induced reactions in during the evolution of the treatments. Comparisons regarding the relative sulfur loss during plasma-surface interactions between the investigated minerals have shown a strong influence of the process parameters whereas, under identical conditions, CuFeS2 reacted up to eight and nineteen times faster reacted than FeS2 or Cu2S. This result represents the basis of the strategy to optimize the flotation of the minerals investigated: The selective generation of oxides on the surface of one component in a mixture of sulphide minerals. In particular, at the separation of CuFeS2/FeS2 mixtures by using the oxide collectors Flotinor Fs-2 in a micro flotation cell, a high selectivity could be achieved. The recovery of CuFeS2 amounted to 100 % while less than 10 % of FeS2 was recovered and no other modifying reagents were used. XPS and XRD analyses indicate the possibility that metal oxide are created upon the CuFeS2 surface, while the formation of iron sulfates upon the FeS2 surface prevented the oxide collector adsorption. An increased intensity of the plasma treatment leads to an increased sulphate formation also on CuFeS2, whereas the recovery, and thus the selectivity of the flotation, was reduced again. It could be shown that this effect can be utilized for the separation of, CuFeS2/MoS2 mixtures by using both, oxide and sulfide collectors, because sulfates are not formed on molybdenum sulfide during Ar/O2 plasmas treatments. By means of the plasma diagnostics used the energy input onto the substrate, the gas temperature and the degree of dissociation of molecular gases were estimated and correlations regarding the surface modification have been worked out. Thereby, the region investigated within the parameter space could be enlarged due to the use of different excitation frequencies, 13.56 MHz and 2.45 GHz, and additional insights have been provided. Further studies, beyond the scope of this work, are, nevertheless, required in order to generate a more comprehensive picture of plasma-mineral interactions and to enable an optimal application of the obtained results.
In this thesis we have revisited the formation of the excitonic insulator (EI), which realizes an exciton condensate. In contrast to optically created exciton condensates, the EI forms in thermal equilibrium and is solely driven by the Coulomb attraction between electrons and holes. The EI phase is anticipated to occur near the semimetal-semiconductor (SM-SC) transition at low temperatures. Depending from which side the EI is approached, it forms due to a BCS-type condensation of electron-hole pairs or a Bose-Einstein condensation (BEC) of excitons. The extended Falicov-Kimball model (EFKM) is the minimal model the EI can be described with. This model describes spinless fermions in two dispersive bands (f band and c band), that interact via a local Coulomb repulsion. The EFKM is also used to describe electronic ferroelectricity (EFE). Both phases, the EI and EFE-type ordering, are characterized by a spontaneous f-c hybridization in the EFKM. We have presented the EI phase, the EFE phase, and the orderings they compete with. Moreover, we have determined the ground-state phase diagram of the EFKM. We have focused particularly on the anticipated BCS-BEC crossover within the EI and have analyzed the formation scenarios. The exciton spectrum and the exciton density in the normal phase close to the critical temperature give information about relevant particles and therefore the nature of the transition. We have demonstrated that the whole EI is surrounded by a halo", that is, a phase composed of electrons, holes and excitons. However, on the SM side, only excitons with a finite momentum exist. These excitons appear only in a small number and barely influence the SM-EI transition. This phase transition is driven by critical electron-hole fluctuations, generated by electrons and holes at the Fermi surface. On the SC side, excitons with arbitrary momenta exist. Most notably, we have found the number of zero-momentum excitons to diverge at the SC-EI transition, signaling the BEC of these particles. Within the EI phase, there is a smooth crossover from the BCS regime to the BEC regime. One of the promising candidates to observe the EI experimentally, is the transition-metal dichalcogenide 1T-TiSe2. Strong evidences were found favoring an EI scenario of the charge-density-wave (CDW) formation in this material. However, some aspects point to a lattice instability to drive the CDW transition. We have addressed this issue by analyzing the recently discovered chiral property of the CDW in 1T-TiSe2. We have found that the EI scenario is insufficient to explain a stable, long range chiral charge ordering. Lattice degrees of freedom must be taken into account. In particular, nonlinear electron-phonon coupling and phonon-phonon interaction are crucial. By estimating appropriate model parameters for 1T-TiSe2, we have suggested a combination of excitonic and lattice instability to drive the CDW transition in this material. Experiments in 1T-TiSe2 and other materials suggest that the coupling to the lattice is non-negligible. We have extended therefore the model by an explicit exciton-phonon interaction, and have analyzed crucial effects of this interaction. While the single-particle spectrum is not modified qualitatively, the electron-hole pair spectrum changes significantly. The inclusion of the phonons lead to a massive collective mode in the ordered ground state in contrast to the case for vanishing exciton-phonon coupling, where the mode is acoustic. We have suggested that a gapless collective mode leads to off-diagonal long range order. This questions that the ground state for finite exciton-phonon coupling represents a condensate.
The thesis describes experimental results based on optical diagnostics of low- pressure discharges. The models, which are necessary for the interpretation of the experimental data, are developed and simulations are done. The contents can be categorized into the following topics: 1) the time-resolved tunable diode laser absorption spectroscopy of excited states of argon in pulsed magnetron discharge and modeling the plasma afterglow; 2) optical emission- and laser absorption spectroscopy of excited states of argon in radio-frequency (rf) discharge and calculation of the escape factor for self-absorption; 3) fast video recording of the oscillatory motion of a dust particle in rf discharge and analysis of the data.
There is a wide variety of Alfvén waves in tokamak and stellarator plasmas. While most of them are damped, some of the global eigenmodes can be driven unstable when they interact with energetic particles. By coupling the MHD code CKA with the gyrokinetic code EUTERPE, a hybrid kinetic-MHD model is created to describe this wave–particle interaction in stellarator geometry. In this thesis, the CKA-EUTERPE code package is presented. This numerical tool can be used for linear perturbative stability analysis of Alfvén waves in the presence of energetic particles. The equations for the hybrid model are based on the gyrokinetic equations. The fast particles are described with linearized gyrokinetic equations. The reduced MHD equations are derived by taking velocity moments of the gyrokinetic equations. An equation for describing the Alfvén waves is derived by combining the reduced MHD equations. The Alfvén wave equation can retain kinetic corrections. Considering the energy transfer between the particles and the waves, the stability of the waves can be calculated. Numerically, the Alfvén waves are calculated using the CKA code. The equations are solved as an eigenvalue problem to determine the frequency spectrum and the mode structure of the waves. The results of the MHD model are in good agreement with other sophisticated MHD codes. CKA results are shown for a JET and a W7-AS example. The linear version of the EUTERPE code is used to study the motion of energetic particles in the wavefield with fixed spatial structure, and harmonic oscillations in time. In EUTERPE, the gyrokinetic equations are discretized with a PIC scheme using the delta-f method, and both full orbit width and finite Larmor radius effects are included. The code is modified to be able to use the wavefield calculated externally by CKA. Different slowing-down distribution functions are also implemented. The work done by the electric field on the particles is measured to calculate the energy transfer between the particles and the wave and from that the growth rate is determined. The advantage of this approach is that the full magnetic geometry is retained without any limiting assumptions on guiding center orbits. Extensive benchmarks have been performed to test the new CKA-EUTERPE code. Three tokamak benchmarks are presented, where the stability of TAE modes are studied as a function of fast particle energy, or in one case as a function of the fast particle charge. The benchmarks show good agreement with other codes. Stellarator calculations were performed for Wendelstein 7-AS and the results demonstrate that the finite orbit width effects tend to be strongly stabilizing.