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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.
In the last decade a new domain has developed in plasma physics: plasma medicine. Despite the successes that have already been achieved in this exciting new field, the interaction of plasmas with “biological materials” is not yet fully understood. Further investigations in particular with respect to the properties of the applied plasmas sources are therefore essential in order to decode this complex interaction process. Currently, a great variety of different discharge types are used in plasma medical investigation which are generally are operated in noble gases like helium and argon or with dry air. In the present work, the main focuses is on the diagnostics of reactive oxygen and nitrogen species (RONS) resulting from the plasma chemistry of an argon radio-frequency (RF) atmospheric pressure plasma jet (APPJ) and its interaction with the ambient atmosphere. To conduct this study, a commercially available plasma device, so-called kinpen is used due to its technical development maturity and its accessibility on the market. As a method of choice, diagnostic techniques are based on optical spectroscopy known to be a reliable tool to investigate plasmas. Consequently, three complementary optical laser diagnostics, namely quantum cascade laser absorption spectroscopy (QCLAS), laser induced fluorescence (LIF) and planar single shot LIF (PLIF), have been successfully applied to the plasma jet itself or its effluent. All of these diagnostics offer a high species selectivity and an excellent spatial and temporal resolution. They are used in this work for i) the characterization of the plasma chemical dynamics with respect to the generation of biological active RONS – in particular for the case of N2 and O2 admixtures. ii) the measurement of the NO density profile in the plasma effluent iii) the investigation of the flow characteristics of the neutral gas component (laminar vs. turbulent) and its influence on the plasma chemistry. Numerical analysis have been carried out in collaboration with PLASMANT (University of Antwerp) via kinetic simulations of the entire plasma chemistry. Expectingly, atomic oxygen (O) and nitric oxide (NO) turn out to be precursors of ozone (O3) and nitric dioxide (NO2). However, it was intriguing to unveil that atomic oxygen and nitrogen metastable (N2(A)) play together a key part --as intermediate species-- in the generation of more stable RONS, e.g. NO. The absolute density of NO space resolved was measured by LIF and absolutely calibrated molecular beam mass spectrometer. LIF was used to determine relative density of OH radical in the plasma plume. 2D-LIF was used to investigate the gas flow pattern with OH as a flow tracer. The results are discussed in details and show different operating mode of the jet, e.g. laminar or turbulent and that the plasma influences these regimes. The first detection and relative measurement by LIF of nitrogen metastable (N2(A)) produced by an argon APPJ is also shortly reported in this work. The outcome of this thesis will bring new insights in the field of argon APPJs chemistry and its interaction with the ambient atmosphere which can be valuable to support plasma modelling and to consider for the applications in plasma medicine.
Magnetic reconnection is a ubiquitous phenomenon observed in a wide range of magnetized plasmas from magnetic confinement fusion devices to space plasmas in the magnetotail. The process enables the release of accumulated magnetic energy by rapid changes in magnetic topology, heating the plasma in the vicinity of the reconnection site, generating fast particles and allowing a wealth of instabilities to grow. This thesis reports on the results from a newly constructed linear, cylindrical and modular guide field reconnection experiment with highly reproducible events, VINETA.II. A detailed analysis of the reconnecting current sheet properties on a macroscopic and microscopic scale in time and space is presented. In the experiment, four parallel axial wires create a figure-eight in-plane magnetic field with an X-line along the central axis, as well as an axial inductive field that drives magnetic reconnection. Particle-in-cell simulations show that the axial current is limited by sheaths at the boundaries and that electrostatic fields along the device axis always set up in response to the induced electric field. Current sheet formation requires an additional electron current source, realized as a plasma gun, which discharges into a homogeneous background plasma created by a rf antenna. The evolution of the plasma current is found to be dominantly set by its electrical circuit. The current response to the applied electric field is mainly inductive, which in turn strongly influences the reconnection rate. The three-dimensional distribution of the current sheet is determined by the magnetic mapping of the plasma gun along the sheared magnetic field lines, as well as by radial cross-field expansion. This expansion is due to a lack of equilibrium in the in-plane force balance. Resistive diffusion of the magnetic field by E=η j is found to be by far insufficient to account for the high reconnection rate E=-dΨ/dt at the X-line, indicating the presence of large electrostatic fields which do not contribute to dissipative reconnection. High-frequency magnetic fluctuations are observed throughout the current sheet which are compared to qualitatively similar observations in the Magnetic Reconnection Experiment (MRX, Princeton). The turbulent fluctuation spectra in both experiments display a spectral kink near the lower hybrid frequency, indicating the presence of lower hybrid type instabilities. In contrast to the expected perpendicular propagation of mainly electrostatic waves, an electromagnetic wave is found in VINETA.II that propagates along the guide field and matches the whistler wave dispersion. Good correlation is observed between the local axial current density and the fluctuation amplitude across the azimuthal plane. Instabilities driven by parallel drifts can be excluded due to the large required drift velocities or low resulting phase velocities that are not observed. It is instead suggested that a perpendicular, electrostatic lower hybrid mode indeed exists that resonantly excites a parallel, electromagnetic whistler wave through linear mode conversion. The resulting fluctuations are found to be intrinsic to the localized current sheet and are independent of the slower reconnection dynamics. Their amplitude is small compared to the in-plane fields, and have a negligible contribution to anomalous resistivity through momentum transport in the present parameter regime.
The collisionless tearing mode is investigated by means of the delta-f PIC code EUTERPE solving the gyrokinetic equation. In this thesis the first simulations of electromagnetic non-ideal MHD modes in a slab geometry with EUTERPE are presented. Linear simulations are carried out in the cases of vanishing and finite temperature gradients. Both cases are benchmarked using a shooting method showing that EUTERPE simulates the linearly unstable tearing mode to a very high accuracy. In the case of finite diamagnetic effects and values of the linear stability parameter Delta of order unity analytic predictions of the linear dispersion relation are compared with simulation results. The comparison validates the analytic results in this parameter range. Nonlinear single-mode simulations are performed in the small- to medium-Delta range measuring the dependency of the saturated island half width on the equilibrium current width. The results are compared with an analytic prediction obtained with a kinetic electromagnetic model. In this thesis the first simulation results in the regime of fast nonlinear reconnection~(medium- to high-Delta range) are presented using the standard gyrokinetic equation. In this regime a nonlinear critical threshold has been found dividing the saturated mode from the super-exponential phase for medium-Delta values. This critical threshold has been proven to occur in two slab equilibria frequently used for reconnection scenarios. Either changing the width of the equilibrium current or the wave number of the most unstable mode makes the threshold apparent. Extensive parameter studies including the variation of the domain extensions as well as the equilibrium current width are dedicated to a comprehensive overview of the critical threshold in a wide range of parameters. Additionally, a second critical threshold for high-Delta equilibria has been observed. A detailed comparison between a compressible gyrofluid code and EUTERPE is carried out. The two models are compared with each other in the linear regime by measuring growth rates over wave numbers of the most unstable mode for two setups of parameters. Analytical scaling predictions of the dispersion relation relevant to the low-Delta regime are discussed. Employing nonlinear simulations of both codes the saturated island half width and oscillation frequency of the magnetic islands are compared in the small-Delta range. Both models agree very well in the limit of marginal instability and differ slightly with decreasing wave vector. Recently, the full polarisation response in the quasi-neutrality equation was implemented in EUTERPE using the Padé approximation of the full gyrokinetic polarisation term. Linear simulation results including finite ratios of ion to electron temperature are benchmarked with the dispersion relation obtained from a hybrid model. Finite temperature effects influence the saturated island width slightly with increasing ion to electron temperature ratio which has been verified by both models.
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.
Magnetic reconnection is a fundamental plasma process where a change in field line connectivity occurs in a current sheet at the boundary between regions of opposing magnetic fields. In this process, energy stored in the magnetic field is converted into kinetic and thermal energy, which provides a source of plasma heating and energetic particles. Magnetic reconnection plays a key role in many space and laboratory plasma phenomena, e.g. solar flares, Earth’s magnetopause dynamics and instabilities in tokamaks. A new linear device (VINETAII) has been designed for the study of the fundamental physical processes involved in magnetic reconnection. The plasma parameters are such that magnetic reconnection occurs in a collision-dominated regime. A plasma gun creates a localized current sheet, and magnetic reconnection is driven by modulating the plasma current and the magnetic field structure. The plasma current is shown to flow in response to a combination of an externally induced electric field and electrostatic fields in the plasma, and is highly affected by axial sheath boundary conditions. Further, the current is changed by an additional axial magnetic field (guide field), and the current sheet geometry was demonstrated to be set by a combination of magnetic mapping and cross-field plasma diffusion. With increasing distance from the plasma gun, magnetic mapping results in an increase of the current sheet length and a decrease of the width. The control parameter is the ratio of the guide field to the reconnection magnetic field strength. Cross-field plasma diffusion leads to a radial expansion of the current sheet at low guide fields. Plasma currents are also observed in the azimuthal plane and were found to originate from a combination of the field-aligned current component and the diamagnetic current generated by steep in-plane pressure gradients in combination with the guide field. The reconnection rate, defined via the inductive electric field, is shown to be directly linked to the time-derivative of the plasma current. The reconnection rate decreases with increasing ratio of the guide field to the reconnection magnetic field strength, which is attributed to the plasma current dependency on axial boundary conditions and the plasma gun discharge. The above outlined results offer insights into the complex interaction between magnetic fields, electric fields, and the localized current flows during reconnection.
Die vorliegende Arbeit liefert Beiträge zur optischen und elektrischen Charakterisierung des dynamischen Verhaltens von Plasmaspezies in Atmosphärendruck-Plasmen insbesondere mit Hinsicht auf den Einsatz in der Plasmamedizin. Dabei wurde ein breites Spektrum verschiedener Diagnostiken angewandt, um die Zugänglichkeit zur Bestimmung weiterer Plasmaparameter an Atmosphärendruck zu prüfen. Diese Arbeit stellt eine neue Methode zur Bestimmung der Ionendichte bei Atmosphärendruck- Bedingungen vor, bei der elektrische Oszillationen ausgewertet werden, deren Ursprung ionenakustische Wellen im Plasma sind. Weiterhin wurden neben relativen optischen Messungen wie der phasenaufgelösten optischen Fotografie (PROI) und der Kreuz- Korrelations-Spektroskopie (CCS) auch absolute optische Messungen mit der interferometrischen Hakenmethode und dem Pockels-Effekt durchgeführt. Anhand von elektrischen Messungen wurde ferner gezeigt, dass mit einer Strom- und Spannungs-Charakteristik der Einfluss von Aufbauparametern, wie der Kapillarposition oder dem Gasfluss, auf das Plasma untersucht werden kann. Gegenstand der Untersuchungen waren verschiedene Plasmaquellen, die für eine Nutzung in der Plasmamedizin entwickelt wurden. Sowohl die elektrischen Messungen des Parametereinflusses als auch die Bestimmung der Ionendichte erfolgten an der selbstpulsenden transienten Funkenentladung in Argon an offener Atmosphäre. Der geringe Filamentdurchmesser und der dennoch hohe Entladungsstrom ermöglichen die Detektion der ionenakustischen Instabilität. Darüber hinaus wurde diese erratisch zündende Entladung räumlich und zeitlich aufgelöst mit der CCS spektroskopisch untersucht. Dabei wird insbesondere die Selbst-Triggerung der CCS ausgenutzt, um einen Zeitbezug trotz des großen Entladungsjitter zu erhalten. Für die PROI wurden die räumlich und zeitlich stabilen Entladungsanordnungen der Nadel-Platte-Geometrie und des Kapillarjets in Helium gewählt. Die Anordnungen wurden mit einer periodischen Sinusspannung betrieben und wiesen Entladungsspalte von d = 5 - 15 mm auf. Eine besondere Anforderung der Messung mit dem Pockels-Effekt ist zu der räumlichen und zeitlichen Stabilität eine dielektrische Gegenelektrode, welche bei der Anordnung des Kapillarjets möglich war. Bei der Anwendung der interferometrischen Hakenmethode kam neben einem Erdgas-Sauerstoff-Mischgasbrenner sowohl eine Mikrowellen-Entladung (Plexc) als auch ein MHz-Plasmajet (kINPen) zur Anwendung. Die Bedeutung der elektrischen Messungen, besonders der Strom- und Spannungscharakteristik einer Entladung, wurde an dem Parametereinfluss der Kapillarposition einer erratisch zündenden transienten Funkenentladung vorgestellt. Es konnte gezeigt werden, dass der Zeitunterschied zwischen dem Stromsignal eines Vorstreamers und der Hauptentladung durch das Einbringen einer Kapillare in den Entladungsspalt deutlich verringert wird. Insbesondere der Beitrag der lokalen elektrischen Feldstärkeerhöhung an der Kapillarkante und der Diffusionsanteil der Umgebungsluft wurden als Ursachen, durch Vergleich einer Feldsimulation mit der Beobachtung der Vorphase an der Kapillarkante in den CCS-Messungen, diskutiert. Anschließend konnte gezeigt werden, dass der Leistungseintrag in die Vorphase durch die Platzierung der Kapillare deutlich reduziert werden kann. Ein wesentliches Ergebnis dieser Arbeit ist die Beobachtung von ionenakustischen Wellen als Oszillationen im Abklingen des Stromsignals einer erratisch zündenden transienten Funkenentladung. Hierzu war es nötig, elektrische Störungen zu erkennen und zu eliminieren. Es konnte ein Erdschleifen-freier Aufbau realisiert werden. In diesem Aufbau zeigt sich, dass die Signale der ionenakustischen Welle ausschließlich in einem bestimmten Gasflussbereich beobachtet werden. Die gemessene Frequenz der Oszillationen wurde als Ionenplasmafrequenz f_{pl ,i} identifiziert und enthält daher Angaben zu den Ionendichten im Bereich von n_{Ar_2^+} = 3•10^{14} cm^{-3} bis 1•10^{12} cm^{-3}. Nach einer Abschätzung der zu erwartenden Elektronendichte, die der gemessenen Ionendichte sehr nahe kommt, wurde die Dispersionsrelation für die vorhandenen Entladungsbedingungen aufgestellt und gelöst. Dabei zeigt sich eine starke zeitliche Dämpfung über die Ionen-Neutralteilchenstöße sowie eine räumliche Verstärkung für die Ionenplasmafrequenz. Aus der Dämpfung der Oszillationsamplituden konnte die Ionen- Neutralteilchen-Stoßfrequenz nu_i = 3•10^7 Hz ermittelt werden. Weiterhin ergibt sich aus der Lösung der Dispersionsrelation ein Existenzbereich für die ionenakustischen Wellen in Abhängigkeit von der Ionendichte und der elektrischen Feldstärke.
There is a wide variety of Alfvén waves in tokamak and stellarator plasmas. While most of them are damped, some of the global eigenmodes can be driven unstable when they interact with energetic particles. By coupling the MHD code CKA with the gyrokinetic code EUTERPE, a hybrid kinetic-MHD model is created to describe this wave–particle interaction in stellarator geometry. In this thesis, the CKA-EUTERPE code package is presented. This numerical tool can be used for linear perturbative stability analysis of Alfvén waves in the presence of energetic particles. The equations for the hybrid model are based on the gyrokinetic equations. The fast particles are described with linearized gyrokinetic equations. The reduced MHD equations are derived by taking velocity moments of the gyrokinetic equations. An equation for describing the Alfvén waves is derived by combining the reduced MHD equations. The Alfvén wave equation can retain kinetic corrections. Considering the energy transfer between the particles and the waves, the stability of the waves can be calculated. Numerically, the Alfvén waves are calculated using the CKA code. The equations are solved as an eigenvalue problem to determine the frequency spectrum and the mode structure of the waves. The results of the MHD model are in good agreement with other sophisticated MHD codes. CKA results are shown for a JET and a W7-AS example. The linear version of the EUTERPE code is used to study the motion of energetic particles in the wavefield with fixed spatial structure, and harmonic oscillations in time. In EUTERPE, the gyrokinetic equations are discretized with a PIC scheme using the delta-f method, and both full orbit width and finite Larmor radius effects are included. The code is modified to be able to use the wavefield calculated externally by CKA. Different slowing-down distribution functions are also implemented. The work done by the electric field on the particles is measured to calculate the energy transfer between the particles and the wave and from that the growth rate is determined. The advantage of this approach is that the full magnetic geometry is retained without any limiting assumptions on guiding center orbits. Extensive benchmarks have been performed to test the new CKA-EUTERPE code. Three tokamak benchmarks are presented, where the stability of TAE modes are studied as a function of fast particle energy, or in one case as a function of the fast particle charge. The benchmarks show good agreement with other codes. Stellarator calculations were performed for Wendelstein 7-AS and the results demonstrate that the finite orbit width effects tend to be strongly stabilizing.
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.
The development of innovative coatings with multifunctional properties is an ambitious task in modification of material surfaces. A novel approach is a hybrid method combining the non-thermal plasma processing with nanotechnology for the development of multifunctional surface coatings. The conception of the hybrid coating process is based on three steps: the preparation of a suspension consisting of an organic liquid and functional nanoparticles, the deposition of the suspension as a thin liquid film on the material surface, and the plasma modification of the liquid organic film to achieve a thin solid composite film with embedded nanoparticles demonstrating multifunctional properties and good adherence on the substrate material. In this work the liquid polydimethylsiloxane (PDMS) was applied as a model system, and the experimental investigations were focused on the PDMS plasma modification. In particular, the specific role of the different plasma components and the influence of the plasma and processing parameters on the PDMS modification were studied. The applied capacitively coupled radio frequency (CCRF) plasma was analyzed by electric probe measurements and optical emission spectroscopy, whereas the molecular changes in PDMS due to plasma-induced chemical reactions were studied by the Fourier transform infrared reflection absorption spectroscopy. Additionally, the photocatalytic activity of thin composite films consisting of plasma cross-linked PDMS with embedded TiO2 nanoparticles was demonstrated. During the investigation it was found that the CCRF discharge modifies efficiently thin liquid PDMS films to solid coatings. The samples were positioned in the plasma bulk at floating potential. The penetration depth of particles like neutrals, ions, electrons and radicals in the film is strongly limited. The heating of samples in the CCRF discharge is weak to modify PDMS by itself and only the plasma radiation is able to transform the liquid bulk to solid one. It is known that the absorption onset of PDMS lies in the VUV region (below 200 nm). The energetic VUV radiation penetrates into the PDMS film on a thickness from several hundred nanometers to few micrometers and initiates photochemical reactions there. Thus, different gases like Ar, Xe, O2, H2O, air and H2 were tested to provide the strongest VUV emission intensity of the CCRF discharge. Discharge pressure and power were varied for all these gases and it was found that at all conditions the H2 plasma demonstrates drastically stronger emission. Thus, H2 gas was selected for the plasma treatment of liquid PDMS films. The IRRAS analysis revealed the transformation process of PDMS with the degradation of CH3 groups, the formation of new groups like SiOH, CH2 and SiH, the formation of the SiOx material and crosslinking. It was found that the modification effect is not uniform across the film thickness. The top region with an initial thickness up to 100 nm loses all CH3 groups, in the underlying region the CH3 concentration increases gradually from zero to the value for PDMS, if the film was thick enough. The methyl-free SiOx top layer contains also SiOH and SiH groups. Furthermore, the SiH groups are concentrated only in a very thin layer with a thickness below 10 nm. The presence of the unscreened polar SiOSi and SiOH groups on the surface causes the adsorption of H2O from the atmosphere, which was also observed by IRRAS. By means of the spectroscopic ellipsometry it was found out that all above described regions experience a shrinking. The reason is the crosslinking and loss of material. The most shrunken layer is the top SiOx layer with the shrinking ratio (final thickness/initial thickness) of 0.55 - 0.60. Further, this ratio gradually rise up to the value of 0.95 in the deeper region, which has the concentration of CH3 groups of about that for PDMS. After the analysis of all results the depth of effective modification was estimated at 300 400 nm for the most optimal conditions. The optimization of the plasma VUV intensity was realized by variation of discharge pressure and power. The strongest plasma emission at studied conditions provided the irradiance of the sample of ca. 13 mW/cm2. However, such strong radiation causes very strong production rate of the gases. These products leave the modifying film slower as they are produced, what causes their accumulation in there. Their pressure grows up leading to formation of bubbles, which later explode. Finally, the film becomes heavily damaged. To avoid this effect the pressure and the RF power were changed to reduce the irradiance to 6 - 7 mW/cm2. This resulted in the absence of any damages.