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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 region surrounding the excitonic insulator phase is a three-component plasma composed of electrons, holes, and excitons. Due to the extended nature of the excitons, their presence influences the surrounding electrons and holes. We analyze this correlation. To this end, we calculate the density of bound electrons, the density of electrons in the correlated state, the momentum-resolved exciton density, and the momentum-resolved density of electron-hole pairs that are correlated but unbound. We find qualitative differences in the electron-hole correlations between the weak-coupling and the strong-coupling regime.
In the present thesis, a systematic study of beam driven Alfvén eigenmodes in high-density and low-temperature plasmas of the W7-AS stellarator is performed. The device went out of operation in 2002 and the study is based on stored experimental data. Alfvén instabilities can roughly be divided into ideal MHD Alfvén eigenmodes and those existing due to kinetic effects. The spectrum of ideal MHD Alfvén waves in toroidal fusion devices consists of a continuum of stable waves that are strongly localized. Weakly damped, discrete eigenmodes can exist in gaps of the continuous spectrum which are formed by plasma inhomogeneities and the coupling of Alfvén continua. This allows an identification of ideal MHD Alfvén eigenmodes in terms of their frequency and mode numbers. Kinetic effects can modify this spectrum and cause additional types of eigenmodes, the kinetic Alfvén eigenmodes (KAE) and energetic particle modes (EPM). The goal of this thesis is twofold: (I) identification and description of fast particle driven Alfvén instabilities in W7-AS, and (II) study of energetic particle losses induced by Alfvén instabilities. The reconstruction of the ideal MHD plasma equilibrium for each discharge with sufficient accuracy is the very foundation of all subsequent steps. This is achieved, based on measured plasma parameter profiles that are further refined by validating them to the measurements of other, independent plasma diagnostics. The applied scheme is inspired by an approach of Integrated Data Analysis (IDA) to combine different diagnostic data and provide combined uncertainties. After mode number analysis and eigenmode identification, the theoretically expected, linear growth rate of the instability is calculated where possible, and the various contributions of the fast particle drive to the instability of the mode are identified. Alfvénic activity recorded by the Mirnov diagnostic is analyzed, which consists of a set of spatially distributed coils that measure magnetic fluctuations. On W7-AS, the probes are arranged in three poloidal arrays at different toroidal positions. The spacing between the probes is non-equidistant. In addition, the signals of one probe array are digitized with a different sample rate. These characteristics prohibit the straight-forward use of standard tools available for harmonic analysis. Instead, a new tool has been developed and thoroughly tested. It is a multi-dimensional extension of the Lomb periodogram, able to provide reliable time-resolved frequency and mode number spectra in the case of uneven datapoint spacing. Numerical studies of this periodogram show a good performance with respect to mode number resolution given the low number of available probes, and robustness against perturbations of the signal. Only two of the probe arrays can be used for the analysis of eigenmodes with frequencies >70 kHz, such that for high-frequency phenomena insufficient information about the mode numbers is available. A total of 133 different Alfvén eigenmodes is studied in discharges from different experimental campaigns. A restriction to discharges from various high-beta campaigns with neutral beam heating is required to allow for a realistic reconstruction of plasma equilibrium and velocity distribution functions of energetic particles. The discharges are characterized by high density, ne = 5 x 1019 m-3 to 2.5 x 1020 m-3 at relatively low temperatures of Te = Ti = 150 ... 600 eV. Alfvén eigenmodes often appear transiently in the startup phase of these discharges, where density and heating power are being ramped up. Occasionally, Alfvén eigenmodes are seen in the stationary, high-beta phase in the presence of considerable neutral beam heating. Most of the Alfvén eigenmodes are successfully classified as ideal MHD eigenmodes. 19 global, 47 toroidicity-induced and 8 ellipticity-induced Alfvén eigenmodes (GAEs, TAEs, and EAEs, respectively) are unambiguously identified by their mode numbers and frequencies. Excellent agreement between experimentally observed mode number spectra and theoretically calculated eigenmode structure is shown for a TAE example. Additional 13 events are found to have frequencies inside the EAE gap and could possibly be EAEs. Evidence for high-frequency Alfvén eigenmodes (mirror- and helicity-induced Alfvén eigenmodes) is seen, but can not be proven rigorously due to uncertain mode numbers and the complexity of the Alfvén continuum. The remaining 41 Alfvén eigenmodes can not be classified to be one of the above cases. Reasons are either high frequencies, mode numbers obscured by far-field effects, or mode numbers that could not be related to ideal MHD Alfvén eigenmodes. A selection of these shows indications of strong non-linear wave-particle interactions and are assumed to be EPMs. Kinetic Alfvén eigenmodes are not expected to exist in the experimental conditions that were studied. The radially resolved velocity distribution function is used to describe the parameter regimes in which the modes are observed in terms of the dimensionless parameters vb/vA (beam velocity normalized to the Alfvén velocity) and ßfast/ßth, where beta is the ratio of plasma pressure to magnetic pressure. The first parameter describes through which of the possible resonance velocities particles can interact with the eigenmode. A peculiarity of the fast particle dynamics in fusion devices is that they can resonantly interact with Alfvén eigenmodes through sideband resonances even if v < vA. The second parameter describes the energy content of the destabilizing fast particle population compared to the potentially stabilizing thermal plasma component. These parameters contain relevant information about the instability of an eigenmode and such diagrams are given for all observed modes. In addition to that, the expected linear growth rate of gap modes is calculated based on a theoretical model that extends the ideal MHD by a perturbative, drift-kinetic description of the energy exchange between waves and circulating particles, neglecting the effects of trapped particles. For the discharges under consideration the thermal electron speed is comparable to vA and the electrons provide a significant Landau damping contribution. Due to strong density gradients near the plasma boundary in most of the discharges, the thermal ions can provide a small drive via the spatial inhomogeneity which does not overcome the electron damping, however. The drive by spatial inhomogeneity of thermal ions requires a certain propagation direction of the mode and is equally stabilizing for opposite mode numbers. The fast particles also contribute to the growth rate via spatial inhomogeneity, velocity gradients and velocity anisotropy terms are negligible in W7-AS. Most of the observed GAE or EAE modes have negative mode numbers, which correspond to a propagation direction for which the spatial inhomogeneity of thermal and beam ions is predicted to be stabilizing. A fast particle drive of these modes is not confirmed, whereas the TAEs are found to be strongly destabilized by neutral beam injection. The distribution of plasma parameters for discharges showing TAEs in terms of the dimensionless stability parameters suggests an instability threshold that is qualitatively confirmed by an exploration of the parameter space with the theoretical model. Wave-induced, resonant losses of energetic ions scale linearly with the wave amplitude. To identify them, correlations between ion loss probe signals and wave amplitudes are searched, where correlation times in the order of the slowing-down time of energetic particles are expected. Significant correlations can be established only exceptionally for 3 of the identified ideal MHD Alfvén eigenmodes. Those Alfvén eigenmodes, however, which are assumed to be EPMs frequently show severe losses of energetic ions that are visible in the time traces of the plasma energy as well.
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.
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.
In dieser Arbeit wurden Experimente an einem DC-Magnetron-Beschichtungsplasma zur (reaktiven) Abscheidung von Ti, TiNx und TiOx-Schichten durchgeführt. Das Ziel war es, durch Korrelation von Messungen des Ionen- und des Energieeinstroms auf das Substrat während des Beschichtungsvorgangs mit Analysen der abgeschiedenen Schichten Aussagen über die Zusammenhänge von Abscheidebedingungen und Schichteigenschaften zu treffen. Von besonderem Interesse waren hierbei die Unterschiede zwischen den beiden Betriebsmodi des eingesetzten Magnetrons (balanced mode und unbalanced mode), da sich über diesen Parameter der Ioneneinstrom auf das Substrat signifikant beeinflussen lässt, sowie der Einfluss hochenergetischer negativ geladener Ionen, die beim Einsatz von Sauerstoff im Gegensatz zu dem von Stickstoff als Reaktivgas auftreten. Die Maxima der mittels energieaufgelöster Massenspektrometrie gemessenen Energieverteilungen aller Ionenspezies liegen im unbalanced mode im Vergleich zum balanced mode bei um etwa 0,2...1 eV höherer Energie. Der im Wesentlichen von den einfach positiv geladenen Argonionen und bei hohem Reaktivgasfluss den molekularen Reaktivgasionen getragene Gesamtioneneinstrom auf das Substrat ist im unbalanced mode deutlich höher als im balanced mode. Der mit Hilfe einer Thermosonde gemessene Energieeinstrom auf das Substrat steigt linear mit der Entladungsleistung an. Im unbalanced mode ist er, bedingt durch den höheren Gesamtioneneinstrom auf das Substrat und die größere mittlere Energie aller Ionenspezies, um mehr als eine Größenordnung höher als im balanced mode. Eine Abhängigkeit des Energieeinstroms vom Reaktivgasfluss wurde nicht beobachtet. Die röntgenreflektometrisch gemessenen Beschichtungsraten steigen über der Entladungsleistung linear an und sind im unbalanced mode trotz geringerer Sputterraten am Target um ca. 10...20 % höher als im balanced mode. Die Begründung hierfür liefert der im unbalanced mode deutlich höhere Energieeinstrom auf das Substrat. Durch diesen erhöhten Energieeintrag in die aufwachsenden Schichten steht im unbalanced mode mehr Energie für Prozesse an der Oberfläche, wie die Oberflächendiffusion, zur Verfügung. Die somit verbesserte laterale Mobilität der Teilchen an der Oberfläche führt dazu, dass diese besser in die wachsende Kristallstruktur eingebaut werden können. Damit ergibt sich letztendlich im unbalanced mode trotz des geringeren Teilcheneinstroms in allen untersuchten Plasmen eine höhere Abscheiderate von Titan auf dem Substrat. Der Energieeinstrom auf das Substrat ist demnach durch seinen signifikanten Einfluss auf die laterale Mobilität der aufwachsenden Teilchen ein bestimmender Parameter für das Schichtwachstum. Die durch die Beimischung von Reaktivgas zum Plasma auftretende Targetnitrierung bzw. –oxidation verursacht ein deutliches Absinken der Sputterraten am Target und damit der Beschichtungsraten über dem Reaktivgasfluss. Messungen der chemischen Zusammensetzungen der Schichten mittels Röntgenphotoelektronenspektroskopie zeigen, dass die Menge des in die Schichten eingebauten Reaktivgases über dessen Konzentration im Beschichtungsplasma zu kontrollieren ist. Im Argon-Stickstoff-Plasma sind die Werte der aus den röntgenreflektometrisch erhaltenen Dichten bei den im unbalanced mode abgeschiedenen Schichten deutlich höher als bei den im balanced mode abgeschiedenen. Untersuchungen mittels Röntgendiffraktometrie zeigen für diese Schichten auch höhere makroskopische Spannungen. Offenbar führt der größere Energieeinstrom hier zu lokalen Temperaturunterschieden, aus denen aufgrund unterschiedlicher Ausdehnungskoeffizienten von Schicht und Substrat beim Abkühlen makroskopische Schichtspannungen resultieren. Insgesamt werden im Argon-Stickstoff-Plasma im unbalanced mode des Magnetrons kompaktere Schichten mit weniger Lücken abgeschieden als unter denselben Bedingungen im balanced mode. Im Argon-Sauerstoff-Plasma wird dieser positive Effekt des höheren Energieeintrags in die aufwachsenden Schichten durch den im unbalanced mode deutlich höheren Beschuss des Substrats mit hochenergetischen negativ geladenen Sauerstoffionen mehr als aufgehoben. Dadurch kommt es in diesem Betriebsmodus des Magnetrons zu einer erhöhten Lückenbildung in den aufwachsenden Schichten, die somit geringere makroskopische Spannungen und geringere mittlere Dichten aufweisen als die im balanced mode abgeschiedenen. Die Summe dieser Ergebnisse zeigt, dass die Eigenschaften der im hier untersuchten DC-Magnetronplasma abgeschiedenen Schichten maßgeblich von der Zusammensetzung des Beschichtungsplasmas und insbesondere von der Art und der Energie der auf das Substrat auftreffenden Ionen abhängen.
The stratospheric aerosol layer plays an important role in the radiative balance of Earth primarily through scattering of solar radiation. The magnitude of this effect depends critically on the size distribution of the aerosol. The aerosol layer is in large part fed by volcanic eruptions strong enough to inject gaseous sulfur species into the stratosphere. The evolution of the stratospheric aerosol size after volcanic eruptions is currently one of the biggest uncertainties in stratospheric aerosol science. We retrieved aerosol particle size information from satellite solar occultation measurements from the Stratospheric Aerosol and Gas Experiment III mounted on the International Space Station (SAGE III/ISS) using a robust spectral method. We show that, surprisingly, some volcanic eruptions can lead to a decrease in average aerosol size, like the 2018 Ambae and the 2021 La Soufrière eruptions. In 2019 an intriguing contrast is observed, where the Raikoke eruption (48∘ N, 153∘ E) in 2019 led to the more expected stratospheric aerosol size increase, while the Ulawun eruptions (5∘ S, 151∘ E), which followed shortly after, again resulted in a reduction in the values of the median radius and absolute distribution width in the lowermost stratosphere. In addition, the Raikoke and Ulawun eruptions were simulated with the aerosol climate model MAECHAM5-HAM. In these model runs, the evolution of the extinction coefficient as well as of the effective radius could be reproduced well for the first 3 months of volcanic activity. However, the long lifetime of the very small aerosol sizes of many months observed in the satellite retrieval data could not be reproduced.
This thesis describes the implementation and first on-line application of a multi-reflection time-of-flight (MR-ToF) mass analyzer for high-resolution mass separation at the ISOLTRAP mass spectrometer at ISOLDE/CERN. On the one hand, the major objective was to improve ISOLTRAPs mass-measurement capabilities with respect to the ratio of delivered contaminating ions to ions of interest. On the other hand, the time necessary to purify wanted from unwanted species should be reduced as much as possible to enable access to even more exotic nuclei. The device has been set up, optimized and tested at the University of Greifswald before its move to ISOLTRAP. The achieved performance comprises mass resolving powers of up to 200000 reached at observation times of 30ms and a contamination suppression of about four orders of magnitude by use of a Bradbury-Nielsen gate. With the characteristics, it outperforms clearly the so far state-of-the-art purification method of a gas-filled Penning trap. To improve the utilization of the MR-ToF mass analyzer, the in-trap lift method has been developed. It simplifies the application and optimization of the device, which is a crucial time factor in an on-line experiment. The device was the first of its kind successfully applied to radioactive ion beams for a mass analysis, for a mass separation (in combination with the Bradbury-Nielsen gate) as a preparatory step for a subsequent Penning-trap mass measurement and as a high-precision mass spectrometer of its own. The later was recently used for the first mass measurement of the neutron-rich calcium isotopes 53Ca and 54Ca. The so-far achieved mass-resolving power of 200000 belongs to the highest reported for time-of-flight mass analyzers at all. The first successful application of the MR-ToF system as the only mass separator at ISOLTRAP resulted in the mass measurement of 82Zn. The new mass value has been compared to mass extrapolations of the most recent Hartree-Fock-Bogoliubov (HFB) mass models, HFB-19 to HFB-21, of the BRUSLIB collaboration. The mass of the nuclide is of high interest for the compositions and depth profile of the outer crust of neutron stars. In the classical model of the outer crust of a cold, non-accrediting and non-rotating neutron star, the sequence of nuclides found within this parts is determined mainly by the binding energy of exotic nuclides. The crustal compositions determined with the three HFB mass models differed with respect to the appearance of a layer of 82Zn, originating from different mass extrapolations of this mass. With the new experimental data, the extrapolations could be evaluated. It was found that the HFB-21 mass value differs less from the experimental data than the ones from HFB-19 and 20. Therefore, in the classical model, 82Zn does not appear anymore in the outer crust. Due to its high resolution and very fast measurement time, the MR-ToF mass analyzer will be an important instruments for future activities at ISOLTRAP, at the ISOLDE facility in general, and at other radioactive ion-beam facilities.
In der vorliegenden Arbeit wurde die Katodenregion einer quecksilberfreien Helium-Xenon Niederdruckentladung im Brennfleckbetrieb experimentell untersucht. Diese Region ist von besonderem Interesse, da sich hier die Elektronenemission, die Erzeugung von Ionen und metastabilen Atomen sowie lebensdauerbegrenzende Prozesse abspielen. Um die Entladung im Brennfleckbetrieb zu realisieren, kam als Katode eine im Rahmen dieser Arbeit entwickelte neuartige planare Geflechtelektrode zum Einsatz. Mit der Methode der ortsaufgelösten Laser-Atom-Absorptionsspektroskopie (LAAS) wurden die absoluten Teilchendichten der zwei untersten angeregten Xe-Atome und die Gastemperatur in der Katodenregion bestimmt. Die Inhomogenität des Spot-Plasmas fand dabei besondere Berücksichtigung. Sowohl die Teilchendichten der zwei untersten angeregten Xe-Atome als auch die Gastemperatur sind unmittelbar vor dem Brennfleck maximal und fallen in axiale und radiale Richtung stark ab. Insbesondere die Gastemperatur beträgt in einem Abstand von 1 mm vor dem Brennfleck circa 650 K und liegt damit deutlich über Raumtemperatur. Des Weiteren ließ sich die Temperatur im Brennfleck auf der Katodenoberfläche mittels optischer Emissionsspektroskopie ermitteln. Dies geschah durch Anpassung des aufgenommenen Spektrums an die Plancksche Strahlungsgleichung. Die Brennflecktemperaturverteilung weißt ein ausgeprägtes Maximum auf, das je nach Entladungsstromstärke maximale Werte zwischen 1414 K bei 40 mA und 1524 K bei 80 mA annimmt. Von diesem Maximum aus wurde ein starker in alle Richtungen nahezu symmetrischer Temperaturabfall festgestellt. Ein technologisch wichtiger Aspekt hinsichtlich der Lebensdauer einer auf Xenon basierenden quecksilberfreien Lampe ist der negative Effekt der Xe-Gasaufzehrung. In dieser Arbeit wird gezeigt, dass die Gasaufzehrung unter Verwendung der planaren Geflechtelektrode im deutlichen Gegensatz zur industriell gefertigten Becherelektrode, wie sie vielfach in Lampen für Lichtwerbung vorkommt, vernachlässigbar klein ist. Dies wird auf die Ausbildung eines heißen Brennflecks und die damit verbundene hohe Katodentemperatur und niedrige Katodenfallspannung zurückgeführt.
Turbulenz ist allgegenwärtig in der Natur. Ein wichtiges Charakteristikum sind Fluktuationen auf einer Vielzahl von räumlichen und zeitlichen Skalen, die sowohl in neutralen Fluiden und gasförmigen Systemen, als auch in Plasmen beobachtet werden. Obwohl der elektromagnetische Charakter von Plasmen eine erhöhte Komplexität von Plasmaturbulenz bedingt, sind die grundlegenden Eigenschaften universell. In magnetisch eingeschlossenen Plasmen führen fluktuierende Plasmaparameter zu turbulentem Transport von Plasmateilchen und Energie, der die Einschlusszeit verringert und wichtige Aspekte zukünftiger Fusionskraftwerke beeinflusst. Der intermittente Charakter dieses konvektiven Teilchenflusses ist verbunden mit turbulenten Strukturen mit großen Amplituden, auch "blobs" genannt, die radial durch das Magnetfeld propagieren. Intermittente Fluktuationen im Randplasma von Experimenten mit linearer Magnetfeldgeometrie werden ebenfalls propagierenden turbulenten Strukturen zugeschrieben. Dabei ist der Mechanismus der radialen Propagation kaum verstanden. In dieser Arbeit wird die Bildung und Propagation von turbulenten Strukturen im linear magnetisierten Helikonexperiment Vineta untersucht. Durch Messungen der Fluktuationen in der azimuthalen Ebene mit multi-dimensionalen Sonden wird gezeigt, dass turbulente Strukturen in Driftwellenturbulenz im Gebiet des maximalen Dichtegradienten entstehen. Die turbulenten Strukturen propagieren hauptsächlich azimuthal in Richtung der Hintergrund ExB-Drift, aber sie besitzen auch eine starke radiale Geschwindigkeitskomponente. Die radiale Propagation wird durch das selbstkonsistente Potential der turbulenten Struktur verursacht, dass zu einem fluktuations-induzierten radialen Transport führt. Im Plasmarand werden die turbulenten Strukturen als intermittente Dichteeruptionen mit großen Amplituden beobachtet. Ein Vergleich der experimentellen Ergebnisse mit numerischen dreidimensionalen Fluid-Simulationen mit abgestimmten Geometrie- und Randbedingungen zeigt Übereinstimmung. Die Bildung der turbulenten Strukturen ist kausal mit einer quasi-kohärenten Driftmode verbunden und ihre radiale Propagation wird durch das selbstkonsistente elektrische Feld verursacht, dass aus der dreidimensionalen Dynamik resultiert. Zum Vergleich wird die Propagation von turbulenten Strukturen im Randplasma vom National Spherical Torus Experiment (NSTX) untersucht und mit theoretischen Propagationsmodellen verglichen.
Abstract
The presented experimental system is a barrier discharge system with plane parallel electrodes. The lateral surface charge distribution being deposited on the dielectric layer during each breakdown is observed optically using the well known electro-optic effect (Pockels effect). The temporal resolution of the surface charge measurement has been increased to 200 ns, and so for the first time it is possible to resolve the charge transfer to the dielectric surface in a single breakdown. In the present measurements, a patterned glow-like barrier discharge is investigated. It is found that the charge reversal in a single discharge spot (microdischarge) starts in the centre and then grows outwards. These experimental findings verify previously unconfirmed predictions from earlier numerical calculations and thereby contribute to a better understanding of the interaction between the plasma and the electrical charge on the electrodes.
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.
Abstract
The surface charge distribution deposited by the effluent of a dielectric barrier discharge driven atmospheric pressure plasma jet on a dielectric surface has been studied. For the first time, the deposition of charge was observed phase resolved. It takes place in either one or two events in each half cycle of the driving voltage. The charge transfer could also be detected in the electrode current of the jet. The periodic change of surface charge polarity has been found to correspond well with the appearance of ionized channels left behind by guided streamers (bullets) that have been identified in similar experimental situations. The distribution of negative surface charge turned out to be significantly broader than for positive charge. With increasing distance of the jet nozzle from the target surface, the charge transfer decreases until finally the effluent loses contact and the charge transfer stops.
Behavior of a porous particle in a radiofrequency plasma under pulsed argon ion beam bombardment
(2010)
The behavior of a single porous particle with a diameter of 250 μm levitating in a radiofrequency (RF) plasma under pulsed argon ion beam bombardment was investigated. The motion of the particle under the action of the ion beam was observed to be an oscillatory motion. The Fourier-analyzed motion is dominated by the excitation frequency of the pulsed ion beam and odd higher harmonics, which peak near the resonance frequency. The appearance of even harmonics is explained by a variation of the particles's charge depending on its position in the plasma sheath. The Fourier analysis also allows a discussion of neutral and ion forces. The particle's charge was derived and compared with theoretical estimates based on the orbital motion-limited (OML) model using also a numerical simulation of the RF discharge. The derived particle's charge is about 7–15 times larger than predicted by the theoretical models. This difference is attributed to the porous structure of the particle.
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.
Infrared laser absorption spectroscopy (IRLAS) employing both tuneable diode and quantum cascade lasers (TDLs, QCLs) has been applied with both high sensitivity and high time resolution to plasma diagnostics and trace gas measurements.
TDLAS combined with a conventional White type multiple pass cell was used to detect up to 13 constituent molecular species in low pressure Ar/H2/N2/O2 and Ar/CH4/N2/O2 microwave discharges, among them the main products such as H2O, NH3, NO and CO, HCN respectively. The hydroxyl radical has been measured in the mid infrared (MIR) spectral range in-situ in both plasmas yielding number densities of between 1011 ... 1012 cm-3. Strong indications of surface dominated formation of either NH3 or N2O and NO were found in the H2-N2-O2 system. In methane containing plasmas a transition between deposition and etching conditions and generally an incomplete oxidation of the precursor were observed.
The application of QCLs for IRLAS under low pressure conditions employing the most common tuning approaches has been investigated in detail. A new method of analysing absorption features quantitatively when the rapid passage effect is present is proposed. If power saturation is negligible, integrating the undisturbed half of the line profile yields accurate number densities without calibrating the system. By means of a time resolved analysis of individual chirped QCL pulses the main reasons for increased effective laser line widths could be identified. Apart from the well-known frequency down chirp non-linear absorption phenomena and bandwidth limitations of the detection system may significantly degrade the performance and accuracy of inter pulse spectrometers. The minimum analogue bandwidth of the entire system should normally not fall below 250 MHz.
QCLAS using pulsed lasers has been used for highly time resolved measurements in reactive plasmas for the first time enabling a time resolution down to about 100 ns to be achieved. A temperature increase of typically less than 50 K has been established for pulsed DC discharges containing Ar/N2 and traces of NO. The main NO production and depletion reactions have been identified from a comparison of model calculations and time resolved measurements in plasma pulses of up to 100 ms. Considerable NO struction is observed after 5 ... 10 ms due to the impact of N atoms.
Finally, thermoelectrically cooled pulsed and continuous wave (cw) QCLs have been employed for high finesse cavity absorption spectroscopy in the MIR. Cavity ring down spectroscopy (CRDS) has been performed with pulsed QCLs and was found to be limited by the intrinsic frequency chirp of the laser suppressing an efficient intensity build-up inside the cavity. Consequently the accuracy and advantage of an absolute internal absorption calibration is not achievable. A room temperature cw QCL was used in a complementary cavity enhanced absorption spectroscopy (CEAS) configuration which was equipped with different cavities of up to ~ 1.3 m length. This spectrometer yielded path lengths of up to 4 km and a noise equivalent absorption down to 4 x 10-8 cm-1Hz-1/2. The corresponding molecular concentration detection limit (e.g. for CH4, N2O and C2H2 at 1303 cm-1/7.66 μm) was generally below 1 x 1010 cm-3 for 1 s integration times and one order of magnitude less for 30 s integration times. The main limiting factor for achieving even higher sensitivity is the residual mode noise of the cavity. Employing a ~ 0.5 m long cavity the achieved sensitivity was good enough for the selective measurement of trace atmospheric constituents at 2.2 mbar.
(A paperback version is published by Logos under ISBN 978-3-8325-2345-9.)
Colossal magneto-resistance manganites are characterized by a complex interplay of charge, spin, orbital and lattice degrees of freedom. Formulating microscopic models for these compounds aims at meeting two conflicting objectives: sufficient simplification without excessive restrictions on the phase space. We give a detailed introduction to the electronic structure of manganites and derive a microscopic model for their low-energy physics. Focusing on short-range electron–lattice and spin–orbital correlations we supplement the modelling with numerical simulations.
Abstract
In this series of two papers we present results about the E-H transition of an inductively coupled oxygen discharge driven at radio frequency (13.56 MHz) for different total gas pressures. The mode transition from the low density E-mode to the high density H-mode is studied using comprehensive plasma diagnostics. The measured electron density can be used to distinguish between the different operation modes. This paper focuses on the determination of the negative atomic ion density and the electronegativity by two experimental methods and global rate equation calculation. As a result, the electronegativity significantly decreases over two orders of magnitude from about 25 in the E-mode to about 0.1 in the H-mode. The temporal behavior of the electronegativity in pulsed ICP shows that the negative atomic ion density reaches a steady state after 10 ms. Negative atomic ions are mainly produced by the dissociative attachment with the molecular ground state. The ion–ion recombination with the positive molecular ions and the collisional detachment with the singlet molecular metastables contribute significantly to the loss of the negative atomic ions.
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.
Abstract
In this series of two papers, the E-H transition in a planar inductively coupled radio frequency discharge (13.56 MHz) in pure oxygen is studied using comprehensive plasma diagnostic methods. The electron density serves as the main plasma parameter to distinguish between the operation modes. The (effective) electron temperature, which is calculated from the electron energy distribution function and the difference between the floating and plasma potential, halves during the E-H transition. Furthermore, the pressure dependency of the RF sheath extension in the E-mode implies a collisional RF sheath for the considered total gas pressures. The gas temperature increases with the electron density during the E-H transition and doubles in the H-mode compared to the E-mode, whereas the molecular ground state density halves at the given total gas pressure. Moreover, the singlet molecular metastable density reaches 2% in the E-mode and 4% in the H-mode of the molecular ground state density. These measured plasma parameters can be used as input parameters for global rate equation calculations to analyze several elementary processes. Here, the ionization rate for the molecular oxygen ions is exemplarily determined and reveals, together with the optical excitation rate patterns, a change in electronegativity during the mode transition.
Computational chemical physics can give important input to astrophysical modelling and other fields of physics, where molecular properties are of importance. Understanding of spectroscopic and reactive behaviour is crucial for many systems of astrophysical interests like stars, interstellar medium and comets. Especially stellar atmospheres are of interest, because the complex physics of stars are not yet completely understood. Stars are in an unstable balance of gravitation and radiation pressure and the atmospheric dynamics have been subject of extensive modelling. Complete and accurate spectroscopic information of the atoms and molecules in these atmospheres is necessary for this attempt. In addition, the only information we have about astrophysical systems is light which is emitted or absorbed by particles in these media. This is not only true for astrophysics. In plasma physics sometimes the usage of invasive diagnostics, like Langmuir probes, is not wanted because they disturb the system. In these cases some information of the system can be regained by passively measuring infrared spectra of the plasma or by active induction of electronic transition like the laser-induced fluorescence method. Another remote sensing application is the measurement of the atmospheric composition on earth. Here, larger particles in the atmosphere as well as greenhouse gases are of current interest. Unfortunately, the experimental spectroscopic data, which is needed for the understanding and interpretation of the measured spectra, is often incomplete. This gap can be, to some extend, filled by computational chemical physics. The aim of this work was to investigate the capabilities and limitations of ab initio based potential energy surfaces for spectroscopic and reactive studies and to apply these methods to problems of rovibrational and rovibronic spectroscopy and reaction dynamics. The choice of ab initio methods and the potential fitting methods is critical for the computational chemical physics, as all further quantities directly depend on their quality. In this work modified versions of the Braams polynomial potential energy surface were used. A high level coupled cluster ab initio method was used to build potentials for a series of small hydrocarbons. Hydrocarbons can be found almost everywhere on earth and in the universe. They exist in laboratory plasmas, stellar and planetary atmospheres and interstellar gases. In all these cases, light emitted or absorbed by the molecules is an important diagnostics of the system. The potential constructed in this work partly included a cluster expansion, which adds reactant configuration spaces to the fits. This could not be done for CH_3 and higher hydrocarbons, because of the limitations of the Coupled Cluster ab initio method, which is well suited for the potential wells, but not for the dissociation regions. The examples of methyl and methane show how the potentials can be used for rovibrational spectroscopy. Results of radiation transport simulations illustrate the importance of as complete-as-possible line lists for radiation transport calculations.\\ The rovibronic spectroscopy of diatomic molecules is another important aspect for the stellar atmospheric modelling. Metal hydrides and oxides add opacity to the atmosphere in the visible light and ultraviolet frequency regions, as well as do the hydrocarbons in the infrared one. In addition the spectra of metal hydrides/oxides can be used to gather information about metal and their isotope abundances. They are used as markers for the conditions in the atmospheres of stars. In this work a new code was developed, that efficiently calculates bound-bound transitions between electronic states and bound-continuum cross sections for diatomic molecules. It also offers an adequate treatment of quasi-bound rovibrational states. One important representative of the diatoms is magnesium hydride, MgH. Before this work, line lists and photodissociation cross section were available involving the three lowest doublet states of MgH. In this work new potential energy curves were calculated and adapted to updated experimental data. This causes changes in the relative energies between the electronic states and therefore shifts in the line lists. These are important, because accurate line positions are needed for the identification of spectral lines. In addition two further electronic states were included in the calculations. This expands the spectral range of MgH into the near ultraviolet region. Radiation transport models showed significant absorption by MgH from the newly added electronic states. A second usage of the diatomic potential energy curves are photodissociation cross sections. As interstellar environments are chemically active, such data is necessary for a complete picture of the ongoing processes. The photodissociation cross sections of MgH reveal a stronger dependence of the underlying potential than the bound-bound lines. In the case of MgH the cross sections are rather weak, besides occasional resonance lines which can be several orders of magnitude stronger. As mentioned, not only spectroscopic, but also reactive behaviour of molecules is important in astrophysics. A current problem connected with this is the abundance of CH^+ in interstellar clouds. Its measured abundances do not fit the predictions from theoretical models. In addition Gerlich and co-workers recently measured low temperature H + CH^+ -> C^+ + H_2 reaction rates, which diverge from the theoretical picture and which could not be explained. In this work a reactive potential energy surface was built for the CH_2^+ system, which was then used to perform extensive calculations with quasi-classical trajectory and quantum scattering methods. It was found out, that the potentials used in previous works are not accurate enough to allow low temperature calculations. Results from these potentials must be taken with care. Furthermore, the results from the new potential energy surface indicate significantly reduced reaction rates compared to previous numerical studies. This is in agreement with the new results of Gerlich and co-workers. Nevertheless, the large error bars in the low temperature range for experimental as well as numerical results strongly suggest refined methods to be developed for both, before a final conclusion can be made. This work demonstrated the possibility of modern computational chemical physics to supply consistent data for spectroscopy and reaction dynamics. These are necessary and important inputs for fields like astrophysics, plasma physics and chemistry.
Advancing Radiation-Detected Resonance Ionization towards Heavier Elements and More Exotic Nuclides
(2022)
RAdiation-Detected Resonance Ionization Spectroscopy (RADRIS) is a versatile method for highly sensitive laser spectroscopy studies of the heaviest actinides. Most of these nuclides need to be produced at accelerator facilities in fusion-evaporation reactions and are studied immediately after their production and separation from the primary beam due to their short half-lives and low production rates of only a few atoms per second or less. Only recently, the first laser spectroscopic investigation of nobelium (Z=102) was performed by applying the RADRIS technique in a buffer-gas-filled stopping cell at the GSI in Darmstadt, Germany. To expand this technique to other nobelium isotopes and for the search for atomic levels in the heaviest actinide element, lawrencium (Z=103), the sensitivity of the RADRIS setup needed to be further improved. Therefore, a new movable double-detector setup was developed, which enhances the overall efficiency by approximately 65% compared to the previously used single-detector setup. Further development work was performed to enable the study of longer-lived (t1/2>1 h) and shorter-lived nuclides (t1/2<1 s) with the RADRIS method. With a new rotatable multi-detector design, the long-lived isotope 254Fm (t1/2=3.2 h) becomes within reach for laser spectroscopy. Upcoming experiments will also tackle the short-lived isotope 251No (t1/2=0.8 s) by applying a newly implemented short RADRIS measurement cycle.
Diese Dissertation beschäftigt sich mit der Erzeugung von edelmetallfreien Katalysatoren für die Sauerstoffreduktion in Brennstoffzellen. Dabei wird ein neuartiger, dualer Plasmaprozess entwickelt, aufgebaut und die so-erzeugten Schichten mit verschiedenen elektrochemischen (CV, RDE und RRDE) und strukturanalytischen Methoden (SEM, EDX, IR, XPS, Leitfähigkeit, XRD, NEXAFS, EXAFS und TEM) untersucht. Auf diese Weise ist es erstmalig gelungen edelmetallfreie Katalysatoren mit einem Plasmaprozess herzustellen, ohne dass eine zusätzliche Pyrolyse benötigt wird. Die katalytische Aktivität der Schichten ist außerdem deutlich höher als die von rein chemisch hergestellten Metall–Polypyrrol-Schichten.
Multiply negatively charged aluminium clusters and fullerenes were generated in a Penning trap using the "electron-bath" technique. Aluminium monoanions were generated using a laser vaporisation source. After this, two-, three- and four-times negatively charged aluminium clusters were generated for the first time. This research marks the first observation of tetra-anionic metal clusters in the gas phase. Additionally, doubly-negatively charged fullerenes were generated. The smallest fullerene dianion observed contained 70 atoms.
Explosive volcanic eruptions emitting large amounts of sulfur can alter the temperature of the lower stratosphere and change the circulation of the middle atmosphere. The dynamical response of the stratosphere to strong volcanic eruptions has been the subject of numerous studies. The impact of volcanic eruptions on the mesosphere is less well understood because of a lack of large eruptions in the satellite era and only sparse observations before that period. Nevertheless, some measurements indicated an increase in mesospheric mid-latitude temperatures after the 1991 Pinatubo eruption. The aim of this study is to uncover potential dynamical mechanisms that may lead to such a mesospheric temperature response. We use the Upper-Atmospheric ICOsahedral Non-hydrostatic (UA-ICON) model to simulate the atmospheric response to an idealized strong volcanic injection of 20 Tg S into the stratosphere (about twice as much as the eminent 1991 Pinatubo eruption). Two experiments with differently parameterized effects of sub-grid-scale orography are compared to test the impact of different atmospheric background states. The simulations show a significant warming of the polar summer mesopause of up to 15–21 K in the first November after the eruption. We argue that this is mainly due to intrahemispheric dynamical coupling in the summer hemisphere and is potentially enhanced by interhemispheric coupling with the winter stratosphere. This study focuses on the first austral summer after the eruption because mesospheric temperature anomalies are especially relevant for the properties of noctilucent clouds, whose season peaks around January in the Southern Hemisphere.
The Mt. Pinatubo eruption in 1991 had a severe impact on the Earth system, with a well-documented warming of the tropical lower stratosphere and a general cooling of the surface. This study focuses on the impact of this event on the mesosphere by analyzing solar occultation temperature data from the Halogen Occultation Experiment (HALOE) instrument on the Upper Atmosphere Research Satellite (UARS). Previous analyses of lidar temperature data found positive temperature anomalies of up to 12.9 K in the upper mesosphere that peaked in 1993 and were attributed to the Pinatubo eruption. Fitting the HALOE data according to a previously published method indicates a maximum warming of the mesosphere region of 4.1 ± 1.4 K and does not confirm significantly higher values reported for that lidar time series. An alternative fit is proposed that assumes a more rapid response of the mesosphere to the volcanic event and approximates the signature of the Pinatubo with an exponential decay function having an e-folding time of 6 months. It suggests a maximum warming of 5.4 ± 3.0 K, if the mesospheric perturbation is assumed to reach its peak 4 months after the eruption. We conclude that the HALOE time series probably captures the decay of a Pinatubo-induced mesospheric warming at the beginning of its measurement period.
The idea of estimating stratospheric aerosol optical thickness from the twilight colours in historic paintings – particularly under conditions of volcanically enhanced stratospheric aerosol loading – is very tantalizing because it would provide information on the stratospheric aerosol loading over a period of several centuries. This idea has in fact been applied in a few studies in order to provide quantitative estimates of the aerosol optical depth after some of the major volcanic eruptions that occurred during the past 500 years. In this study we critically review this approach and come to the conclusion that the uncertainties in the estimated aerosol optical depths are so large that the values have to be considered questionable. We show that several auxiliary parameters – which are typically poorly known for historic eruptions – can have a similar effect on the red–green colour ratio as a change in optical depth typically associated with eruptions such as, for example, Tambora in 1815 or Krakatoa in 1883. Among the effects considered here, uncertainties in the aerosol particle size distribution have the largest impact on the colour ratios and hence the aerosol optical depth estimate. For solar zenith angles exceeding 80∘, uncertainties in the stratospheric ozone amount can also have a significant impact on the colour ratios. In addition, for solar zenith angles exceeding 90∘ the colour ratios exhibit a dramatic dependence on solar zenith angle, rendering the estimation of aerosol optical depth highly challenging. A quantitative determination of the aerosol optical depth may be possible for individual paintings for which all relevant parameters are sufficiently well constrained in order to reduce the related errors.
Plasmapolymerisation mit einem Atmosphärendruck-Mikroplasma-Jet zur Bildung funktioneller Schichten
(2012)
In Rahmen dieser Arbeit wurde die Plasmapolymerisation von aminogruppenhaltigen und perfluorierten Kohlenwasserstoffen mit einem Atmosphärendruck Mikroplasma Jet untersucht, mit dem Ziel einer erstmaligen erfolgreichen Abscheidung von Teflon-artigen und aminogruppenhaltigen Schichten. Hierzu wurde ein Versuchsaufbau zur Schichtabscheidung mit einem Mikroplasma-Jet bei Atmosphärendruckbedingungen konzipiert und aufgebaut. Dieser besteht im Wesentlichen aus dem Plasma-Jet und der ihn umgebenden Glaskuppel, welche die Erzeugung definierter Umgebungsatmosphären bei Normaldruck gestattet sowie vor eventuell entstehenden toxischen Reaktionsprodukten schützt. Als erste Aufgabe wurde die Deposition mit den aminogruppenhaltigen Präkursoren Cyclopropylamin (CPA) und Ethylendiamin (EDA) bearbeitet. Es zeigte sich, dass die Abscheidung im selbstorganisierten Jet-Modus möglich war. Die abgeschiedenen Schichten besitzen trotz eines kuppelförmigen Abscheidungsprofils eine homogene chemische Struktur mit einem Stickstoffgehalt von bis zu 20%, wie durch Profilometrie beziehungsweise XPS ermittelt wurde. Es wurden Werte von [NH2]/[C] zwischen 5,5 % und 3 % (EDA) sowie 4 % und 1 % (CPA) erreicht, abhängig von der Behandlungszeit der Substrate und der verwendeten Umgebungsatmosphäre. Die Schutzgasatmosphäre, bestehend aus einem Gemisch aus Stickstoff und Wasserstoff, welche dazu gedacht war die Bildung primärer Aminogruppen zu unterstützen, hatte einen negativen Effekt auf die Abscheidung. Im Vergleich zu einem Prozess an Luft wurde die Depositionsrate halbiert. Weiterhin konnte ein positiver Effekt auf den Anteil der Aminogruppen nur bei CPA festgestellt werden. Bezüglich der chemischen Zusammensetzung der Schichten wird ein erstes Modell der Plasmapolymerisationsreaktionen vorgestellt, welches auf dem wiederholten Vorgang der Abspaltung einer Aminogruppe und der nachfolgenden Reaktion der so entstandenen Radikale basiert. Bei der Bearbeitung der zweiten Aufgabe, der Deposition von fluorierten Plasmapolymer-Schichten, wurde ein spezielles Entladungsregime des Jets entdeckt. Die hierbei identifizierten Konditionen ermöglichten erstmalig die Abscheidung von C:F-Schichten mit einem Atmosphärendruck Jet. Hierbei wurden mit Octafluorcyclobutan (c-C4F8) als Präkursor, mit hohen Wachstumsraten (bis zu 43 nm/s mit N2-Atmosphäre) Schichten erzeugt. In diesen wurde mitttels XPS eine homogene chemische Struktur mit einem [F]/[C]-Verhältnis von 1,4 und einem sehr geringen Gehalt an Stickstoff und Sauerstoff nachgewiesen. Fits des hoch aufgelöst gemessenen C 1s Peaks zeigen einen Vernetzungsgrad von 44 % und ein [CF2]/[CF3]-Verhältnis von rund 1,8. Der statische Wasserkontaktwinkel bei diesen Schichten lag im Bereich von 100° – 135°. Die geforderte Hydrophobie der Schichten wurde damit erreicht. Luft als Umgebungsatmosphäre während des Beschichtungsprozesses führt nicht zu einem überwiegend ätzenden Plasmaprozess, reduziert jedoch die Depositionsrate um Faktor vier. Änderungen der chemischen Zusammensetzung der Schicht im Vergleich zur Schutzgasatmosphäre wurden nicht festgestellt. Die Verwendung von Octafluorpropan (C3F8) als Präkursor ergab nur ein minimales Schichtwachstum unter Schutzgas- und kein Wachstum unter Luft-Atmosphäre. Basierend auf den Beobachtungen anderer Autoren, wurde dies durch für die Plasmapolymerisation ungünstigere Fragmentierung des Präkursors erklärt. Das spezielle Entladungsregime, die eingeschnürte und lokalisierte bogenähnliche Entladung, wird als die Ausprägung einer --Modus Atmosphärendruck Entladung erklärt, bei der das Substrat als zweite geerdete Elektrode fungiert. Hierzu ist eine ausreichende Leitfähigkeit des Substrats notwendig. Anhand eines vereinfachten Ersatzschaltbildes werden die beobachteten Abhängigkeiten von Substratmaterial und Entladungsregime modelliert
Response of Osteoblasts to Electric Field Line Patterns Emerging from Molecule Stripe Landscapes
(2022)
Molecular surface gradients can constitute electric field landscapes and serve to control local cell adhesion and migration. Cellular responses to electric field landscapes may allow the discovery of routes to improve osseointegration of implants. Flat molecule aggregate landscapes of amine- or carboxyl-teminated dendrimers, amine-containing protein and polyelectrolytes were prepared on glass to provide lateral electric field gradients through their differing zeta potentials compared to the glass substrate. The local as well as the mesoscopic morphological responses of adhered osteoblasts (MG-63) with respect to the stripes were studied by means of Scanning Ion Conductance Microscopy (SICM) and Fluorescence Microscopy, in situ. A distinct spindle shape oriented parallel to the surface pattern as well as a preferential adhesion of the cells on the glass site have been observed at a stripe and spacing width of 20 μm. Excessive ruffling is observed at the spindle poles, where the cells extend. To explain this effect of material preference and electro-deformation, we put forward a retraction mechanism, a localized form of double-sided cathodic taxis.
Electromagnetic Drift Waves
(2010)
In the rf-plasma of the linear magnetized VINETA experiment, different types of low-frequency waves are observed. The emphasis in this work is on the interaction mechanism between drift waves on the one and kinetic Alfven waves on the other hand. In the peaked density profile of the plasma column drift waves occur as modulation of the plasma density. As gradient driven instability, they draw their energy from the radial density gradients. Alfven waves as magnetic field fluctuations are stable in the present configuration. They are launched by a magnetic excitation antenna. Parallel conduction currents in the plasma are common to both wave phenoma. A B-dot probe as standard diagnostic tool is used to detect the fluctuating magnetic fields of both wave types. The challenge are the small induced voltages due to the low wave frequency. The probe design with an integrated amplifier close to the probe head takes this into acount. The developed B-dot probe is mounted to different positioning systems to characterize both wave phenomena. For Alfven waves, the dispersion relation is recorded experimentally. It is found to be in good agreement with the prediction of the Hall-MHD theory with included resistive term, accounting for the cold collisional plasma. The fluctuating magnetic field pattern is recorded with azimuthal scans. The current density is obained by Amperes law. It is concentrated in helically twisted current filaments. For the unstable drift waves, similar investigations are done with simultaneously recorded density fluctuations. In the azimuthal plane, the locations of the parallel current filaments and the fluctuating density are found to be in phase, supporting the predicted drive of parallel currents by pressure gradients. A mutual influence of the two wave types is observed in an interaction experiment. Assuming parallel currents as coupling quantity, an interpretation of the experimental findings is given based on the linear theory of drift waves.
Abstract
The presented work highlights the role of residual weakly-bound surface electrons acting as an effective seed electron reservoir that favors the pre-ionization of diffuse barrier discharges (BDs). A glow-like BD was operated in helium at a pressure of 500 mbar in between two plane electrodes each covered with float glass at a distance of
3 mm.The change in discharge development due to laser photodesorption of surface electrons was studied by electrical measurements and optical emission spectroscopy. Moreover, a 1D numerical fluid model of the diffuse discharge allowed the simulation of the laser photodesorption experiment, the estimation of the released surface electrons, and the understanding of their impact on the reaction kinetics in the volume. The breakdown voltage is clearly reduced when the laser beam at photon energy of 2.33 eV hits the cathodic dielectric that is charged with residual electrons during the discharge pre-phase. According to the adapted simulation, the laser releases only a small amount of surface electrons in the order of
10 pC. Nevertheless, this significantly supports the pre-ionization. Using a lower photon energy of 1.17 eV, the transition from the glow mode to the Townsend mode is induced due to a much higher electron yield up to 1 nC. In this case, both experiment and simulation indicate a retarded stepwise release of surface electrons initiated by the low laser photon energy.
Abstract
Single self-stabilized discharge filaments were investigated in the plane-parallel electrode configuration. The barrier discharge was operated inside a gap of 3 mm shielded by glass plates to both electrodes, using helium-nitrogen mixtures and a square-wave feeding voltage at a frequency of 2 kHz. The combined application of electrical measurements, ICCD camera imaging, optical emission spectroscopy and surface charge diagnostics via the electro-optic Pockels effect allowed the correlation of the discharge development in the volume and on the dielectric surfaces. The formation criteria and existence regimes were found by systematic variation of the nitrogen admixture to helium, the total pressure and the feeding voltage amplitude. Single self-stabilized discharge filaments can be operated over a wide parameter range, foremost, by significant reduction of the voltage amplitude after the operation in the microdischarge regime. Here, the outstanding importance of the surface charge memory effect on the long-term stability was pointed out by the recalculated spatio-temporally resolved gap voltage. The optical emission revealed discharge characteristics that are partially reminiscent of both the glow-like barrier discharge and the microdischarge regime, such as a Townsend pre-phase, a fast cathode-directed ionization front during the breakdown and radially propagating surface discharges during the afterglow.
Surface charge measurements on different dielectrics in diffuse and filamentary barrier discharges
(2017)
Abstract
Previously, we reported on the measurement of surface charges during the operation of barrier discharges (BDs) using the electro-optic Pockels effect of a bismuth silicon oxide (BSO) crystal. With the present work, the next milestone is achieved by making this powerful method accessible to various dielectrics which are typically used in BD configurations. The dynamics and spatial distribution of positive and negative surface charges were determined on optically transparent borosilicate glass, mono-crystalline alumina and magnesia, respectively, covering the BSO crystal. By variation of the nitrogen admixture to helium and the pressure between 500 mbar and 1 bar, both the diffuse glow-like BD and the self-stabilized discharge filaments were operated inside of a gas gap of 3 mm. The characteristics of the discharge and, especially, the influence of the different dielectrics on its development were studied by surface charge diagnostics, electrical measurements and ICCD camera imaging. Regarding the glow-like BD, the breakdown voltage changes significantly by variation of the cathodic dielectric, due to the different effective secondary electron emission (SEE) coefficients. These material-specific SEE yields were estimated using Townsend’s criterion in combination with analytical calculations of the effective ionization coefficient in helium with air impurities. Moreover, the importance of the surface charge memory effect for the self-stabilization of discharge filaments was quantified by the recalculated spatio-temporal behavior of the gap voltage.
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 present experimental work investigates plasma turbulence in the edge region of magnetized high-temperature plasmas. A main topic is the turbulent dynamics parallel to the magnetic field, where hitherto only a small data basis existed, especially for very long scale lengths in the order of ten of meters. A second point of special interest is the coupling of the dynamics parallel and perpendicular to the magnetic field. This anisotropic turbulent dynamics is investigated by two different approaches. Firstly, spatially and temporally high-resolution measurements of fluctuating plasma parameters are investigated by means of two-point correlation analysis. Secondly, the propagation of signals externally imposed into the turbulent plasma background is studied. For both approaches, Langmuir probe arrays were utilized for diagnostic purposes. The main findings can be summarized as follows: Greatly elongated fluctuation structures exist in plasma edge turbulence. The structures are aligned along the confining magnetic field (k|| = 0). The correlation degree of fluctuations for a short connection length of 0.75m is greater than 80%. For much longer connection lengths of 23m and 66m, the correlation degree is reduced to approximately 40%. A conceptual interpretation of these observations is the coexistence of two different fluctuation components. One component has a correlation length parallel to the magnetic field below 20m and the other component a correlation length greater than 70m. Sine signals in the frequency range 1-100 kHz were injected into the turbulent plasma background. The propagation parallel and perpendicular to the magnetic field of the signals was studied. In poloidal direction, an asymmetry is observed, that can be explained by a copropagation of the signal with the background E × B-rotation of the plasma. The signal propagation parallel to the magnetic field shows no such asymmetry. As an advanced approach, spatio-temporal wave patters were injected into the edge plasma. The waves launched that way can be seen as test waves' in a turbulent background. The coupling strength of the imposed wave patterns to the background turbulence relies on the match of the imposed waves to the dynamics of turbulent structures. If the propagation direction of the imposed waves is parallel to the propagation direction of the background plasma, improved coupling is observed. This finding underlines the importance of the background plasma rotation for future attempts of controlling the plasma edge turbulence. Further optimization of frequency and wave vector of the imposed waves is probably a promising approach for achieving a significant and systematic influence of turbulence. Taking into account the present experimental state-of-the-art, for a deeper insight into the mechanism of the plasma edge turbulence of magnetized high-temperature plasmas a joint effort of numerical modeling and experimental results is a valuable approach. Such a cooperation should cover the explanation of the correlation observations as well as the experiments on signal injection into background turbulence. A quantitative comparison between the results presented in this work and a dedicated numerical drift wave simulation would be a significant step forward to a better understanding of plasma edge turbulence.
Abstract
We propose a setup enabling electron energy loss spectroscopy to determine the density of the electrons accumulated by an electropositive dielectric in contact with a plasma. It is based on a two-layer structure inserted into a recess of the wall. Consisting of a plasma-facing film made out of the dielectric of interest and a substrate layer, the structure is designed to confine the plasma-induced surplus electrons to the region of the film. The charge fluctuations they give rise to can then be read out from the backside of the substrate by near specular electron reflection. To obtain in this scattering geometry a strong charge-sensitive reflection maximum due to the surplus electrons, the film has to be most probably pre-n-doped and sufficiently thin with the mechanical stability maintained by the substrate. Taking electronegative CaO as a substrate layer we demonstrate the feasibility of the proposal by calculating the loss spectra for Al2O3, SiO2, and ZnO films. In all three cases we find a reflection maximum strongly shifting with the density of the surplus electrons and suggest to use it for charge diagnostics.
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.
Collisional absorption of dense fully ionized plasmas in strong high-frequency laser fields is investigated in the non-relativistic case. Quantum statistical methods are used as well as molecular dynamics simulations. In the quantum statistical expressions for the electrical current density and the electron-ion collision frequency–valid for arbitrary field strength–strong correlations are taken into account. In addition, molecular dynamic simulations were performed to calculate the heating of dense plasmas in laser fields. Comparisons with the analytic results for different plasma parameters are given. Isothermal plasmas as well as two-temperature plasmas are considered.
We present a Green's function based treatment of the effects of electron-phonon coupling on transport through a molecular quantum dot in the quantum limit. Thereby we combine an incomplete variational Lang-Firsov approach with a perturbative calculation of the electron-phonon self energy in the framework of generalised Matsubara Green functions and a Landauer-type transport description. Calculating the ground-state energy, the dot single-particle spectral function and the linear conductance at finite carrier density, we study the low-temperature transport properties of the vibrating quantum dot sandwiched between metallic leads in the whole electron-phonon coupling strength regime. We discuss corrections to the concept of an anti-adiabatic dot polaron and show how a deformable quantum dot can act as a molecular switch.
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.
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.
We examine the turbulence driven by the ion and electron temperature gradients in selected magnetic configurations of the Wendelstein 7-X (W7-X) stellarator. The inherent flexibility in the configuration space of W7-X enables us to find candidate configurations manifesting low turbulent transport. We follow insights gained by stellarator optimization techniques, in order to identify key geometric features, which are directly related to the ion and electron heat fluxes produced by plasma turbulence. One such a feature is the flux expansion at locations where the curvature is particularly unfavourable. Starting from a configuration routinely used in the W7-X experiment, we end up with an optimized configuration. Based on this equilibrium, we select a configuration from W7-X configuration database with a similar feature as the optimized one. With the help of nonlinear gyrokinetic simulations, we show that the heat flux in this configuration is less stiff than in the initial configuration, both for ion temperature gradient and electron temperature gradient turbulence.
AbstractWe propose a new scattering mechanism of Rydberg excitons, i.e., those with high principal quantum numbers, namely scattering by coupled LO phonon-plasmon modes, which becomes possible due to small differences in energies of the states due to different quantum defects. Already in very low-density electron–hole plasmas these provide a substantial contribution to the excitonic linewidth. This effect should allow determining plasma densities by a simple line shape analysis. Whenever one expects that low-density electron–hole plasma is present the plasmon induced broadening is of high significance and must be taken into account in the interpretation.
Abstract
We present experiments on the luminescence of excitons confined in a potential trap at milli-Kelvin bath temperatures under continuous-wave (cw) excitation. They reveal several distinct features like a kink in the dependence of the total integrated luminescence intensity on excitation laser power and a bimodal distribution of the spatially resolved luminescence. Furthermore, we discuss the present state of the theoretical description of Bose–Einstein condensation of excitons with respect to signatures of a condensate in the luminescence. The comparison of the experimental data with theoretical results with respect to the spatially resolved as well as the integrated luminescence intensity shows the necessity of taking into account a Bose–Einstein condensed excitonic phase in order to understand the behaviour of the trapped excitons.
We investigate local THz field generation using spintronic THz emitters to enhance the resolution for micrometer-sized imaging. Far-field imaging with wavelengths above 100 µm limits the resolution to this order of magnitude. By using optical laser pulses as a pump, THz field generation can be confined to the area of laser beam focusing. The divergence of the generated THz beam due to laser beam focusing requires the imaged object to be close to the generation spot at a distance below the THz field wavelength. We generate THz-radiation by fs-laser pulses in CoFeB/Pt heterostructures, based on spin currents, and detect them by commercial low-temperature grown-GaAs (LT-GaAs) Auston switches. The spatial resolution of THz radiation is determined by applying a 2D scanning technique with motorized stages allowing step sizes in the sub-micrometer range. Within the near-field limit, we achieve spatial resolution in the dimensions of the laser spot size on the micrometer scale. For this purpose, a gold test pattern is evaporated on the spintronic emitter separated by a 300 nm SiO2 spacer layer. Moving these structures with respect to the femtosecond laser spot, which generates THz radiation, allows for resolution determination. The knife-edge method yields a full-width half-maximum beam diameter of 4.9 +- 0.4 µm at 1 THz. The possibility to deposit spintronic emitter heterostructures on simple glass substrates makes them attractive candidates for near-field imaging in many imaging applications.
In our study, we determine the alignment of magnetic domains in a CoFeB layer using THz radiation. We generate THz pulses by fs laser pulses in magnetized CoFeB/Pt heterostructures based on spin currents. An LT-GaAs Auston switch detects the radiation phase sensitively and allows us to determine the magnetization alignment. Our scanning technique with motorized stages, with step sizes in the sub-micrometer range, allows us to image two dimensional magnetic structures. Theoretically, the resolution is restricted to half of the wavelength if focusing optics in the far-field limit are used. By applying near-field imaging, the spatial resolution is enhanced to the single digit micrometer range. For this purpose, spintronic emitters in diverse geometric shapes, e.g., circles, triangles, squares, and sizes are prepared to observe the formation of magnetization patterns. The alignment of the emitted THz radiation can be influenced by applying unidirectional external magnetic fields. We demonstrate how magnetic domains with opposite alignment and different shapes divided by domain walls are created by demagnetizing the patterns using minor loops and imaged using phase sensitive THz radiation detection. For analysis, the data are compared to Kerr microscope images. The possibility of combining this method with THz range spectroscopic information of magnetic texture or antiferromagnets in direct vicinity to the spintronic emitter makes this detection method interesting for a much wider range of applications probing THz excitation in spin systems with high resolution beyond the Abbe diffraction limit, limited solely by the laser excitation area.
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.
Tunable Diode Laser Absorption Spectroscopy in the mid InfraRed spectral range (IR-TDLAS) has been applied to investigate the behaviour of CF, CF2 and C2F4 species produced in pulsed CF4/H2 capacitively coupled radio frequency plasmas (13.56 MHz CCP). This experimental technique was shown to be suitable for temporally resolved measurements of the absolute number density of the target molecules in the studied fluorocarbon discharges. The temporal resolution of about 20…40 ms typically achieved in the standard data acquisition mode (“stream mode”) was sufficient for the real-time measurements of CF2 and C2F4, but not of CF whose kinetics was observed to be much faster. Therefore, a more sophisticated approach (“burst mode”) providing a temporal resolution of 0.94 ms was established and successfully applied to CF density measurements. In order to enable the TDLAS measurements of the target species, preliminary investigations on their spectroscopic data had been carried out. In particular, pure C2F4 has been produced in laboratory by means of vacuum thermal decomposition (pyrolysis) of polytetrafluoroethylene and used as a reference gas. Therefore, an absorption structure consisting of several overlapping C2F4 lines around 1337.11 cm-1 was selected and carefully calibrated, which provided the first absolute measurements of the species by means of the applied experimental technique. The absolute number density traces measured for CF, CF2 and C2F4 in the studied pulsed plasmas were then analysed, in which two differential balance equations were proposed for each of the species to describe their behaviour during both “plasma on” and “plasma off” phases. Analytical solutions of the balance equations were used to fit the experimental data and hence to deduce important information on the kinetics of the studied molecules. In particular, during the “plasma off” phase, the self-recombination of CF2 (CF2 + CF2 (+M) → C2F4 (+M)) was found to be dominant in the kinetics of the radical, but of minor importance for C2F4 production. A rapid consumption of CF observed within 7…25 ms after switching off the plasma was explained mainly by volume reaction with other species (most likely with CF3), whereas diffusion of the radical towards the reactor walls followed by sticking on the surfaces was found to contribute only at relatively low pressures (<10 Pa). Under certain discharge conditions, measured CF density traces exhibited significant overshoots in 50…150 ms after the plasma ignition, which had not been known from literature before. The electron impact fragmentation of C2F4 was shown to be essential for CF production at the beginning of the “plasma on” phase and therefore for formation of the observed CF density overshoots. Finally, the broad band FTIR spectroscopy was applied in order to better characterize the gas phase composition of the studied plasmas. Thus, absorption bands of CF4, C2F4, C2F6, C3F8, CHF3 and HF stable molecules were detected in the FTIR spectra recorded between 400 and 4000 cm-1. The spectra were then successfully deconvolved and the absolute concentration of the detected species was estimated. In particular, the absolute number density of C2F4 obtained from the FTIR measurements was in a good agreement with that achieved by means of the IR-TDLAS technique. The work was supported by the German Research Foundation (DFG) within the framework of the Collaborative Research Centre Transregio 24 “Fundamentals of Complex Plasmas” (SFB/TRR24, project section B5).
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.
In magnetisierten Plasmen kommt dem Verständnis von magnetischen Fluktuationen eine tragende Rolle hinsichtlich der Plasmadynamik zu. Diese Fluktuationen treten in Form linearer und nichtlinearer Wellenphänomene oder auch als Änderung der magnetischen Topologie auf. Im Rahmen der vorliegenden Dissertation wurde der Einfluß von niederfrequenten elektromagnetischen Wellen und der von topologischen Magnetfeldänderungen durch magnetische Rekonnektion auf die Dynamik der Ionen experimentell untersucht. In dem linearen magnetisierten Laborexperiment VINETA wurden kinetische Alfvénwellen angeregt und durch detaillierte Messung der Dispersion mittels magnetischer Fluktuationsdiagnostiken eindeutig identifiziert. Für das Verständnis des Dispersionsverhaltens müssen die Berandung der Wellen und der Einfluß von Stößen einbezogen werden. Mittels laserinduzierter Fluoreszenz (LIF) wurde die Ionenenergieverteilungsfunktion (IEVF) gemessen. Dabei wurde das Schema dahingehend erweitert, daß bei periodischen Störungen des Plasmas phasenaufgelöste Messungen der IEVF durchgeführt werden können. Die elektrischen Felder der durch vergleichsweise kleine magnetische Störungen angeregten linearen Alfvénwellen sind jedoch in der Regel zu klein, um einen signifikanten Einfluß auf die Ionendynamik zu nehmen. Anders verhält es sich jedoch bei einem stark nicht-linearem Anregungssschema: Die Welle-Teilchen Wechselwirkung konnte für nichtlineare Anregung Alfvénischer Wellen durch amplitudenmodulierte Helikoneigenmoden mittels LIF nachgewiesen werden. In dem toroidalen Experiment VTF kann magnetische Rekonnektion periodisch und unter reproduzierbaren Bedingungen angetrieben werden. Diese Voraussetzungen ermöglichen systematische Untersuchungen der Rückwirkung magnetischer Rekonnektion auf die Ionendynamik mittels LIF. Dabei ist es zum ersten Mal gelungen, eine Ionenheizung als Folge von Rekonnektion direkt nachzuweisen. Ferner konnte gezeigt werden, daß diese Heizung stark lokalisiert ist und nur am magnetischen X-Punkt, dem Ort der Rekonnektion, auftritt. Mittels zeitaufgelöster Messungen konnte ein kausaler Zusammenhang zwischen der Rekonnektionsrate und der Ionenheizung gezeigt werden. Desweiteren wurden starke nicht-thermische Komponenten der IEVF diagnostiziert, die mit der beobachteten Ionenheizung korrelieren. Numerische Simulationen, basierend auf einem kinetischen Einteilchenbild, zeigen einen Transfer von magnetischer Energie zu kinetischer Energie der Ionen, der konsistent mit dem experimentell beobachteten Anstieg der Ionentemperatur ist.
Diese Dissertation präsentiert experimentelle Untersuchungen zu vertikalen und lateralen Strukturen von Polyelektrolytmultischichten (PEM) adsorbiert auf festen Oberflächen. Zur Herstellung von PEM werden Polykationen (Poly-(allylamin)hydrochlorid (PAH) oder Poly-(diallyldimethylammonium)chlorid, PDADMAC) und Polyanionen (Polys-(styren)sulfonat (PSS)) aus einer wässrigen Lösung auf eine hydrophile Siliziumdioxid-Oberfläche sequentiell adsorbiert. Um nicht–elektrostatische (sekundäre) Kräfte während der Adsorption zu untersuchen, wird Reichweite und Stärke der elektrostatischen Wechselwirkung durch eine definierte Konzentration monvalenten Salzes (c_ads) in den Polyelektrolyt (PE)-Lösungen eingestellt. Schichtdicke, und Homogenität der Multischichten entlang der PEM-Normalen werden mit Röntgenreflexion gemessen. Dies ist in Übereinstimmung mit veröffentlichten Daten und wird auf die elektrostatische Abschirmung, beschrieben durch die Debye-Hückel Theorie zurückgeführt. Komplementär wird Neutronenreflexion genutzt, um die Interpenetration einzelner Polyelektrolytschichten zu quantifizieren. Hierzu wird ein PEM aus zwei Blöcken unterschiedlicher Streulängendichte (SLD) hergestellt. Der SLD-Kontrast wird durch Verwendung von protonierten und deuterierten PSS realisiert. Durch Variation der Anzahl protonierter und deuterierter PE-Schichten wird die Breite der inneren Grenzflächen positionsabhängig entlang der PEM-Normalen vermessen. So ist erstmals eine eindeutige Bestimmung der Interpenetration (inneren Rauigkeit, sigma_int) benachbarter Polykat-/Polyananiondoppelschichten möglich. Die PEM-Dicke skaliert mit der Wurzel der Salzkonzentration in der Adsorptionslösung. Sowohl für PAH/PSS als auch für PDADMAC/PSS-Multischichten ist sigma_int nahe an der Film/Luft-Grenzfläche am geringsten und steigt mit zunehmendem Abstand. Für das PAH/PSS-System ist die Zunahme monoton, während beim PDADMAC/PSS-System sigma_int zunächst anwächst und sich dann eine konstante innere Rauigkeit (sigma_int, max) einstellt. Bei PADMAC/PSS steigt sigma_int,max mit zunehmendem c_ads. Erklärt wird diese Beobachtung durch eine höhere extrinsische Ladungsträgerkompensation der Polyelektrolytketten und eine verringerte elektrostatische Wechselwirkung, letzteres führt zu einer erhöhten Flexibilität der Polyelektrolytketten. Die Änderung von sigma_int wird über ein 1-dimensionales Diffusionsmodell quantifiziert. Zusätzlich wird der Polymerisationsgrad (Anzahl Monomere pro Kette) des Polykations variiert. Bei einer Vergrößerung des Polymerisationsgrades und großem c_ads nimmt die maximale innere Rauigkeit ebenfalls zu. Dies weist auf kooperative Effekte zwischen Polykat- und Polyanion hin, da nur das PSS deuteriert ist. Bei geeignetem c_ads nimmt die Dicke pro adsorbierter Polykation/Polyanion-Doppelschicht (d_Bl) zu. Während für den salzfreien Fall (c_ads = 0) die Parameter d_Bl und Polymerisationsgrad entkoppelt sind, wird die Kopplung mit steigendem c_ads immer deutlicher. Dies wird mit einer PE-Schicht erklärt, in der die PE-Ketten bei der Adsorption eine flache (c_ads = 0) bzw. geknäulte (c_ads > 0) Konformation einnehmen. In diesem Fall steigt sigma_int bei großem Polymerisationsgrad rapide nahe der PEM/Luft-Grenzfläche, d.h. die Diffusionskonstante wächst. An dieser Stelle wird die These aufgestellt, daß entropische Kräfte und Stressrelaxation die Interpenetration verursachen. sigma_int, max stellt einen metastabilen Gleichgewichtszustand dar. Da die Diffusionskonstante einer Kette invers mit der Anzahl der Segmente skaliert, erklärt Stressrelaxation warum die Diffusionskonstante mit steigendem Polymerisationsgrad zunimmt.
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 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.
The multi-cell Penning–Malmberg trap concept has been proposed as a way to accumulate and confine unprecedented numbers of antiparticles, an attractive but challenging goal. We report on the commissioning and first results (using electron plasmas) of the World's second prototype of such a trap, which builds and improves on the findings of its predecessor. Reliable alignment of both ‘master’ and ‘storage’ cells with the axial magnetic field has enabled confinement of plasmas, without use of the ‘rotating wall’ (RW) compression technique, for over an hour in the master cell and tens of seconds in the storage cells. In the master cell, attachment to background neutrals is found to be the main source of charge loss, with an overall charge-confinement time of 8.6 h. Transfer to on-axis and off-axis storage cells has been demonstrated, with an off-axis transfer rate of 50% of the initial particles, and confinement times in the storage cells in the tens of seconds (again, without RW compression). This, in turn, has enabled the first simultaneous plasma confinement in two off-axis cells, a milestone for the multi-cell trap concept.
Ion trajectories have been simulated for an assembly of a linear quadrupole ion-filter and a linear Paul trap with additional pin electrodes for MS SPIDOC, a project in preparation for the study of biomolecules by single-particle imaging with X-ray pulses. The ion-optical components are based on digital RF guiding and trapping fields. In order to carefully handle biomolecules over a wide mass-over-charge range, the module presented consists of separate components for filtering and accumulation/trapping in order to select the ions of interest and to convert the beam from a continuous ion source to ion bunches, respectively, as required for the experiments downstream. The present analysis focuses on the transmission efficiency and mass resolving power of the filter, as well as the buffer-gas-pressure-dependent ion capture and thermalization in the trap for the example of a mass-to-charge ratio equivalent to hemoglobin 15+ ions. The resulting optimized ion bunch delivered by the assembly is characterized.
The combination of a linear quadrupole ion-filter and linear Paul trap operated with a rectangular guiding field for the filtering and accumulation of ions within the Mass Spectrometry for Single Particle Imaging of Dipole Oriented protein Complexes (MS SPIDOC) prototype [T. Kierspel et al., Anal. Bioanal. Chem., published online] is characterized. Using cationic caesium-iodide clusters, the ion-separation performance, ion accumulation, cooling, and ejection via in-trap pin electrodes is evaluated. Furthermore, proof-of-principle measurements are performed with 64 kDa multiply-charged non-covalent protein complexes of human hemoglobin and 804 kDa non-covalent complex of GroEL, to demonstrate that the module meets the criteria to handle high-mass ions which are the main objective of the MS SPIDOC project. The setup's performance is found to be in line with previous results from ion-trajectory simulations [F. Simke et al., Int. J. Mass Spectrom.473 (2022) 116779].
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.
AbstractAnalytical results for the dielectric function in RPA are derived for three-, two-, and one-dimensional semiconductors in the weakly-degenerate limit. Based on this limit, quantum corrections are derived. Further attention is devoted to systems with linear carrier dispersion and the resulting Dirac-cone physics.
Abstract
The anomalous Hall effect (AHE) is a fundamental spintronic charge‐to‐charge‐current conversion phenomenon and closely related to spin‐to‐charge‐current conversion by the spin Hall effect. Future high‐speed spintronic devices will crucially rely on such conversion phenomena at terahertz (THz) frequencies. Here, it is revealed that the AHE remains operative from DC up to 40 THz with a flat frequency response in thin films of three technologically relevant magnetic materials: DyCo5, Co32Fe68, and Gd27Fe73. The frequency‐dependent conductivity‐tensor elements σxx and σyx are measured, and good agreement with DC measurements is found. The experimental findings are fully consistent with ab initio calculations of σyx for CoFe and highlight the role of the large Drude scattering rate (≈100 THz) of metal thin films, which smears out any sharp spectral features of the THz AHE. Finally, it is found that the intrinsic contribution to the THz AHE dominates over the extrinsic mechanisms for the Co32Fe68 sample. The results imply that the AHE and related effects such as the spin Hall effect are highly promising ingredients of future THz spintronic devices reliably operating from DC to 40 THz and beyond.
Abstract
Identifying materials with an efficient spin-to-charge conversion is crucial for future spintronic applications. In this respect, the spin Hall effect is a central mechanism as it allows for the interconversion of spin and charge currents. Spintronic material research aims at maximizing its efficiency, quantified by the spin Hall angle and the spin-current relaxation length . We develop an all-optical contact-free method with large sample throughput that allows us to extract and . Employing terahertz spectroscopy and an analytical model, magnetic metallic heterostructures involving Pt, W and Cu80Ir20 are characterized in terms of their optical and spintronic properties. The validity of our analytical model is confirmed by the good agreement with literature DC values. For the samples considered here, we find indications that the interface plays a minor role for the spin-current transmission. Our findings establish terahertz emission spectroscopy as a reliable tool complementing the spintronics workbench.
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.
Die vorliegende Arbeit widmet sich der plasmachemischen Herstellung und physikalisch-chemischen Charakterisierung von dünnen organischen Schichten auf der Basis von Ethylenglykol (Präkursor). Die Oberflächen können die Adsorption von Proteinen minimieren und daher als neues biokompatibles Material getestet werden. Im Schwerpunkt der Arbeit liegt die Entwicklung eines plasmachemischen RF-Reaktors (genannt Nevada) und einer innovativen Beschichtungstechnologie TFPD (Temperature Forced Plasma Deposition) als Erweiterung der PECVD (Plasma Enhanced Chemical Vapour Deposition). Ein Gemisch aus Ethylenglykol und Argon wurde als Modellsystem untersucht. Die Plasmabedingungen und die Oberflächentemperatur wurden optimiert und an Phaseneigenschaften von Ethylenglykol angepasst. Die komplexe Polykondensation steht für die plasmagestützte Umwandlung der Kondensatschicht zu einem stabilen Poly(Ethylenglykol)-ähnlichen Plasmapolymer. Der Prozess wurde durch die simultane Temperaturkontrolle und in situ FTIR-Spektroskopie analysiert. Zur Untersuchung der erzeugten Proben wurden weitere ex situ Oberflächenanalysen wie XPS, AFM, TDS, MALDI, XRD und die optische Ellipsometrie verwendet. Durch die neue Methode TFPD entstehen extrem glatte, amorphe und wasserunlösliche Schichten mit einem Potenzial für die Biokompatibilität.
Synopsis
Polyanionic metal clusters are produced by electron attachment in both Paul and Penning traps. After size and charge-state selection, the cluster properties are further investigated by various methods including photo-dissociation. Depending on the particular cluster species various decay modes are observed.
AbstractGas puff modulation experiments are performed at ASDEX Upgrade in L-mode plasmas. We model the discharge with the ASTRA transport code in order to determine transport coefficients outside of a normalized radius of ρ
pol = 0.95. The experimental data is consistent with a range of particle diffusivities and pinch velocities of the order of D = (0.20 ± 0.13) m2 s−1 and v = (−1 ± 2) m s−1, respectively. The electron temperature response caused by the gas modulation permits to estimate also that heat diffusivity χ
e increases almost linearly when collisionality rises due to fuelling. The fuelling particle flux is amplified by recycling, overcompensating losses.
In this work we will analyse the capabilities of several numerical techniques for the description of different physical systems. Thereby, the considered systems range from quantum over semiclassical to classical and from few- to many-particle systems. For each case we address an interesting, partly unsolved question. Despite the different topics we address in the individual chapters, the problems under study are somehow related because we focus on the time evolution of the system. In chapter 1 we investigate the behaviour of a single quantum particle in the presence of an external disordered background (static potentials). Starting from the quantum percolation problem, we address the fundamental question of a disorder induced (Anderson-) transition from extended to localised single-particle eigenstates. Distinguishing isolating from conducting states by applying a local distribution approach for the local density of states (LDOS), we detect the quantum percolation threshold in two- and three-dimensions. Extending the quantum percolation model to a quantum random resistor model, we comment on the possible relevance of our results to the influence of disorder on the conductivity in graphene sheets. Furthermore, we confirm the localisation properties of the 2D percolation model by calculating the full quantum time evolution of a given initial state. For the calculation of the LDOS as well as for the Chebyshev expansion of the time evolution operator, the kernel polynomial method (KPM) is the key numerical technique. In chapter 2 we examine how a single quantum particle is influenced by retarded bosonic fields that are inherent to the system. Within the Holstein model, these bosonic degrees of freedom (phonons) give rise to an infinite dimensional Hilbert space, posing a true many-particle problem. Constituting a minimal model for polaron formation, the Holstein model allows us to study the optical absorption and activated transport in polaronic systems. Using a two-dimensional variant of the KPM, we calculate for the first time quasi-exactly the optical absorption and dc-conductivity as a function of temperature. Concerning the numerical technique, the close relation to the time evolution in the other chapters get clear if we identify temperature with an imaginary time. In chapter 3 we come back to the time evolution of a quantum particle in an external, static potential and investigate the capability of semiclassical approximations to it. Considering various one-dimensional geometries, we address basic quantum effects as tunneling, interference and anharmonicity. The question is, to which extend and at which numerical costs, several semiclassical methods can reproduce the exact result for the quantum dynamics, calculated by Chebyshev expansion. To this end we consider the linearised semiclassical propagator method, the Wigner-Moyal approach and the recently proposed quantum tomography. A conceptually very interesting aspect of the compared semiclassical methods is their relation to different representations of quantum mechanics (wave function/density matrix, Wigner function, quantum tomogram). Finally, in chapter 4 we calculate the dynamics of a classical many-particle system under the influence of external fields. Considering a low-temperature rf-plasma, we investigate the interplay of the plasma dynamics and the motion of dust particles, immersed into the plasma for diagnostic reasons. In addition to the huge number of involved particles, the numerical description of this systems faces the challenge of a large range of involved time and length scales. Exploiting the mass differences of plasma constituents and dust particles allows for separating the PIC description of the plasma from the MD simulation of the dust particles in the effective surrounding plasma.
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.
Two main aspects concerning drift wave dynamics in linear, magnetized plasma devices are addressed in the work: In part I of the thesis, drift waves are studied in a helicon plasma. The plasma parameter regime is characterized by comparably high collision frequencies and comparably high plasma-p exceeding the electron-ion mass ratio. Single Langmuir probes and a poloidal probe array are used for spatiotemporal studies of drift waves as well as for characterization of background plasma parameters. The main goals are the identification of a low-frequency instability and its major destabilization mechanisms. All experimentally observed features of the instability were found to be consistent with drift waves. A new code, based on a non-local cylindrical linear model for the drift wave dispersion, was used to gain more insight into the dominating destabilzation mechanisms, and also into dependencies of mode frequencies and growth rates on different parameters. In the experiment and in the numerical model, poloidal mode structures were found to be sheared. Part II of the thesis reports about mode-selective spatiotemporal synchronization of drift wave dynamics in a low-P plasma. Active control of the fluctuations is achieved by driving a preselected drift mode to the expense of other modes and broadband turbulence. It is demonstrated that only if a resonance between the driver signal and the drift waves in both space and time is reached, the driver has a strong influence on the drift wave dynamics. The synchronization effect is qualitatively well reproduced in a numerical simulation based on a Hasegawa-Wakatani model.
This study evaluated the impact of a defined plasma treated water (PTW) when applied to various stages within fresh-cut endive processing. The quality characteristic responses were investigated to establish the impact of the PTW unit processes and where PTW may be optimally applied in a model process line to retain or improve produce quality. Different stages of application of PTW within the washing process were investigated and compared to tap water and chlorine dioxide. Fresh-cut endive (Cichorium endivia L.) samples were analyzed for retention of food quality characteristics. Measurements included color, texture, and nitrate quantification. Effects on tissue surface and cell organelles were observed through scanning electron and atomic force microscopy. Overall, the endive quality characteristics were retained by incorporating PTW in the washing process. Furthermore, promising results for color and texture characteristics were observed, which were supported by the microscopic assays of the vegetal tissue. While ion chromatography detected high concentrations of nitrite and nitrate in PTW, these did not affect the nitrate concentration of the lettuce tissue post-processing and were below the concentrations within EU regulations. These results provide a pathway to scale up the industrial application of PTW to improve and retain quality characteristic retention of fresh leafy products, whilst also harnessing the plasma functionalized water as a process intervention for reducing microbial load at multiple points, whether on the food surface, within the process water or on food-processing surfaces.
Solar Activity Driven 27‐Day Signatures in Ionospheric Electron and Molecular Oxygen Densities
(2022)
Abstract
The complex interactions in the upper atmosphere, which control the height‐dependent ionospheric response to the 27‐day solar rotation period, are investigated with the superposed epoch analysis technique. 27‐day signatures describing solar activity are calculated from a solar proxy (F10.7) and wavelength‐dependent extreme ultraviolet (EUV) fluxes (Thermosphere Ionosphere Mesosphere Energetics and Dynamics/Solar EUV Experiment), and the corresponding 27‐day signatures describing ionospheric conditions are calculated from electron density profiles (Pruhonice ionosonde station) and O2 density profiles (Global‐scale Observations of the Limb and Disk). The lag analysis of these extracted signatures is applied to characterize the delayed ionospheric response at heights from 100 to 300 km and the impact of major absorption processes in the lower (dominated by O2) and upper ionosphere (dominated by O) is discussed. The observed variations of the delay in these regions are in good agreement with model simulations in preceding studies. Additionally, the estimated significance and the correlation of the delays based on both ionospheric parameters are good. Thus, variations such as the strong shift in 27‐day signatures for the O2 density at low heights are also reliably identified (up to half a cycle). The analysis confirms the importance of ionospheric and thermospheric coupling to understand the variability of the delayed ionospheric response and introduces a method that could be applied to additional ionosonde stations in future studies. This would allow to describe the variability of the delayed ionospheric response spatially, vertically and temporally and therefore may contribute further to the understanding of processes and improve ionospheric modeling.
Abstract
Based on the analysis of electron density Ne profiles (Grahamstown ionosonde), a case study of the height‐dependent ionospheric response to two 27‐day solar rotation periods in 2019 is performed. A well‐defined sinusoidal response is observed for the period from 27 April 2019 to 24 May 2019 and reproduced with a Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model simulation. The occurring differences between model and observations as well as the driving physical and chemical processes are discussed based on the height‐dependent variations of Ne and major species. Further simulations with an artificial noise free sinusoidal solar flux input show that the Ne delay is defined by contributions due to accumulation of O+ at the Ne peak (positive delay) and continuous loss of O2+ ${\mathrm{O}}_{2}^{+}$ in the lower ionosphere (negative delay). The neutral parts' 27‐day signatures show stronger phase shifts. The time‐dependent and height‐dependent impact of the processes responsible for the delayed ionospheric response can therefore be described by a joint analysis of the neutral and ionized parts. The return to the initial ionospheric state (and thus the loss of the accumulated O+) is driven by an increase of downward transport in the second half of the 27‐day solar rotation period. For this reason, the neutral vertical winds (upwards and downwards) and their different height‐dependent 27‐day signatures are discussed. Finally, the importance of a wavelength‐dependent analysis, statistical methods (superposed epoch analysis), and coupling with the middle atmosphere is discussed to outline steps for future analysis.
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.
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.
Abstract
The spectral properties of three-dimensional dust clusters confined in gaseous discharges are investigated using both a fluid mode description and the normal mode analysis (NMA). The modes are analysed for crystalline clusters as well as for laser-heated fluid-like clusters. It is shown that even for clusters with low particle numbers and under presence of damping fluid modes can be identified. Laser-heating leads to the excitation of several, mainly transverse, modes. The mode frequencies are found to be nearly independent of the coupling parameter and support the predictions of the underlying theory. The NMA and the fluid mode spectra demonstrate that the wakefield attraction is present for the experimentally observed Yukawa balls at low pressure. Both methods complement each other, since NMA is more suitable for crystalline clusters, whereas the fluid modes allow to explore even fluid-like dust clouds.
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.
We report on the effect of solar variability at the 27-day and the 11-year timescales on standard phase height measurements in the ionospheric D region carried out in central Europe. Standard phase height corresponds to the reflection height of radio waves (for constant solar zenith distance) in the ionosphere near 80 km altitude, where NO is ionized by solar Lyman radiation. Using the superposed epoch analysis (SEA) method, we extract statistically highly significant solar 27-day signatures in standard phase heights.
The 27-day signatures are roughly inversely correlated to solar proxies, such as the F10.7 cm radio flux or the Lyman-flux. The sensitivity of standard phase height change to solar forcing at the 27-day timescale is found to be in good agreement with the sensitivity for the 11-year solar cycle,suggesting similar underlying mechanisms. The amplitude of the 27-day signature in standard phase height is larger duringsolar minimum than during solar maximum, indicating that the signature is not only driven by photoionization of NO.We identified statistical evidence for an influence of ultra-long planetary waves on the quasi 27-day signature of standard phase height in winters of solar minimum periods.
Abstract
We have demonstrated efficient injection and trapping of a cold positron beam in a dipole magnetic field configuration. The intense 5 eV positron beam was provided by the NEutron induced POsitron source MUniCh facility at the Heinz Maier-Leibnitz Zentrum, and transported into the confinement region of the dipole field trap generated by a supported, permanent magnet with 0.6 T strength at the pole faces. We achieved transport into the region of field lines that do not intersect the outer wall using the
drift of the positron beam between a pair of tailored plates that created the electric field. We present evidence that up to 38% of the beam particles are able to reach the intended confinement region and make at least a 180° rotation around the magnet where they annihilate on an insertable target. When the target is removed and the
plate voltages are switched off, confinement of a small population persists for on the order of 1 ms. These results lend optimism to our larger aims to apply a magnetic dipole field configuration for trapping of both positrons and electrons in order to test predictions of the unique properties of a pair plasma.
Application of quantum cascade laser absorption spectroscopy to studies of fluorocarbon molecules
(2009)
The recent advent of quantum cascade lasers (QCLs) enables room-temperature mid-infrared spectrometer operation which is particularly favourable for industrial process monitoring and control, i.e. the detection of transient and stable molecular species. Conversely, fluorocarbon containing radio-frequency discharges are of special interest for plasma etching and deposition as well as for fundamental studies on gas phase and plasma surface reactions. The application of QCL absorption spectroscopy to such low pressure plasmas is typically hampered by non-linear effects connected with the pulsed mode of the lasers. Nevertheless, adequate calibration can eliminate such effects, especially in the case of complex spectra where single line parameters are not available. In order to facilitate measurements in fluorocarbon plasmas, studies on complex spectra of CF4 and C3F8 at 7.86 μm (1269 – 1275 cm-1) under low pressure conditions have been performed. The intra-pulse mode, i.e. pulses of up to 300 ns, was applied yielding highly resolved spectral scans of ∼ 1 cm-1 coverage. Effective absorption cross sections were determined and their temperature dependence was studied in the relevant range up to 400 K and found to be non-negligible.
Fluorocarbon containing capacitively coupled radio frequency (cc-rf) plasmas are widely used in technical applications and as model systems for fundamental investigations of complex plasmas. Absorption spectroscopy based on pulsed quantum cascade lasers (QCL) was applied in the mid-IR spectral range of 1269-1275 cm-1. Absolute densities of the precursor molecule CF4 and of the stable product C3F8 were measured with a time resolution of up to 1 ms in pulsed CF4/H2 asymmetrical cc-rf (13.56 MHz) discharges. For this purpose both the non-negligible temperature dependence of the absorption coefficients and the interference of the absorption features of CF4 and C3F8 had to be taken into account in the target spectral range. Therefore, at two different spectral positions composite absorption spectra were acquired under the same plasma conditions in order to discriminate between CF4 and C3F8 contributions. A total consumption of∼ 12 % was observed for CF4 during a 1 s plasma pulse, whereas C3F8 appeared to be produced mainly from amorphous fluorocarbon layers deposited at the reactor walls. A gas temperature increase by ∼ 100 K in the plasma pulse was estimated from the measurements. Additionally, not yet identified unresolved absorption (potentially from the excited CF4 molecule) was found during the àon-phase'.
In order to clarify the physics of the crossover from a spin-density-wave (SDW) Mott insulator to a charge-density-wave (CDW) Peierls insulator in one-dimensional (1D) systems, we investigate the Hubbard-Holstein Hamiltonian at half filling within a density matrix renormalisation group (DMRG) approach. Determining the spin and charge correlation exponents, the momentum distribution function, and various excitation gaps, we confirm that an intervening metallic phase expands the SDW-CDW transition in the weak-coupling regime.
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.
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.
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.
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.
The nature and origin of electronic nematicity remains a significant challenge in our
understanding of the iron-based superconductors. This is particularly evident in the
iron chalcogenide, FeSe, where it is currently unclear how the experimentally
determined Fermi surface near the M point evolves from having two electron pockets
in the tetragonal state, to exhibiting just a single electron pocket in the nematic state. This
has posed a major theoretical challenge, which has become known as the missing electron
pocket problem of FeSe, and is of central importance if we wish to uncover the secrets
behind nematicity and superconductivity in the wider iron-based superconductors. Here,
we review the recent experimental work uncovering this nematic Fermi surface of FeSe
from both ARPES and STM measurements, as well as current theoretical attempts to
explain this missing electron pocket of FeSe, with a particular focus on the emerging
importance of incorporating the dxy orbital into theoretical descriptions of the nematic
state. Furthermore, we will discuss the consequence this missing electron pocket has on
the theoretical understanding of superconductivity in this system and present several
remaining open questions and avenues for future research.
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.
AbstractPulsed streamer discharges submerged in water have demonstrated potential in a number of applications. Especially the generation of discharges by short high-voltage pulses in the nanosecond range has been found to offer advantages with respect to efficacies and efficiencies. The exploited plasma chemistry generally relies on the initial production of short-lived species, e.g. hydroxyl radicals. Since the diagnostic of these transient species is not readily possible, a quantification of hydrogen peroxide provides an adequate assessment of underlying reactions. These conceivably depend on the characteristics of the high-voltage pulses, such as pulse duration, pulse amplitude, as well as pulse steepness.A novel electrochemical flow-injection system was used to relate these parameters to hydrogen peroxide concentrations. Accordingly, the accumulated hydrogen peroxide production for streamer discharges ignited in deionized water was investigated for pulse durations of 100 ns and 300 ns, pulse amplitudes between 54 kV and 64 kV, and pulse rise times from 16 ns to 31 ns. An independent control of the individual pulse parameters was enabled by providing the high-voltage pulses with a Blumlein line. Applied voltage, discharge current, optical light emission and time-integrated images were recorded for each individual discharge to determine dissipated energy, inception statistic, discharge expansion and the lifetime of a discharge.Pulse steepness did not affect the hydrogen peroxide production rate, but an increase in amplitude of 10 kV for 100 ns pulses nearly doubled the rate to (0.19 ± 0.01) mol l−1 s−1, which was overall the highest determined rate. The energy efficiency did not change with pulse amplitude, but was sensitive to pulse duration. Notably, production rate and efficiency doubled when the pulse duration decreased from 300 ns to 100 ns, resulting in the best peroxide production efficiency of (9.2 ± 0.9) g kWh−1. The detailed analysis revealed that the hydrogen peroxide production rate could be described by the energy dissipation in a representative single streamer. The production efficiency was affected by the corresponding discharge volume, which was comprised by the collective volume of all filaments. Hence, dissipating more energy in a filament resulted in an increased production rate, while increasing the relative volume of the discharge compared to its propagation time increased the energy efficiency.
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.
Beams of ions and electrons are a source of free energy which can be transferred to waves via an instability. Beams exist in almost all plasma environments, but their instabilities are particularly important for the dynamics of space plasmas. In the absence of collisions, the instability drives waves to large amplitudes and forms nonlinear structures such as solitary waves. The electric fields in these waves can scatter particles in the background plasma, or disrupt currents. Both of these effects are important for the overall dynamics of the plasma. In this thesis, both electron and ion beam plasma instabilities have been investigated in the linear plasma device VINETA and using a Particle-in-Cell simulation. The electron beam instability has been demonstrated by previous authors to be a useful diagnostic for the plasma density. The spatial resolution of previous results was confirmed at a few millimetres, and a temporal resolution of 1ms was shown for the first time. An ion beam was generated with a double plasma discharge. Compared to space, this environment and indeed most laboratory plasmas have considerably higher collisionality and a limited spatial extent which introduces gradients in the plasma. Gradients perpendicular to the beam propagation direction are linked to a decrease of both the wavelength and amplitude of the instability. It was observed in both experiment and simulation that gradients in sheaths at the boundaries of the plasma not only affect the time averaged plasma parameters, but also excite instabilities. Fluctuations within the sheath spread the beam in velocity space, effectively increasing its temperature. Warmer beams require a higher drift velocity to excite an instability. This was also confirmed by experimental and numerical results. Collisions are shown to be the dominant damping force for the electron beam instability. For ions, collisions play an important role in the simulation, but appear to be overshadowed by Landau damping from impurities in the experiment. When boundary conditions are removed from the simulation, wave amplitudes increase and nonlinear effects become important. Saturation by particle trapping and coalescence of phase space holes is observed, which could eventually lead to the solitary waves as they are observed in space plasmas.
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.
Abstract
Experimental studies on dusty plasmas containing systems of (super-)paramagnetic dust particles are presented. In our experiments, external (homogeneous as well as inhomogeneous) magnetic fields in the mT range are applied to study the effect on single particles or few-particle systems that are trapped inside the sheath region. The behavior of the paramagnetic dust particles is considerably different than that of dielectric plastic particles, which are widely used in dusty plasmas. It is revealed that especially non-magnetic contributions play an important role in the interaction between superparamagnetic particles.
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.
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.
The confinement of energy has always been a challenge in magnetic confinement fusion devices. Due to their toroidal shape there exist regions of high and low magnetic field, so that the particles are divided into two classes - trapped ones that are periodically reflected in regions of high magnetic field with a characteristic frequency, and passing particles, whose parallel velocity is high enough that they largely follow a magnetic field line around the torus without being reflected. The radial drift that a particle experiences due to the field inhomogeneity depends strongly on its position, and the net drift therefore depends on the path taken by the particle. While the radial drift is close to zero for passing particles, trapped particles experience a finite radial net drift and are therefore lost in classical stellarators. These losses are described by the so-called neoclassical transport theory. Recent optimised stellarator geometries, however, in which the trapped particles precess around the torus poloidally and do not experience any net drift, promise to reduce the neoclassical transport down to the level of tokamaks. In these optimised stellarators, the neoclassical transport becomes small enough so that turbulent transport may limit the confinement instead. The turbulence is driven by small-scale-instabilities, which tap the free energy of density or temperature gradients in the plasma. Some of these instabilities are driven by the trapped particles and therefore depend strongly on the magnetic geometry, so the question arises how the optimisation affects the stability. In this thesis, collisionless electrostatic microinstabilities are studied both analytically and numerically. Magnetic configurations where the action integral of trapped-particle bounce motion, J, only depends on the radial position in the plasma and where its maximum is in the plasma centre, so-called maximum-J configurations, are of special interest. This condition can be achieved approximately in quasi-isodynamic stellarators, for example Wendelstein 7-X. In such configurations the precessional drift of the trapped particles is in the opposite direction from the direction of propagation of drift waves. Instabilities that are driven by the trapped particles usually rely on a resonance between these two frequencies. Here it is shown analytically by analysing the electrostatic energy transfer between the particles and the instability that, thanks to the absence of the resonance, a particle species draws energy from the mode if the frequency of the mode is well below the charateristic bounce frequency. Due to the low electron mass and the fast bounce motion, electrons are almost always found to be stabilising. Most of the trapped-particle instabilities are therefore predicted to be absent in maximum- J configurations in large parts of parameter space. Analytical theory thus predicts enhanced linear stability of trapped-particle modes in quasi-isodynamic stellarators compared with tokamaks. Moreover, since the electrons are expected to be stabilising, or at least less destabilising, for all instabilities whose frequency lies below the trapped-electron bounce frequency, other modes might benefit from the enhanced stability as well. In reality, however, stellarators are never perfectly quasi-isodynamic, and the question thus arises whether they still benefit from enhanced stability. Here the stability properties of Wendelstein 7-X and a more quasi-isodynamic configuration, QIPC, are investigated numerically and compared with another, non-quasiisodynamic stellarator, the National Compact Stellarator Experiment (NCSX) and a typical tokamak. In gyrokinetic simulations, performed with the gyrokinetic code GENE in the electrostatic and collisionless approximation, several microinstabilities, driven by the density as well as both ion and electron temperature gradients, are studied. Wendelstein 7-X and QIPC exhibit significantly reduced growth rates for all simulations that include kinetic electrons, and the latter are indeed found to be stabilising when the electrostatic energy transfer is analysed. In contrast, if only the ions are treated kinetically but the electrons are taken to be in thermodynamic equilibrium, no such stabilising effect is observed. These results suggest that imperfectly optimised stellarators can retain most of the stabilising properties predicted for perfect maximum-J configurations. Quasi-isodynamic stellarators, in addition to having reduced neoclassical transport, might therefore also show reduced turbulent transport, at least in certain regions of parameter space.
Es wurde ein Versuchsaufbau für die Behandlung von Titanproben mittels der Plasma-Immersions-Ionen-Implantation konzipiert, konstruiert und aufgebaut. Im Unterschied zu üblichen PIII-Anlagen wurde hier eine kapazitiv gekoppelte RF-Entladung als Hintergrundentladung mit zwei koplanaren Elektroden direkt über den zu behandelnden Titanproben benutzt. Auf diese Weise war es möglich unter Verwendung von Kupfer für die Elektroden und den Probenhalter, die Titanproben mit Kupfer zu dotieren und parallel, durch die Wahl geeigneter Prozessgase, zu oxidieren. Zusätzlich waren neben Prozessgaskontrollern auch Flüssigkeitskontroller vorhanden, so dass mit diesem Versuchsaufbau Gase, verdampfte Flüssigkeiten und erodierte Metalle in verschiedensten Kombinationen gleichzeitig als Prozessgas für die PIII zur Verfügung gestellt werden konnte. Wie bei PIII-Prozessen erforderlich, waren die Hochspannungshöhe und -dauer einstellbar. Der Strom wurde so gemessen, dass er die Ionendosis wider gibt und für die Temperaturmessung der Substratoberfläche wurde ein in-situ Pyrometer verwendet. Die relevanten elektrischen Parameter wurden mittels eines Oszilloskops bestimmt. Es stellte sich heraus, dass ein Gesamtionenstrom von bis zu 48mA implantiert werden konnte. Dies entspricht bis zu 3.5 x 1015 Ionen cm-2 s-1 und dementsprechend einem Energiestrom von bis zu 5 J cm-2 s-1. Die daraus resultierenden Temperaturen der Substratoberfläche betrugen bis zu 600 °C. Diese Parameter bewegen sich durchaus im Parameterfeld herkömmlicher Anlagen. Der erste inhaltliche Arbeitsgegenstand dieser Arbeit bestand darin, die Oberfläche der Titanproben unter Verwendung von Sauerstoff als Prozessgas mit einem ausreichend dicken und idealerweise kristallinen Titandioxid zu modifizieren. Auf diese Weise wurde die undefinierte natürliche Oxidschicht durch ein definiertes Oxid ausgetauscht. Röntgen-Diffraktometrie Messungen der modifizierten Proben ergaben neben alpha-Titan auch Rutil als primäre Kristallstruktur in der Oberfläche. Durch Variation der Prozessparameter, speziell des Duty-Cycle und damit des Ionenstroms bzw. der Temperatur der Titanproben, konnte die Konzentration an Rutil direkt gesteuert werden. Der zweite inhaltliche Arbeitsgegenstand bestand in der Modifikation des Prozesses und des Versuchsaufbaus zur Dotierung des Titans mit einem antimikrobiell wirksamen Metall, wobei Kupfer aufgrund seiner biologischen Eigenschaften favorisiert wurde. Um die positiven physikochemischen Eigenschaften der Titandioxidoberfläche bestmöglich zu nutzen, wurde parallel zur Dotierung mit Kupfer eine definierte Titandioxid-Schicht erzeugt. Als Prozessgas wurden dafür sauerstoffhaltige Gase, speziell Sauerstoff und Wasserdampf verwendet. Es zeigte sich, dass mit Sauerstoff aufgrund des hohen atomaren O Anteils CuO und TiO2 erzeugt, während mit Wasserdampf, aufgrund der reduzierenden Wirkung des Wasserstoffs, bis zu 30% metallisches Kupfer in die TiO2 Matrix eingebracht werden konnte. Zusätzlich konnte ein Übergang von alpha-Titan für kleine Ionendosen zu Rutil für Dosen oberhalb von 4 x 1018 Ionen cm-2 und 450 °C festgestellt werden. Weiterhin zeigten die Diffraktogramme Ti3O als Übergangsmodifikation, jedoch keine Kupfer- oder Kupferoxidkristalle. Durch geeignete Prozessparameterwahl ist es daher selbst bei geringen Implantationsspannungen von 10 kV möglich, einen bis zu 200nm dicken Verbund aus kristallinem TiO2 (Rutil) angereichert mit metallischem Kupfer zu erzeugen. Ein zusätzlicher Arbeitsgegenstand bestand in exemplarischen Zelltests mit Staphylococcus aureus (MRSA) und MG-63 Zellen als Vertreter für problematische Krankenhauskeime bzw. als Modellorganismus für Knochenzellen. Parallel zu diesen Versuchen wurde das Kupfer-Release der mit Kupfer dotierten Titanoberfläche bestimmt. Mit höheren Ionendosen während der Ionenimplantation konnten die Proben dahingehend modifiziert werden, dass eine höhere Kupferkonzentration aus dem Verbund ausgelöst wurde. Dieser Verlauf spiegelte sich auch in den Zelltests wieder. Während die Vitalität der MG-63 Zellen mit steigender Dosis abnahm, stieg die antibakterielle Wirkung an. So konnten über 95% der Bakterien getötet werden, wobei die Zellvitalität mit 80% im Vergleich zur Zelle auf dem Deckgläschen immer noch sehr hoch war. Weiterhin wurde eine brauchbare numerische Simulation erstellt, die ein besseres Verständnis der physikalischen Prozesse auf und unter der Oberfläche der zu modifizierenden Proben ermöglichte. Zusätzlich kann, basierend auf dieser Simulation, eine Prozessabstimmung geschehen. In die Simulation gingen die mit der Software TRIM simulierten Tiefenprofile, Sputterraten der einzelnen Oberflächenelemente, Elementanteile der Oberfläche sowie der einfliegenden Ionen und deren Energie mit ein. Dabei zeigte sich, dass die experimentell erhaltenen Tiefenprofile mit diesem Modell bis zu einem gewissen Genauigkeitsgrad qualitativ und quantitativ erklärt werden können.
In this thesis wave propagation in the whistler wave frequency range ωci≤ω≤ωce in the linear magnetized plasma experiment VINETA is investigated. The plasma is generated by a helicon antenna and has a diameter of about 10 cm. Whistler waves are launched by a loop antenna with a diameter of 4.5 cm and the fluctuating magnetic field is mapped by Ḃ-probes. Experiments are carried out for plasma parameters γ≤1/ √ 2 under which the only transversal polarized wave according to plane wave dispersion theory is the whistler wave. Due to the small collision frequencies ν≪1 cyclotron damping of whistler waves in this parameter regime is dominant and depends only on the electron plasma-β. The influence of the inhomogeneous plasma profile and excitation by a loop antenna is investigated by measurements of the fluctuating magnetic field perpendicular to the ambient magnetic field in azimuthal and radial axial planes. A mode characterized by the number of wave lengths m in the azimuthal direction is found. The mode structure is modified by the specific shape of the plasma density profile. Profiles with a homogeneous density inside the plasma radius are found to posses a comparably simple mode structure. An agreement in the mode structure of full-wave simulations in three dimensions, including a Gaussian density profile and excitation of the wave by a loop antenna, with the experimental results is found. Conclusions on the spatial structure of the excited mode are drawn using the simulations which predict excitation of an m=2 mode. The wave is found to be ducted within the plasma radius over a wide parameter range. A Helmholtz decomposition of the simulations electric field exhibits the fluctuating space charge as the dominant source for the electric field, while the contribution due to induction is negligible. The magnetic field is given partially by the electron and displacement current. Both contributions to the magnetic field are of the same order of magnitude. The frequency dependency of the excited modes spatial damping increment is investigated using measurements of the magnetic fluctuations along the symmetry axis of the plasma. In order to illustrate the parameter dependency, the electron plasma-β is varied over two orders in magnitude in the range β = 4·10-4 - 2.4·10-2. The experimental result for the spatial damping increment of the mode yields a strong damping for wave frequencies ω/ωce > 0.5 at maximum plasma-β, which shifts to higher frequencies with decreasing β. The parameter dependency of the damping for a fixed frequency is studied in an axial ambient magnetic field gradient. In both cases an excellent agreement between the experimental result and predictions for cyclotron damping from plane wave dispersion theory is found.
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.
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.
Magnetooptical properties of one-dimensional aperiodic structures formed by stacking together magnetic and nonmagnetic layers according to the Kolakoski self-generation scheme are studied theoretically using the 4x4 transfer matrix method. The effect of the generation stage of the sequence, and the helicity and direction of light propagation through the magneto-photonic crystals on the transmission/reflection spectra as well as Faraday and ellipticity rotations, have been investigated. Our results reveal that this kind of aperiodic magneto-photonic crystals can be used for the fabrication of multifrequency laser cavities, and optical filters/sensors.
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.