Refine
Year of publication
- 2021 (24) (remove)
Document Type
- Article (15)
- Doctoral Thesis (9)
Has Fulltext
- yes (24)
Is part of the Bibliography
- no (24)
Keywords
- - (13)
- dusty plasma (4)
- Komplexes Plasma (2)
- Wendelstein 7-X (2)
- electron–hole plasma (2)
- 3D (1)
- 4D flow MRI (1)
- 7755384-6 (1)
- ASDEX Upgrade (1)
- Bayes-Verfahren (1)
- Biomolecules (1)
- DIT (1)
- Detachment (1)
- Digital ion trap (1)
- Dirac-cone physics (1)
- Dissertation (1)
- Divertor (1)
- Elektronenstreuung (1)
- FTIR spectroscopy (1)
- Festkörperphysik (1)
- Fusion , Plasma , Plasmaphysik (1)
- Fusionsreaktor (1)
- Heat flux (1)
- Hybridisierungstheorie (1)
- Ion thrusters (1)
- Langmuir probe diagnostics (1)
- Least-squares method (1)
- Lichtstreuung (1)
- L–H transition (1)
- MR-ToF device (1)
- Madden-Julian Oscillation (1)
- Mie-Theorie (1)
- Modell (1)
- N incorporation (1)
- OLR-based MJO Index (1)
- OMI (1)
- PIC (1)
- PWV (1)
- Particle flux (1)
- Photoströme (1)
- Plasma (1)
- Plasmadiagnostik (1)
- Plasmaphysik (1)
- Plasmasonde (1)
- Plasmonik (1)
- Python (1)
- Quadrupole mass filter (1)
- RF mass Spectrometry (1)
- RNS (1)
- Rydberg excitons (1)
- S/XB coefficient (1)
- Seebeck effect (1)
- Sheath transmission coefficient (1)
- Simulation (1)
- Stellarator (1)
- Strikeline (1)
- Ti-Cu-N coating (1)
- Transmission electron microscopy (1)
- WSS (1)
- X-ray diffraction (1)
- X-ray photoelectron spectroscopy (1)
- actin cytoskeleton (1)
- actin quantification (1)
- anti-adhesive surface (1)
- aortic arch (1)
- atherosclerosis (1)
- atmosphere (1)
- atmospheric pressure plasma (1)
- cell spreading (1)
- cell-material interaction (1)
- charge measurement (1)
- climate (1)
- cold physical plasma (1)
- combination therapy (1)
- coupled phonon-plasmon modes (1)
- density limit (1)
- dielectric response (1)
- dust-density waves (1)
- energy-resolved ion mass spectrometry (1)
- flow dynamics (1)
- fluorescence (1)
- food quality (1)
- ftir spectroscopy (1)
- hollow cathode discharge (1)
- inflammatory/immunological response (1)
- intramuscularly implantation (1)
- intraseasonal variation (1)
- laser spectroscopy (1)
- leafy greens (1)
- low-temperature plasma polymerization (1)
- magnetic fields (1)
- magnetron sputtering (1)
- magnetron sputtering discharge (1)
- mapping (1)
- mathematical modeling (1)
- medical gas plasma technology (1)
- melamine-formaldehyde (1)
- microwave-driven discharge (1)
- mouse (1)
- nanowire (1)
- networks (1)
- neural (1)
- non-thermal processing (1)
- oncology (1)
- osteoblasts (1)
- particle (1)
- phase separation (1)
- photocurrent (1)
- plaque characteristics (1)
- plasma-fluorocarbon-polymer (1)
- positively biased anode (1)
- quantum corrections (1)
- quasi-two-dimensional systems (1)
- reactive oxygen species (1)
- ready-to-eat produce (1)
- reconstruction (1)
- scanning ion conductance microscopy (1)
- scattering processes (1)
- separatrix (1)
- similarity scaling (1)
- spin-polarized current (1)
- surface charge sensing (1)
- terahertz ellipsometry (1)
- terahertz spintronics (1)
- terahertz time‐domain spectroscopy (1)
- titanium (Ti) alloys (1)
- tokamak (1)
- topological insulator (1)
- topologische Isolatoren (1)
- tumor immunology (1)
- turbulence (1)
- vision (1)
- weather forecasting (1)
Institute
- Institut für Physik (24) (remove)
Publisher
- IOP Publishing (6)
- MDPI (5)
- Elsevier (1)
- Frontiers Media S.A. (1)
- Software Sustainability Institute (1)
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.
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.
AbstractThe performance of a positively biased external ring anode in combination with a hollow cathode (HC) discharge or a magnetron sputtering (MS) discharge, both with a Ti cathode and with Ar as working gas, is investigated. Plasma and floating potential increase as function of anode voltage. Energy-resolved mass spectrometry reveals that the kinetic energy of argon and titanium ions is enhanced by a positive anode voltage allowing for an effective energy control of plasma ions.
AbstractInfrared IR absorption spectra of melamine-formaldehyde (MF) microparticles confined in an rf plasma are studied at different plasma conditions. Several absorption peaks have been analysed in dependence of plasma power and their temporal evolution. For comparison, the IR absorption spectra of heated MF microparticles without plasma exposition are used to determine the general influence of the temperature on the IR spectra. Measuring the temperature of the particles inside the plasma shows that the temperature is not the only process changing the particles’ IR spectra. Chemical changes of the MF particles with increasing plasma power influence the absorption peak structure.
AbstractThe efficient operation of a tokamak is limited by several constraints, such as the transition to high confinement or the density limits occurring in both confinement regimes. These particular boundaries of operation are derived in terms of a combination of dimensionless parameters describing interchange-drift-Alfvén turbulence without any free adjustable parameter. The derived boundaries describe the operational space at the separatrix of the ASDEX Upgrade tokamak, which is presented in terms of an electron density and temperature existence diagram. The derived density limits are compared against Greenwald scaling. The power threshold and role of ion heat flux for the transition to high confinement are discussed.
Fast 3D particle reconstruction using a convolutional neural network: application to dusty plasmas
(2021)
AbstractWe present an algorithm to reconstruct the three-dimensional positions of particles in a dense cloud of particles in a dusty plasma using a convolutional neural network. The approach is found to be very fast and yields a relatively high accuracy. In this paper, we describe and examine the approach regarding the particle number and the reconstruction accuracy using synthetic data and experimental data. To show the applicability of the approach the 3D positions of particles in a dense dust cloud in a dusty plasma under weightlessness are reconstructed from stereoscopic camera images using the prescribed neural network.
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.
Growth, ageing and atherosclerotic plaque development alter the biomechanical forces acting on the vessel wall. However, monitoring the detailed local changes in wall shear stress (WSS) at distinct sites of the murine aortic arch over time has been challenging. Here, we studied the temporal and spatial changes in flow, WSS, oscillatory shear index (OSI) and elastic properties of healthy wildtype (WT, n = 5) and atherosclerotic apolipoprotein E-deficient (Apoe−/−, n = 6) mice during ageing and atherosclerosis using high-resolution 4D flow magnetic resonance imaging (MRI). Spatially resolved 2D projection maps of WSS and OSI of the complete aortic arch were generated, allowing the pixel-wise statistical analysis of inter- and intragroup hemodynamic changes over time and local correlations between WSS, pulse wave velocity (PWV), plaque and vessel wall characteristics. The study revealed converse differences of local hemodynamic profiles in healthy WT and atherosclerotic Apoe−/− mice, and we identified the circumferential WSS as potential marker of plaque size and composition in advanced atherosclerosis and the radial strain as a potential marker for vascular elasticity. Two-dimensional (2D) projection maps of WSS and OSI, including statistical analysis provide a powerful tool to monitor local aortic hemodynamics during ageing and atherosclerosis. The correlation of spatially resolved hemodynamics and plaque characteristics could significantly improve our understanding of the impact of hemodynamics on atherosclerosis, which may be key to understand plaque progression towards vulnerability.
The Madden-Julian Oscillation (MJO) is a prominent feature of the intraseasonal variability of the atmosphere. The MJO strongly modulates tropical precipitation and has implications around the globe for weather, climate and basic atmospheric research. The time-dependent state of the MJO is described by MJO indices, which are calculated through sometimes complicated statistical approaches from meteorological variables. One of these indices is the OLR-based MJO Index (OMI; OLR stands for outgoing longwave radiation). The Python package mjoindices, which is described in this paper, provides the first open source implementation of the OMI algorithm, to our knowledge. The package meets state-of-the-art criteria for sustainable research software, like automated tests and a persistent archiving to aid the reproducibility of scientific results. The agreement of the OMI values calculated with this package and the original OMI values is also summarized here. There are several reuse scenarios; the most probable one is MJO-related research based on atmospheric models, since the index values have to be recalculated for each model run.
This thesis discusses three publications in the field of dusty plasmas.
In the first section, measurements of the ir absorption of silica nanoparticles confined in an argon radiofrequency plasma discharge using a Fourier transform infrared spectrometer have been performed. By varying the gas pressure of the discharge and duty cycle of the applied radiofrequency voltage, a shift of the absorption peak of silica is observed. This shift is attributed to charge-dependent absorption features of silica. The charge-dependent shift has been calculated for silica particles, and from comparisons with the experiment the particle charge has been retrieved using the infrared phonon resonance shift method. With the two different approaches of changing the gas pressure and altering the duty cycle, one is able to deduce a relative change of the particle charge with pressure variations and an absolute estimate of the charge with the duty cycle.
In the second part, infrared (IR) absorption spectra of melamine-formaldehyde (MF) microparticles confined in an rf plasma are studied at different plasma conditions. Several absorption peaks have been analysed in dependence of plasma power and their temporal evolution. For comparison, the IR absorption spectra of heated MF microparticles without plasma exposition are used to determine the general influence of the temperature on the IR spectra. Measuring the temperature of the particles inside the plasma shows that the temperature is not the only process changing the particles' IR spectra. Chemical changes of the MF particles with increasing plasma power influence the absorption peak structure.
Finally, experiments on dust clusters trapped in the sheath of a radio frequency discharge have been performed for different magnetic field strengths ranging from a few milliteslas to 5.8 T. The dynamics of the dust clusters are analyzed in terms of their normal modes. From that, various dust properties such as the kinetic temperature, the dust charge, and the screening length are derived. It is found that the kinetic temperature of the cluster rises with the magnetic field, whereas the dust charge nearly remains constant. The screening length increases slightly at intermediate magnetic field strengths. Generally, the dust properties seem to correlate with magnetization parameters of the plasma electrons and ions, however only to a small degree.
This thesis describes how the data of the Langmuir probes in the Wendelstein 7-X (W7X) Test Divertor Unit (TDU) were evaluated, checked for consistency with other diagnostics and used to analyse plasma detachment.
Langmuir probes are an electronic diagnostic, and were among the first to be used in plasma physics to determine particle fluxes, potentials, temperatures and densities.
W7X is a large, advanced stellarator, magnetic confinement fusion experiment, operated at the Max-Planck-Institut for Plasma Physics(IPP) in Greifswald, Germany.
Its TDU is an uncooled graphite component, shaped and positioned to intercept the convective heat load of the plasma.
Detachment describes a desirable operation state of strongly reduced loads on this component.
The evaluation of Langmuir probe data relies heavily on models of the sheath, formed at the interface between plasma and a solid surface, to infer plasma parameters from the directly measured quantities.
Multiple such models are analysed, generalised, and adapted to our use case.
A detailed comparison is made to determine the most suitable model, as this choice strongly affects the predicted parameters.
Special attention is paid to uncertainties on the parameters, which are determined using a Bayesian framework.
From the inferred parameters, heat and particle fluxes are calculated.
These are also indirectly measured by two other, camera-based diagnostic systems.
Observations are compared to test the validity of assumptions and calculations in the evaluation of all three diagnostics by checking their results for consistency.
The first comparison, with the infrared emission camera system, shows good agreement with theoretical predictions and reported measurements of the sheath transmission factor, for which we derive and measure a value in W7X.
Parameter dependencies in the quality of this agreement hint at remaining issues.
The second comparison, with the Hydrogen alpha photon flux camera system, shows significant discrepancy with expectations.
These are argued to originate from systematic differences in the measurement locations, which are quantified and related to the magnetic topology.
Langmuir probe observations of individual discharges are analysed to discuss conditions under which detachment occurs, transition into that state and fluctuations observed prior to and during it.
A spatial parametrisation of the data is developed and used to facilitate this.
These observations contribute to the larger aim of understanding particle balance control and fusion plasma edge processes.
Three-dimensionally extended dusty plasmas containing mixtures of two particle species of different size have been investigated on parabolic flights. To distinguish the species even at small size disparities, one of the species is marked with a fluorescent dye, and a two-camera video microscopy setup is used for position determination and tracking. Phase separation is found even when the size disparity is below 5%. Particles are tracked to obtain the diffusion flux, and resulting diffusion coefficients are in the expected range for a phase separation process driven by plasma forces. Additionally, a measure for the strength of the phase separation is presented that allows to quickly characterize measurements. There is a clear correlation between size disparity and phase separation strength.
Molecular dynamics simulations of binary dusty plasmas have been performed and their behavior with respect to the phase separation process has been analyzed. Here as well, it is found that even the smallest size disparities lead to phase separation. The separation is due to the force imbalance on the two species and the separation becomes weaker with increasing mean particle size.
In the second part of the thesis, Experiments on self-excited dust-density waves under various magnetic fields have been performed. For that purpose, different dust clouds of micrometer-sized dust particles were trapped in the sheath of a radio frequency discharge. The self-excited dust-density waves were studied for magnetic field strengths ranging from 0 mT to about 2 T. It was observed that the waves are very coherent at the lowest fields (B < 20 mT). At medium fields (20 mT < B < 300 mT), the waves seem to feature a complex competition between different wave modes before, at even higher fields, the waves become more coherent again. At the highest fields (B > 1 T), the wave activity is diminished. The corresponding wave frequencies and wavenumbers have been derived. From the comparison of the measured wave properties and a model dispersion relation, the ion density and the dust charge are extracted. Both quantities show only little variation with magnetic field strength.
Modern space missions depend more and more on electric propulsion devices for in-space
flights. The superior efficiency by ionizing the feedgas and propelling them using electric
fields with regard to conventional chemical thrusters makes them a great alternative. To
find optimized thruster designs is of high importance for industrial applications. Building
new prototypes is very expensive and takes a lot of time. A cheaper alternative is to rely
on computer simulations to get a deeper understanding of the underlying physics. In order
to gain a realistic simulation the whole system has to be taken into account including the
channel and the plume region. Because numerical models have to resolve the smallest time
and spatial scales, simulations take up an unfeasible amount of time. Usually a self-similarity
scaling scheme is used to greatly speed up these simulations. Until now the limits of this
method have not been thoroughly discussed. Therefore, this thesis investigates the limits
and the influence of the self-similarity scheme on simulations of ion thrusters. The aim
is to validate the self-similarity scaling and to look for application oriented tools to use
for thruster design optimization. As a test system the High-Efficiency-Multistage-Plasma
thruster (HEMP-T) is considered.
To simulate the HEMP-T a fully kinetic method is necessary. For low-temperature plasmas,
as found in the HEMP-T, the Particle-in-Cell (PIC) method has proven to be the best
choice. Unfortunately, PIC requires high spatial and temporal resolution and is hence
computationally costly. This limits the size of the devices PIC is able to simulate as well
as limiting the exploration of a wider design space of different thrusters. The whole system
is physically described using the Boltzmann and Maxwell equations. Using these system
of equations invariants can be derived. In the past, these invariants were used to derive a
self-similarity scaling law, maintaining the exact solution for the plasma volume, which is
applicable to ion thrusters and other plasmas. With the aid of the self-similarity scaling
scheme the computation cost can be reduced drastically. The drawback of the geometrical
scaling of the system is, that the plasma density and therefore the Debye length does not
scale. This expands the length at which charge separation occurs in respect to the system
size. In this thesis the limits of this scaling are investigated and the influence of the scaling
at higher scaling factors is studied. The specific HEMP-T design chosen for these studies is
the DP1.
Because the application of scaling laws is limited by the increasing influence of charge separation with increased scaling, PIC simulations still are computationally costly. Another approach to explore a wider design space is given using Multi-Objective-Design-Optimization
(MDO). MDO uses different tools to generate optimized thruster designs in a comparatively
short amount of time. This new approach is validated using the PIC method. During this
validation the drawback of the MDO surfaces. The MDO calculations are not self-consistent
and are based on empirical values of old thruster designs as input parameters, which not
necessarily match the new optimized thruster design. By simulating the optimized thruster
design with PIC and recalculate the former input parameters, a more realistic thruster design is achieved. This process can be repeated iteratively. The combination of self-consistent
PIC simulations with the performance of MDO is a great way to generate optimized thruster
designs in a comparatively short amount of time. The proof of concept of such a combination
is the pinnacle of this thesis.
The first Therapeutic ROS and Immunity in Cancer (TRIC) meeting was organized by the excellence research center ZIK plasmatis (with its previous Frontiers in Redox Biochemistry and Medicine (FiRBaM) and Young Professionals’ Workshop in Plasma Medicine (YPWPM) workshop series in Northern Germany) and the excellence research program ONKOTHER-H (Rostock/Greifswald, Germany). The meeting showcased cutting-edge research and liberated discussions on the application of therapeutic ROS and immunology in cancer treatment, primarily focusing on gas plasma technology. The 2-day hybrid meeting took place in Greifswald and online from 15–16 July 2021, facilitating a wide range of participants totaling 66 scientists from 12 countries and 5 continents. The meeting aimed at bringing together researchers from a variety of disciplines, including chemists, biochemists, biologists, engineers, immunologists, physicists, and physicians for interdisciplinary discussions on using therapeutic ROS and medical gas plasma technology in cancer therapy with the four main sessions: “Plasma, Cancer, Immunity”, “Plasma combination therapies”, “Plasma risk assessment and patients studies”, and “Plasma mechanisms and treated liquids in cancer”. This conference report outlines the abstracts of attending scientists submitted to this meeting.
Surface Stoichiometry and Depth Profile of Tix-CuyNz Thin Films Deposited by Magnetron Sputtering
(2021)
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.
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.
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.
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.