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Institute
- Institut für Physik (160) (remove)
Modern space missions depend more and more on electric propulsion devices for in-space
flights. The superior efficiency by ionizing the feedgas and propelling them using electric
fields with regard to conventional chemical thrusters makes them a great alternative. To
find optimized thruster designs is of high importance for industrial applications. Building
new prototypes is very expensive and takes a lot of time. A cheaper alternative is to rely
on computer simulations to get a deeper understanding of the underlying physics. In order
to gain a realistic simulation the whole system has to be taken into account including the
channel and the plume region. Because numerical models have to resolve the smallest time
and spatial scales, simulations take up an unfeasible amount of time. Usually a self-similarity
scaling scheme is used to greatly speed up these simulations. Until now the limits of this
method have not been thoroughly discussed. Therefore, this thesis investigates the limits
and the influence of the self-similarity scheme on simulations of ion thrusters. The aim
is to validate the self-similarity scaling and to look for application oriented tools to use
for thruster design optimization. As a test system the High-Efficiency-Multistage-Plasma
thruster (HEMP-T) is considered.
To simulate the HEMP-T a fully kinetic method is necessary. For low-temperature plasmas,
as found in the HEMP-T, the Particle-in-Cell (PIC) method has proven to be the best
choice. Unfortunately, PIC requires high spatial and temporal resolution and is hence
computationally costly. This limits the size of the devices PIC is able to simulate as well
as limiting the exploration of a wider design space of different thrusters. The whole system
is physically described using the Boltzmann and Maxwell equations. Using these system
of equations invariants can be derived. In the past, these invariants were used to derive a
self-similarity scaling law, maintaining the exact solution for the plasma volume, which is
applicable to ion thrusters and other plasmas. With the aid of the self-similarity scaling
scheme the computation cost can be reduced drastically. The drawback of the geometrical
scaling of the system is, that the plasma density and therefore the Debye length does not
scale. This expands the length at which charge separation occurs in respect to the system
size. In this thesis the limits of this scaling are investigated and the influence of the scaling
at higher scaling factors is studied. The specific HEMP-T design chosen for these studies is
the DP1.
Because the application of scaling laws is limited by the increasing influence of charge separation with increased scaling, PIC simulations still are computationally costly. Another approach to explore a wider design space is given using Multi-Objective-Design-Optimization
(MDO). MDO uses different tools to generate optimized thruster designs in a comparatively
short amount of time. This new approach is validated using the PIC method. During this
validation the drawback of the MDO surfaces. The MDO calculations are not self-consistent
and are based on empirical values of old thruster designs as input parameters, which not
necessarily match the new optimized thruster design. By simulating the optimized thruster
design with PIC and recalculate the former input parameters, a more realistic thruster design is achieved. This process can be repeated iteratively. The combination of self-consistent
PIC simulations with the performance of MDO is a great way to generate optimized thruster
designs in a comparatively short amount of time. The proof of concept of such a combination
is the pinnacle of this thesis.
Organic molecules are the carbon-based complex of several atoms, is an innovative and essential element to create nano-structural platforms, as a building block in the
field of organic electronics and organic spintronics. Because of its variety and functionality via widely studied synthetic methods, molecules have played an important role in electronics as not only a transport channel in bulk form but also a tuning layer
at the interface of hetero structures. The potential of molecular layers has also stood out in spintronics, owing to its mass-low composition producing long spin life time.
Organic materials can be employed in spintronics applications, benefiting from their low cost, ease of processing, and chemical tunability. Beyond this advantage, the configuration
of molecules on a metal film displays unique phenomena as it can control the molecular spins and interfacial coupling between them, resulting in the emergence
of molecular spinterface.
This thesis work focuses on identifying the interfacial properties between the ferromagnet and the Phenalenyl (PLY) based metal complexes. The growth morphology study of the copper-phenalenyl Cu-PLY based molecules influence the electronic coupling between the molecular layer and the ferromagnet. Zinc- Phenalenyl (ZMP) molecule already have been studied [1] by demonstrate the formation of a spinterface,
resulting interface magneto resistance (IMR) close to room temperature. The
spinterface formation leads to the unique property, that a magnetic tunnel junction
with a ZMP barrier requires only one ferromagnetic metal layer, while the other ferromagnetic layer is formed in the organic barrier directly at the ferromagnet/organic
barrier interface. Here we compare Phenaleny, Copper-Phenaleny Cu-PLY and Zincmethyl- phenaleny molecule based MTJ electrical and magnetic properties which will
be suitable for tunnel barrier and can be used for stable memory devices. We tune the magnetic property of ferromagnet and forma hybrid interface without any oxide layers in between the ferromagnet and molecular layers. The tuning of magnetic properties
via the molecular approach will certainly extend versatile functionalities of organic spinterfaces.
Development of an Electrostatic Ion Beam Trap for Laser Spectroscopy of Short-lived Radionuclides
(2021)
Due to its high accuracy and resolution, collinear laser spectroscopy (CLS) is a powerful tool to measure nuclear ground state properties such as nuclear spins, electromagnetic moments and mean-square charge radii of short-lived radionuclides. Performing CLS with fast beams (>30 keV) provides an excellent spectral resolution approaching the natural linewidth. However, its fluorescence-light detection limits its successful application to nuclides with yields of more than several 100 to 10,000 ions/s, depending on the specific case and spectroscopic transition. To extend its reach to the most exotic nuclides with very low production yields far away from stability, more sensitive methods are needed. For this reason, the novel Multi Ion Reflection Apparatus for CLS (MIRACLS) is currently under development at ISOLDE/CERN. This setup aims to combine the high resolution of conventional fluorescence based CLS with a high experimental sensitivity, enhanced by a factor of 30 to 700 depending on the mass and lifetime of the studied nuclide. By repetitively reflecting the ion beam between the electrostatic mirrors of an electrostatic ion beam trap, often also called Multi-Reflection Time of Flight (MR-ToF) device, the laser beam probes the ion bunch during each revolution. Therefore, the observation time is extended and the experimental sensitivity is enhanced compared to conventional single-passage CLS. As part of this thesis, a MIRACLS proof-of-principle apparatus has been constructed around an MR-ToF system, operating at ~1.5 keV beam energy, which has been upgraded for the purpose of CLS. The goal of this setup is to demonstrate the potential of the MIRACLS concept, to benchmark simulations that are employed to design a future device operating at 30 keV, and to further develop the technique. For this purpose, CLS measurements with ions of stable magnesium and calcium isotopes are performed. This data serves to characterise the performance of the new method, especially in terms of gain in sensitivity and measurement accuracy.
In this thesis, the transport properties of topological insulators are investigated. In contrast to trivial insulators, topological insulators possess conducting boundary states which cross the bulk energy gap that separates the highest occupied electronic band from the lowest unoccupied band. The materials used in this thesis are three-dimensional topological insulators with time-reversal symmetry. Their associated helical surface states are protected against elastic backscattering by Kramers degeneracy. The unique properties of the helical surface states can be utilized to generate spin-polarized currents at the surface of topological insulators and to control their propagation direction. This makes them a promising material class for the field of spintronics.
Here, we perform photocurrent scans of topological insulator Hall bar and nanowire devices. From these measurements, we obtained two-dimensional maps of the polarization-independent and helicity-dependent components of the photocurrents.
We find that the polarization-independent component is dominated by the Seebeck effect and thus driven by thermoelectric currents. On the other hand, the helicity-dependent component is driven by the spin-polarized currents that emerge from the topologically non-trivial helical surface states via the circular photogalvanic effect.
First and foremost, our experiments demonstrate that topological insulator nanowires provide a promising platform for the generation of spin-polarized currents, whose direction can be controlled via the helicity of the excitation light. They also highlight the importance of analysing the spatial distribution of the photocurrent, as we observe a strong enhancement of the spin-polarized current and the thermoelectric current at the interface between the nanowire and the metallic contacts. As our analysis shows, the thermoelectric current is enhanced by the Schottky effect and the spin-polarized current is amplified by the spin Nernst effect. In addition, the spin Nernst effect is also present in Hall bar devices and manifest as an enhancement of the spin-polarized current along the Hall bar sides.
Motiviert durch den Vorschlag einer direkten, optischen Ladungsmessung an Staubteilchen wird die Lichtstreuung an den dielektrischen Kern-Schale-Teilchen tiefgehend untersucht.
Das Streuregime wird durch Analyse des Nah- und Fernfeldes unter Verwendung von Methoden, die für homogene Teilchen entwickelt wurden, eingehend charakterisiert und eine Verallgemeinerung der dazu verwendeten Funktionen auf ein k-fach beschichtetes Teilchen angegeben. Dabei werden die sich im Teilcheninneren manifestierenden Effekte der Hybridisierung der beiden Oberflächenphononen des Kern-Schale-Teilchens herausgearbeitet und visualisiert.
Die vorliegende Untersuchung der unterschiedlichen Kenngrößen ermöglicht ein detailliertes und umfangreiches Verständnis der Lichtstreuung an dielektrischen Kern-Schale-Teilchen und der Art und Weise, wie sich die Hybridisierung der Oberflächenphononen auf diese auswirkt.
Die dabei analysierte Interferenzstruktur des elektromagnetischen Feldes in der Teilchenschale, berechnet mittels der vollen Mie-Rechnung, passt zur Interpretation der optischen Antwort des Kern-Schale-Teilchens mithilfe der Hybridisierungstheorie.
Dieses Hybridisierungsbild und somit die Subsysteme und ihre Wechselwirkung werden in dieser Arbeit aus den analytisch exakten Mie-Koeffizienten heraus präpariert, um die neue Sichtweise mit der alten Mie-Theorie zusammenzubringen.
Die Idee einer spektroskopische Ladungsmessung wird im Hinblick auf die Bestimmung der Wandladung aufgegriffen. Die bisherigen Methoden zur Ladungsmessung sind zwar vielfältig, bieten jedoch nur Zugang zur absoluten Wandladung und liefern keine Informationen über ihre Verteilung senkrecht zur Oberfläche oder über die Dynamik der Aufladung.
Beides wäre jedoch für ein mikroskopisches Verständnis der Plasma-Wand-Wechselwirkung notwendig, sodass die Elektronenenergieverlustspektroskopie zur Ladungsbestimmung vorgeschlagen wird. Die Methode wird zunächst anhand einer lokalen Antworttheorie für verschiedene in die Wand eingesetzte Schichtstrukturen ausgelotet und aufgrund vielversprechender Resultate anschließend mittels der im betrachteten Parameterbereich notwendigen nichtlokalen Antworttheorie eingehend untersucht. Diese Theorie erfasst die Anregung von Resonanzen höherer Moden, die sich als besonders sensitiv auf die zusätzlichen Ladungsträger erweisen. Insgesamt wird ein experimenteller Aufbau mit einer geeigneten, in die Plasmakammerwand einsetzbaren Schichtstruktur vorgeschlagen, mit dem die Wandladung durch Elektronenenergieverlustspektroskopie bestimmt werden könnte.
This work presents the first experimental investigation of the gas balance on the optimized modular stellarator Wendelstein 7-X (W7-X). A balance of all injected and removed particles and a measurement of internal particle reservoirs allows inference of the bound particle reservoir in the wall, which is of interest due to its effects on plasma density control and fuel retention. Different scenarios of the gas balance are presented with data from the operation campaign 1.2 with an inertially cooled graphite divertor. Both net outgassing and net retention scenarios are presented and W7-X is found to operate stable in a wide range of scenarios with varying wall conditions.
Since fusion experiments are conducted in ultra-high vacuum, suitable gauges are required for total and partial pressure measurement. The challenges and opportunities of the operation of pressure gauges in the steady magnetic field extending beyond plasma pulses are discussed. The performance of newly improved neutral pressure gauges, based on crystal cathode emitters is quantified. These provide improved operational robustness since they can be operated for long periods of time in strong magnetic fields. A crystal cathode setup and and its operation performance is presented along with a fast calibration scheme.
Partial pressure measurements provide additional important information complementing the total neutral pressure measurements, and allowing additional physics insights. As part of this thesis work, a new diagnostic of this kind was implemented on W7-X, the so-called diagnostic residual gas analyzer (DRGA). It provides a wealth of information on various neutral gas species, with a relatively high time resolution - of order a few seconds. The diagnostic setup and its first results are presented in this thesis.
In this thesis, I present work motivated, in part, by a series of upcoming laboratory experiments (APEX), which seeks to uncover some of the inner workings of turbulence and stability in electron- positron plasmas in closed field-line systems. I present the results of several distinct, but connected, problems addressing the theory of electron-positron plasmas.
This work is partitioned into several parts, which loosely correspond to different particulars of the APEX experiment and the different theoretical physics problems which reside within.
I begin with the derivation of a kinetic theory for plasmas which are optically thin to cyclotron emission, as indeed, experimental pair plasmas are expected to be. The results of this section include: (1) the derivation of a general kinetic theory of cyclotron radiation in electron-ion plasmas; (2) a calculation showing that cyclotron emission results in strongly anisotropic distribution functions on the radiation timescale; (3) calculation of the evolution of the distribution function under collisional scattering which, in the absence of any radiation terms, acts to drive the plasma towards a Maxwellian; (4) generalisation of this theory to more exotic geometries; (5) specialisation of this theory to pair plasmas of experimental interest; and (6) presentation of the applications and the limitations of this theory.
The second project is primarily concerned with non-neutral plasmas. We begin with gyrokinetic theory and a novel extension of this theoretical framework to plasmas with arbitrary degree of neutrality in straight field-line geometry. I go on to discuss the gyrokinetic stability theory of such plasmas in this simplified geometry. I conclude this project with a discussion of some further
nuances in the theory of singly-charged non-neutral plasmas, this time concerning global features. Namely, I declare an interest in the equilibria such plasmas might be able to attain.
The final project pertains to plasmas which are in state of Maxwellian equilibrium i.e., electron- positron plasmas with sufficiently large number densities of each species to attain a stationary quasineutral plasma. To this end, I present gyrokinetic flux-tube simulations of electron-positron plasmas in complex, and experimentally relevant, magnetic geometries on the road towards a study of gyrokinetic turbulence. The results of this work include: (1) the first simulations of electron- positron plasmas in a stellarator and ring-dipole geometry; (2) an analytic theory of trapped particle modes in electron-positron plasmas, a result which can also be verified numerically; and (3) extension of several important theoretical results in electron-positron plasmas to experimentally relevant geometries. The culmination of this project is the roadmap ahead towards demonstration of the so-called “inward pinch” effect in electron-positron plasmas in a magnetic Z-pinch.
In this work, studies with respect to the exhaust problem were performed
in the stellarator experiment Wendelstein 7-X with different target concepts and different magnetic field geometries. Different infrared cameras were used to study the heat flux from the plasma onto the PFC. In the first publication, the limiter set-up was used with a simpler magnetic topology in the plasma edge. The radial fall-off of the parallel heat flux for inboard limiters in W7-X shows, similar to inboard limiters in tokamaks, two different radial fall-off lengths, a short (narrow) one, characterizing the near-SOL, and a long (broad) characterizing the far-SOL. For the far-SOL, the heating power and connection length have been identified as the main scaling parameters, while for the near-SOL, the electron temperature close to the LCFS has been identified as the main scaling parameter. The two fall-off lengths differ by a factor 10, and the found scalings for both regimes differ from known models and experimental scalings in tokamaks. A turbulent-driven feature was discussed in the publication as a possible explanation for the behavior of the fall-off length in W7-X.
The gained information and data have been further used to support many
other publications, covering the symmetry of the heat loads, the
energy balance of the machine, and seeding experiments.
The heat exhaust in W7-X with an island divertor was studied in the second
and third publication. Definitions of parameters such as peaking factor and
wetted area were applied for the heterogeneous heat flux pattern on the
W7-X divertor. It was shown that the island divertor concept is capable
of spreading out the heat efficiently, resulting in large wetted areas of up to 1.5 m2. The reached values for the wetted area are comparable to the ones of the larger tokamak JET but with a much smaller ratio of wetted
area to the area of the last closed flux surface. Furthermore, a positive
scaling of the wetted area with the power in the SOL was observed. This
scaling is beneficial for future reactors but needs further investigation of the involved transport processes. The peaking factor (discussed in the second publication) describes how concentrated the heat load is within the region of the strike line. It was shown that this factor is decreasing for increasing densities without affecting the wetted area. The present work paves the way for further analysis of the transport processes of the heat flux towards the island divertor of Wendelstein 7-X.
The active screen plasma nitrocarburizing (ASPNC) technology is a state-of-the-art plasma-assisted heat treatment for improving surface hardness and wear resistance of metallic workpieces based on thermochemical diffusion. In comparison to conventional plasma nitrocarburizing, the use of an active screen (AS) improves thermal homogeinity at the workload and reduces soot formation. Further it can serve as a chemical source for the plasma processes, e.g. by use of an AS made of carbon-fibre reinforced carbon. This compilation of studies investigates the plasma-chemical composition of industrial- and laboratory-scale ASPNC plasmas, predominantly using in-situ laser absorption spectroscopy with lead-salt tuneable diode lasers, external-cavity quantum cascade lasers, and a frequency comb. In this way the temperatures and concentrations of the dominant stable molecular species HCN, NH3, CH4, C2H2, and CO, as well as of less prevelant species, were recorded as functions of e.g. the pressure, the applied plasma power, the total feed gas flow and its composition. Additionally, the diagnostics were applied to a chemically similar plasma-assisted process for diamond deposition.
Resulting from this thesis are new insights into the practical application of an AS made of CFC, the plasma-chemistry involving hydrogen, nitrogen, and carbon, and the particular role of CO as an indicator for reactor contamination. The effect of the feed gas composition on the resulting nitrogen- and carbon-expanded austenite layers was proven by combination of in-situ laser absorption spectroscopy with post-treatment surface diagnostics. Furthermore this work marks the first use of frequency comb spectroscopy with sub-nominally resolved Michelson interferometry for investigation of a low-pressure molecular discharge. This way the rotational bands of multiple species were simultaneously measured, resulting in temperature information at a precision hitherto not reached in the field of nitrocarburizing plasmas.
Es wurde eine Methode zur Herstellung ultradünner Filme aus Metall bzw. metallischen Verbindungen (Legierungen) etabliert. Die Struktur und die physikalischen Eigenschaften der Filme wurden untersucht. Die entwickelte Präparationsmethode beruht auf induzierter Filmkontraktion nach erzwungener Benetzung (iFCaFW). Die Filme bestehen aus ultradünnen vertikal heterostrukturierten Multischichten (2D-VHML), sie entstehen durch den Beschichtungsvorgang und bestehen aus jeweils einer nm-dicken metallischen Schicht (M) eingebettet zwischen zwei Metall(hydr)oxidschichten (MOxHy) im nm- bis sub-nm Bereich. Dieser vertikal heterostrukturierte Aufbau wurde bei allen untersuchten Filmmaterialien beobachtet. Alle in dieser Arbeit vorgestellten Schichtsysteme wurden unter atmosphärischem Druck hergestellt. Es konnten Substrate aus Silicium und Muskovit sowie aus Borosilikat- und Kalk-Natron-Glas (Objektträger) beschichtet werden. Jede, aus flüssigem Metall bzw. flüssiger Legierung hergestellte Schicht verfügt über eine feste (Hydr)oxidschicht an der Luftgrenzfläche. Diese feste (Hydr)oxidschicht fungiert als Substrat für die nächste darüber aufgebrachte Schicht aus flüssigem Metall bzw. flüssiger Legierung. Somit entstehen vertikal heterostrukturierte Multischichten durch identische Wiederholung des Beschichtungsvorgangs. Dies ist eine innovative und vergleichsweise umweltfreundliche Methode, um transparente, elektrisch leitfähige und lateral homogene nm-dünne ein- oder mehrschichtige Metallfilme herzustellen. Verwendet wurden Metalle mit sehr niedriger Schmelztemperatur (kleiner als 300 °C), wie Bismut, Gallium, Indium, Zinn und ihre Legierungen. Die hohe Oberflächenspannung der geschmolzenen Metalle und Legierungen sowie die Adhäsion mit der die (Hydr)oxidhaut dieser Metalle und Legierungen auf verschiedenen Substraten haftet ermöglicht die Beschichtungsmethode.
This thesis contains studies on a special class of topological insulators, so called anomalous Floquet topological insulators, which exclusively occur in periodically driven systems. At the boundary of an anomalous Floquet topological insulator, topologically protected transport occurs even though all of the Floquet bands are topologically trivial. This is in stark contrast to ordinary topological insulators of both static and Floquet type, where the topological invariants of the bulk bands completely determine the chiral boundary states via the bulk-boundary correspondence. In anomalous Floquet topological insulators, the boundary states are instead characterized by bulk invariants that account for the full dynamical evolution of the Floquet system.
Here, we explore the interplay between topology, symmetry, and non-Hermiticity in two-dimensional anomalous Floquet topological insulators. The central results of this exploration are (i) new expressions for the topological invariants of symmetry-protected anomalous Floquet topological phases which can be efficiently computed numerically, (ii) the construction of a universal driving protocol for symmetry-protected anomalous Floquet topological phases and its experimental implementation in photonic waveguide lattices, (iii) the discovery of non-Hermitian boundary state engineering which provides unprecedented possibilities to control and manipulate the topological transport of anomalous Floquet topological insulators.
Experience in the construction of optimized stellarators shows the coil system is a significant challenge. The precision necessary allow the generation of accurate flux surfaces in recent experiments affected both cost and schedule negatively. Moreover, recent experiments at Wendelstein 7-X have shown that small field corrections were necessary for the operation of specific desired magnetic configurations. Therefore, robust magnetic configurations in terms of coil geometry and assembly tolerances have a high potential to facilitate swifter and less expensive construction of future, optimized stellarators. We present a new coil optimization technique that is designed to seek out coil configurations that are resilient against 3D coil displacements. This stochastic version of stellarator coil optimization uses the sampling average approach to incorporate an iterative perturbation analysis into the optimization routine. The result is a robust magnetic configuration that simultaneously reproduces the target magnetic field more accurately and leads to a better fusion performing coil configuration.
The non-renewable energy sources coal, oil and natural gas that contribute the major share of the world's energy, will be running out in the next 40-80 years. With the growing energy demands especially in developing countries, which is likely to surpass that of the developed countries in next 50 years, an alternate energy source is the need to the hour. The nuclear fusion energy is foreseen as one of the potential candidates to solve the current global energy crisis. One of the major challenges faced by the fusion community is the problem of power exhaust. With the larger fusion devices to be built in the future, the heat load on the plasma facing components are expected to grow higher. The present work explores two numerical studies performed on the Wendelstein 7-X, the world's largest stellarator type fusion device, to cope with this problem.
The first project on `'Numerical Studies on the impact of Connection Length in Wendelstein 7-X'' identifies magnetic configuration with long connection lengths, which could bring down the peak heat fluxes onto the divertor to manageable levels, by greater role of cross-field transport which may assist to get a wider heat deposition profile. The second project on `'Development of Heating Scenario to Reduce the Impact of Bootstrap Currents in Wendelstein 7-X'' advocates a novel self-consistent approach to reach high plasma density at full heating power without overloading the divertor during the transient phase of the evolution of the toroidal plasma current, by controlling two parameters; density and power. The aim of both the projects is to contribute to tackling the challenge of the tremendous power exhaust from fusion plasma which, if solved, will be a large step closer to a fusion power plant.
An experimental investigation of particle parallel flows has been carried out at Wendelstein 7-X (W7-X), one of the most advanced stellarators in the world. The studies are restricted to the outermost plasma region, the scrape-off layer (SOL), which is shaped to tackle the exhaust problem in vision of future fusion reactors based on plasma magnetic confinement. The aim of the measurements is to set the basis for a physics analysis of the SOL dynamics by obtaining direct information on convective heat transport, together with the assessment of the predominant flow directions of the main plasma ions and of fusion-products or wall-released impurities. In this way, a better comprehension of the interplay between the transport parallel and perpendicular to the SOL field lines can be achieved, contributing to the understanding of the effectiveness of the island divertor configuration.
The chosen instrument for the experimental studies is the Coherence Imaging Spectroscopy (CIS) diagnostic, a camera-based interferometer capable of measuring 2D Doppler particle flows associated with a selected visible line from the plasma. The diagnostic is distinguished by its high time resolution and spatial coverage, allowing the visualisation and measurements of flow velocities for a full module of W7-X simultaneously. A CIS diagnostic has been fully designed for W7-X with an improved level of accuracy achieved thanks to the implementation of a new calibration source, a continuous-wave-emission tunable laser. The laser allowed a full characterization of the diagnostic and a frequent precise calibration, making the CIS system reliable for parallel flow investigations during the operational campaign OP1.2. The validity and importance of the CIS measurements have been further confirmed with dedicated simulation of the SOL plasma parameters by the EMC3-EIRENE code, and by comparisons with other edge diagnostics. The CIS results show the effects related to dynamical changes in the SOL due to impurity gas puffs or the development of a plasma current. Moreover, CIS can be used as a powerful tool to test the limits of the current theoretical models, for example in the case of forward and reversed field experiments.
Barrier corona (BC) arrangements are employed in different plasma-based applications such as material surface and exhaust gas treatments. However, a comprehensive study about the discharge behavior and properties in such strongly asymmetric arrangements is still missing. This dissertation is devoted to the detailed investigation of single microdischarges (MDs) in a sinusoidally driven BC discharge in air at atmospheric pressure. The discharge arrangement consist of a sharp metal pin and a dielectric-covered hemispherical electrode. It is the first study of volume BC discharges, in which phasially-resolved spatio-temporal development of the MDs are recorded using a multi-dimensional time-correlated single photon counting (TC-SPC) technique. The morphology of the MDs is recorded using an ICCD camera. A voltage probe and a current probe are employed to measure the applied voltage and current pulses. Furthermore, phase-resolved current measurements and statistical studies of current pulse amplitudes are realized using an oscilloscope.
Due to the asymmetric geometry and material of the electrodes, discharge behavior in the two polarities of the applied sinusoidal voltage is significantly different. For the voltage amplitude being applied, mostly two MDs appear in the anodic pin half-cycles. It is observed that the breakdown mechanism in both MDs is a positive streamer starting near the anode, similar to the single MDs in symmetric dielectric barrier discharges (DBDs). However, the second MDs have different properties, such as longer duration of the bulk plasma and broader current pulses. It is considered that the differences are mainly due to the positive surface charges deposited by the first MDs on the dielectric. It is proposed, for the first time, that the current pulse derivative maximum corresponds to the arrival of the streamer head at the cathode surface. This is used to synchronize the spatio-temporal development of the MDs with their current pulses. The accuracy of the synchronization is limited to the rise-time of the current probe (350 ps). In each cathodic pin half-cycle, only one major MD appears. The appearance and amplitude of the MDs are more erratic compared to the anodic pin polarity. The TC-SPC recordings show that the MDs appearing at low applied voltages have a similar spatio-temporal development to the MDs of the anodic pin polarity. On the other hand, at high applied voltages a development similar to transient sparks, i.e. a double-streamer starting near the tip of the pin (cathode), is observed. The statistical study shows that in DBD-like MDs the current pulse amplitude is not dependent on the appearance phase (or applied voltage), but this is not the case for the transient sparks.
Since BC reactors are also used for air cleaning, a set of experiments is done with 35 ppm toluene additive. It is observed that adding toluene results in 500~V lower breakdown voltage. Hence, the discharge in the presence of toluene is operated under over-voltage condition, resulting in stronger MDs in the anodic pin, and earlier-appearing as well as weaker MDs in the cathodic pin half-cycles.
The results of this dissertation about the spatio-temporal development and statistical behavior of the single MDs are foreseen to be employed in the study and optimization of plasma reactors, such as "Stacked DBD Reactor," which are developed for exhaust gas and material surface treatment. Furthermore, the results are a benchmark for the study of a novel discharge arrangement with a rotating dielectric electrode.
With this thesis, studies which form the bedrock for the long term goal of first wall heat load control and optimization for the advanced stellarator Wendelstein 7-X are developed, described and put into context. It is laid out how reconstruction of features of the edge magnetic field from plasma facing component heat loads is an important first step and can successfully be achieved by artificial neural networks. A detailed study of plasma facing component heat load distribution, potential overloads and overload mitigation possibilities is made in first order approximation of the impact of the main plasma dynamic effects.
This thesis describes recent developments in multi-reflection time-of-flight mass spectrometry (MR-ToF MS) with ions exhibiting large masses and mass differences at an MR-ToF setup at the University of Greifswald. A series of in-trap manipulation techniques to selectively retain or eject ion bunches of multiple species with disparate mass-to-charge ratios is investigated. These highlight the possibility to correct long-term flight-time drifts using a reference ion species far away in mass from the species of interest and also the ability to use such a pair to perform single-reference precision mass determinations. In both cases, the results obtained with disparate-mass ion pairs are comparable to those known from operation with isobaric species.
In addition, an in-trap photoexcitation technique is developed and applied to study the dissociation behavior of atomic bismuth clusters (systems of some number of bismuth atoms). Compared to previous works by other groups, the probed cluster-size range is expanded for both ion polarities, resulting in a more comprehensive picture of the underlying dissociation pathways. The known significance of neutral-tetramer breakoff is confirmed, however, evidence is also found for the loss of larger neutral fragments.
Lastly, the principle of tandem high-resolution MR-ToF MS is introduced. This new method allows the study of the change in dissociation behavior of the cationic bismuth octamer resulting from substituting one of its atoms for lead. It is found that the lead-doping opens new preferential fragmentation pathways that outstrip the dominant tetramer breakoff for this specific precursor cluster size. As a first proof-of-principle experiment, the case of the cationic octamer shows that tandem MR-ToF MS is well-suited for the investigation of compound clusters.
The importance of ion propulsion devices as an option for in-space propulsion of space
crafts and satellites continues to grow. They are more efficient than conventional chemi-
cal thrusters, which rely on burning their propellant, by ionizing the propellant gas in a
discharge channel and emitting the heavy ions at very high velocities. The ion emission
region of a thruster is called the plume and extends several meters axially and radially
downstream from the exit of a thruster. This region is particularly important for the effi-
ciency of a thruster, because it determines energy and angular distribution of the emitted
ions. It also determines the interaction with the carrier space craft by defining the electric
potential shape and the fluxes and energies of the emitted high energy ions, which are the
key parameters for sputter erosion of satellite components such as solar panels. Developing
new ion thrusters is expensive because of the high number of prototypes and testing cycles
required. Numerical modeling can help to reduce the costs in thruster development, but
the vastly differing length and time scales of the system, particularly the large differences of
scales between the discharge chamber and the plume, make a simulation challenging. Often
both regions are considered to be decoupled and are treated with different models to make
their simulation technically feasible. The coupling between channel and plume plasmas and
its influence on each other is disregarded, because there is no interaction between the two
regions. Therefore, this thesis investigates the physical effects which arise from this cou-
pling as well as models suitable for an integrated simulation of the whole coupled problem
of channel and plume plasmas. For this purpose the High Efficiency Multistage Plasma
Thruster (HEMP-T) ion thruster is considered.
For the discharge channel plasma, a fully kinetic model is required and the Particle-in-Cell
(PIC) method is applied. The PIC method requires very high spatial and temporal resolu-
tions which makes it computationally costly. As a result, only the discharge channel and the
near-field plume close to the channel exit can be simulated. In the channel, the results show
that electrons are magnetized and follow the magnetic field lines. The orientation of the
magnetic field there is mostly parallel to the symmetry axis and the channel walls which re-
sults in a high parallel electron transport and leads to a flat electric potential and a reduced
plasma-wall sheath. Only at the magnetic cusps, which are characteristic of HEMP-Ts the
electrons are guided towards the wall, with ions following due to quasineutrality, where a
classical plasma-wall sheath develops. The ion-wall contact is thus limited to the cusp re-
gion. The small radial drop of the potential towards the wall gives rather low energies of
ions impinging at the wall and minimizes erosion in the HEMP-T.
In the near-field plume, which extends from the thruster exit plane to some centimeters
downstream, the ion emission characteristics is defined. The ratio of radial and axial elec-
tric field components in this region determines the ion emission angle which should be
minimized for maximum thruster efficiency. The plasma discharge in the channel produces
high plasma densities and the subsequent drop from plasma to vacuum potential occurs
further downstream for higher densities. This increases the ratio of radial and axial electric
field components because the plasma expands radially outside of the confinement from the
dielectric discharge channel walls. The potential structure in the near-field plume impacts
also the supply of electrons for the channel discharge because the electrons enter the channel
from the plume. An effect which arises from this coupling is the breathing mode oscilla-
tion. It is an oscillation which is observed in all plasma quantities and is located near the
thruster exit. The oscillation frequency measured in the simulation is in good agreement
with a predator-prey estimate which validates this ansatz. However, the electron tempera-
ture, assumed constant in the predator-prey model, correlates inversely with the oscillation,
i.e. it is minimal at the current maximum and vice versa, which contributes to the observed
oscillations. Because of the oscillation of the plasma number density, the potential drop also
oscillates in the exit region and thus the ratio of radial to axial electric field components,
which results in the oscillation of the mean ion emission angle.
Regarding suitable models for a combined simulation of channel and plume plasmas, the
PIC model for channel and near-field plume is explicitly coupled to a hybrid fluid-PIC
model for the plume. The latter treats the electrons as a fluid, hence increasing the effective
spatial and temporal resolutions which can be applied in the plume simulations at the cost
of reduced accuracy of the electron model. Plasma densities decrease by two orders of
magnitude two meters downstream from the channel exit. The explicitly coupled kinetic
and hybrid PIC models are well suited for the computation of a HEMP-T and its plume
expansion, but they disregard the coupling of channel and plume plasmas for which other
methods are necessary. For this purpose a new approach is presented with a proof-of-
principle validation. The limited spatial resolution in the plume can be overcome with the
mesh-coarsening method, which increases the resolution in regions of low plasma density
without numerical artifacts. Sub-cycling for the electrons in the plume can then be used
to increase the temporal resolution in the plume. The combination of both methods, called
the sub-cycling mesh-coarsening (SMC) algorithm in the scope of this work, promises high
savings in computational cost which can make a combined simulation of plume and channel
plasmas feasible.
In this doctoral thesis, algorithms are presented that are designed for the investigation in the mesopause region between the upper Mesosphere and Lower Thermosphere (MLT). The photochemical models are proposed and applied to represent the oxygen airglow and the oxygen photochemistry in the MLT. Atomic oxygen, O, in the ground state, O(3P), is of special interest because it is a reactive trace gas actively contributing to the Earth’s airglow. The retrievals of O(3P) concentrations, [O(3P)], are based on the nightglow time series of the green line emission measured remotely as in the first part of this thesis and the individual profiles of multiple nightglow emissions of O and molecular oxygen (O2) measured in situ as in the second part of this thesis. To process the complete spectral time series measured by using the satellite-borne instrument SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY), an intricate set of algorithms is developed and applied with the regularized total least squares minimization approach to estimate a set of the optimal regularization parameters and to retrieve a corresponding set of vertical Volume Emission Rate (VER) profiles. Furthermore, these algorithms take emissions of another origin and the Earth's shape into account. Considering not identified states of O2, the established photochemical models are adjusted resulting in two model modifications. Both model modifications are employed to retrieve the [O(3P)] time series on the basis of the VER time series in the MLT. The model input parameters vary in the atmosphere that motivated to propose these two model modifications and to employ available sources of the input parameters. One semi-empirical model, one general circulation model and the satellite-borne instrument SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) are employed as sources of the reference [O(3P)] and input parameters time series. The SABER instrument employed as a source of the input parameters is preferred according to the comparison of the retrieved and reference [O(3P)] time series. Studying the impact of the 11-year solar cycle on O(3P) in the MLT, an algorithm is developed and applied with the Levenberg-Marquardt algorithm to estimate the optimal fit parameters step-wise. The results of the O(3P) sensitivity analysis obtained with respect to the solar activity forcing at the 11 year and 27 day time scales and the lunar gravitational forcing agree with the reference model simulations. The hypothesis regarding vertical shifts between different of Meinel bands at least partly caused by the hydroxyl radical (OH*) quenching with O(3P) is confirmed experimentally. Based on the conclusion drawn in the first part of this thesis that the data sets’ self-consistency is high as for the averaged SABER and SCIAMACHY measurements, a comprehensive set of available data with a higher level of the data sets’ self-consistency is employed in the second part of this thesis. Multiple airglow emissions measured in situ during four campaigns are employed to propose the Multiple Airglow Chemistry (MAC) model. Processed emissions are the Herzberg I, Chamberlain, Atmospheric and Infrared Atmospheric band emissions of O2 and the green line emission of O. Considering all widely known and additionally complemented reactions, the MAC model is proposed to represent the oxygen airglow and the oxygen photochemistry in the MLT. The presented MAC model is based on the hypothesis of Slanger et al. (2004) stating that higher excited states of O2 are coupled with each other through vibronic de-excitation caused by collisions among molecules of this group of O2 states in the MLT. This hypothesis is modified excluding the singlet Herzberg state of O2 from the group of O2 states considered by Slanger et al. (2004). The MAC calculations are carried out sequentially starting with higher excited O2 states to provide the retrieved output concentrations of these O2 states as the input concentrations to the next calculation steps. The final step is only based on concentrations of all species, whereas each of the earlier steps is based on a corresponding VER profile besides of the input concentrations. The oxygen photochemistry in the MLT is represented by all species considered at the final step that makes it possible to adopt the MAC reactions in a general circulation model. Four modifications of the MAC model, i.e. including or excluding the triplet Herzberg states of O2 and including or excluding ozone and odd hydrogen (hydrogen, OH* and hydroperoxy radical), lead to negligible differences in the retrieved [O(3P)] profiles. Based on the MAC calculations verified and validated on the basis of the four rocket campaigns, one of the effective modifications of the MAC model (excluding the triplet Herzberg states of O2, ozone and odd hydrogen) is further reduced to the most effective modification. This implies that for the [O(3P)] retrieval only the O2 Atmospheric band emission, temperature and concentrations of molecular nitrogen (N2) and O2 are sufficient to apply. Calculations carried out by using the most effective modification of the MAC model are verified and validated on the basis of self-consistent in situ measurements obtained simultaneously. The MAC model enables identifying precursors of (1) the three lowest O2 valence states and (2) the second excited O state responsible for (1) the Atmospheric and Infrared Atmospheric band emissions of O2 and (2) the green line emission of O, respectively. Particularly, the singlet Herzberg state of O2 is identified as the major precursor of the second excited O state resulting in the green line emission. In focus of potential further research is an extension of the MAC model with vibrationally excited states of O2 and ionized species.
In this work, we theoretically investigate both aspects of charge-transferring atom-surface collisions: local-moment-type correlations and emission of secondary electrons from surfaces. Ideally, one chooses an approach that keeps as many electronic and lattice degrees of freedom at an ab-initio level as possible. In practice, however, this sophistication is hard to maintain. In this work, we do not aim to perform a description from first principles which could utilize density functional theory or quantum-chemical techniques. Instead, we keep only the most important degrees of freedom of the scattering process and use effective models for them. These are basically the Anderson-impurity model leading to time-dependent Anderson-Newns Hamiltonians and Gadzuk’s semiempirical approach to describe the projectile-target interaction from classical image shifts. In direct comparison with the description from first principles, the semiempirical approach offers a flexible basis for the modeling of a great variety of projectile-target combinations. The addition of further effective models to increase the general quality of the results is possible since the approach is very modular. The clear physical interpretation of each effective model, as well as the requirement for only a few and generally available parameters are further advantages of this approach. Rewritten in terms of Coleman’s pseudo-particle operators, the model is then numerically analyzed. This is done within a non-crossing approximation for the hybridization self-energies which are utilized by contour-ordered Green functions for each relevant electronic state of the projectile.