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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.
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.