Universität Stuttgart
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Item Open Access Simulation of Electron Bernstein Waves in FLiPS with various numerical methods(2021) Rumiantsev, Kirill; Hirth, Thomas (Prof. Dr.)The plasma generation and heating by microwaves is an important research topic in the field of controlled nuclear fusion. All modern fusion plasma devices such as Wendelstein 7-X use microwave heating. The microwave plasma-heating primarily occurs at the resonances, where the microwaves are efficiently absorbed. The heating scenario must be designed such that the microwaves can reach the resonance. When the plasma exceeds the cutoff density, the microwaves will be reflected, and the resonance becomes inaccessible. However, it is possible to perform heating by Electron Bernstein Waves (EBWs), since these electrostatic waves propagate even in overdense plasmas, unlike the electromagnetic plasma waves. EBWs cannot propagate in the vacuum and must be created through a coupling process. Both O- and X-mode can couple to EBWs. The thesis investigates the coupling of the O- and X-mode to EBWs as well as the EBW propagation with various numerical methods. The application of only one numerical method is not sufficient as the coupling involves very different wavelength scales. The optimal coupling scheme for the expected plasma parameters was determined using a Finite-Difference Time-Domain (FDTD) code. Since EBWs are not included in the code, a Boundary-Value Problem (BVP) code was developed. Using the BVP code, the effect of the collisions on EBWs was studied. The field amplification at the upper-hybrid resonance (UHR), where EBWs couple to the electromagnetic waves, and the effect of the magnetic field on EBWs could be directly visualized. The propagation of the EBW was investigated using the novel ray-tracing code RiP. The ray-tracing simulations provided a clear picture of the essential features of the wave propagation. For the O- and X-mode coupling, the importance of the axial plasma inhomogeneity was shown. For the first time, the method of the Wigner function was applied to calculate the intensity distribution of EBWs. Both, ray-tracing and the Wigner function simulations showed that the inhomogeneous magnetic can cause focusing of EBWs. The focusing effect can have practical applications e.g. for controlled local heating of the plasma. Additionally, the focusing effect can cause a parametric decay due to the field enhancement in the focal regions. In this thesis, the simulations were focused on excitation and propagation of EBWs in the geometry of the linear plasma device FLiPS located at the University of Stuttgart. Measurements were carried out to study the predicted focusing of the EBWs in the FLiPS plasma with monopole antennas. The measurements provided the density profile used in the simulations. The expected amplification of the signal at the UHR was not detected, indicating either the complete collisional absorption of the X-mode at the upper-hybrid resonance, or the turbulent plasma density oscillations that reduce the coupling efficiency to EBWs. These effects can be studied further using the developed tools since they provide a complete toolbox to study the full coupling process to EBWs in an actual experimental geometry.Item Open Access Influence of the ion energy on generation and properties of thin barrier layers deposited in a microwave plasma process(2012) Ramisch, Evelyn Christine; Stroth, Ulrich (Prof. Dr.)The demand for environment-friendly energy sources increases more and more, which is not only caused by the energy turnaround initialized by the Federal Government. In this context, the focus is set mainly on the development of wind power and solar energy with competitive production costs. Above all, this is a problem for solar cells, which, today, are mainly fabricated out of crystalline silicon and, therefore, are in competition with semiconductor industry. Hence, the development of solar cells based on alternative materials like e.g. copper-indium-gallium-diselenide (CIGS) is of great interest. Because of the lower layer thickness needed for this material, these solar cells can be fabricated on flexible substrates like metal foils. This possibility offers a broader spectrum of applications. For reaching low production costs, the applicability of unpolished steel foil, which exhibits scratches on the µm scale, is investigated as substrate for the solar cells in this work. The use of any metal as substrate requires a barrier layer between the substrate and the solar cells to prevent short-circuits between the separate cells of a solar module and to prevent the diffusion of undesired substrate elements into the solar cells. In this work, siliconoxide and silicon-nitride coatings are deposited as barrier layers in a microwaveplasma process in a gas mixture of HMDSO (hexamethyldisiloxane) and oxygen or monosilane and ammonia. To have the opportunity of influencing the layer growth by high-energetic ions, an additional substrate bias is applied during the deposition, which leads to a capacitive discharge superimposing the microwave one. The high-energetic ions impinging on the layer surface lead to a layer smoothing and melting, especially at positions of indentations in the substrate surface. Hence, the barrier properties of the coating are improved clearly, which was identified by insulation measurements of the deposited film. The layer growth modification is analyzed on the basis of substrates with a well-defined rough surface structure in the µm range experimentally as well as by simulations with the Monte-Carlo Code SDTrimSP-2D, which allows a detailed analysis of the local layer growth mechanisms contributing to the deposition. Additionally, the impinge of the energetic ions affects the molecular structure and composition of the coatings as well. These parameters are an important indicator for the layer material properties like adhesion, hardness and diffusion properties. The molecular composition of the deposited layers is analyzed in detail by Fourier- ransform infrared (FTIR) spectroscopy. From the layer composition and their refractive index, conclusions on the diffusion behavior of the coatings are drawn. In case of applying the substrate bias, the spectra indicate a denser and harder film in case of silicon oxide. Hence, these layers are more diffusion preventing compared to the unbiased ones. On the other hand, the silicon-nitride coatings show contrary properties: They offer more porous layers, when the substrate bias is applied, and, therefore, they assist diffusion.Item Open Access Particle dynamics simulation and diagnostics of the PECVD processes in fluorocarbon rf discharges(2010) Barz, Jakob Philipp; Lunk, Achim (Prof. Dr. rer. nat. habil.)The present work deals with the investigation of fluorocarbon plasmas by different experimental methods and supporting numerical analysis of the plasma with an emphasis on plasma-chemical interactions. Several insights could be gained from the combined experimental and numerical approaches, especially concerning the conclusiveness of the results and previous observations from the literature. Plasma diagnostics were performed with non-invasive methods, such as UI probe measurements, microwave interferometry, laser-induced fluorescence, UV absorption measurements, and mass spectrometry. The complementary numerical simulations accounted for the electron-neutral interactions, discharge dynamics, and chemical reactions. From the excitation and ionization cross sections of argon as well as the dissociation, ionization, and attachment cross sections of trifluoromethane, the field-dependence of transport parameters were obtained. These transport parameters were used as input data for fluid-modeling of the discharge. For the plasma dynamics simulation, the Boltzmann-equation was solved numerically for transport of mass, momentum, and energy in a time-dependent two-term approach. The so-obtained electron density and the power-voltage characteristics were compared to measurements with microwave interferometry and the UI probe, respectively. An overall good agreement of the numerical and measured electron densities was obtained over a large variation range of plasma power, gas composition, and pressure. The power-voltage characteristics showed a good agreement between numerical results and data obtained right after ignition of plasma. It was further found that the measured data showed time-dependent developments from which strong deviations resulted. The time scales of changes were typically in the range of milliseconds to seconds after ignition. It was concluded that compositional changes in the gas phase were the reason. The high abundance of oligomers as well as small molecules like HF in the gas phase on one hand, and the loss of molecules by polymer deposition on the other hand affect the charge carrier mobilities and the ionic composition, such result in the changes observed. Furthermore, from this investigation, the major fragmentation processes were identified. For the investigation of the reaction-diffusion processes, investigations by laser-induced fluorescence were carried out. In order to obtain best resolution along the axial direction of the plasma reactor, the conventional crossed-beam technique was modified. Such, a resolution of up to 60 micrometers became possible. Thus, highly-resolved axial densities of two plasma abundant intermediates, fluoromethylidine and difluorocarbene, were obtained. For the analysis of the gas phase kinetics, a numerical chemical-diffusion model was set up. To complete the analysis of the plasma dynamics, the deposition of plasma polymer onto substrates was examined. The deposition rate was determined, and changes in the surface chemistry at the transition form uncovered substrates to closed films were revealed. For the identification of the deposition precursors, results from the chemical-diffusion model were adopted for the analysis. The oligomer molecules, which are produced at high results according to the simulation, were shown to correlate well with the polymer deposition rate. It was found by electron spin resonance (ESR) that chemical reactions took place within the deposited polymer films. The restructuring of the polymer by these reactions resulted in highly cross-linked films according to x-ray photoelectron spectroscopy (XPS). Further, it was found that the amount of fluorine in the polymer was lower than could be expected from the oligomers formed according to the chemical model. Such, it was suggested that ejection of fluorine containing species was taking place especially during the plasma glow, promoted by electron and ion bombardment, and radiation. Moreover, the ejection of fluorine containing species was tentatively ascribed to the production of difluorocarbene at the surface of the plasma chamber as observed by LIF. Concluding, radical and metastable fluxes from the electrodes, combined with isotropic gas phase reactions, determine the density profiles of several species from trifluoromethane plasmas. They strongly feed back the plasma chemistry, which itself feeds back the plasma particle dynamics. According to models, the deposition occurs via formation of oligomers in the gas phase, which deposit on the surface either as neutrals or ions, and become crosslinked by subsequent reactions. The origin of the particle fluxes at the electrodes is not yet identified, but indications were found for the chemical cross-linking processes being the cause.Item Open Access The role of MHD instabilities in the improved H-mode scenario(2009) Flaws, Asher; Stroth, Ulrich (Prof.)Recently a regime of tokamak operation has been discovered, dubbed the improved H-mode scenario, which simultaneously achieves increased energy confinement and stability with respect to standard H-mode discharges. It has been suggested that magnetohydrodynamic (MHD) instabilities play some role in establishing this regime. In this thesis MHD instabilities were identified, characterised, and catalogued into a database of improved H-mode discharges in order to statistically examine their behaviour. The onset conditions of MHD instabilities were compared to existing models based on previous H-mode studies. Slight differences were found, most notably a reduced $\beta_N$ onset threshold for the frequently interrupted regime for neoclassical tearing modes (NTM). This reduced threshold is due to the relatively low magnetic shear of the improved H-mode regime. This study also provided a first-time estimate for the seed island size of spontaneous onset NTMs, a phenomenon characteristic of the improved H-mode scenario. Energy confinement investigations found that, although the NTM impact on confinement follows the same model applicable to other operating regimes, the improved H-mode regime acts to mitigate the impact of NTMs by limiting the saturated island sizes for NTMs with toroidal mode number $n \geqslant 2$. Surprisingly, although a significant loss in energy confinement is observed during the sawtooth envelope, it has been found that discharges containing fishbones and low frequency sawteeth achieve higher energy confinement than those without. This suggests that fishbone and sawtooth reconnection may indeed play a role in establishing the high confinement regime. It was found that the time evolution of the central magnetic shear consistently locks in the presence of sawtooth and fishbone reconnection. Presumably this is due to the periodic redistribution of the central plasma current, an effect which is believed to help establish and maintain the characteristic current profile required for improved H-mode operation. A similar effect was proposed for the NTM instability whereby the magnetic island drives an additional toroidal current which flattens the central current density profile. However, it was found that the NTM impact on the toroidal current density could be accounted for purely in terms of the $3$ conventional current contributions, namely: ohmic, bootstrap, and auxiliary heating current drive, without requiring an additional current source.Item Open Access Strukturentstehung in Driftwellenturbulenz toroidaler Plasmen(2009) Manz, Peter; Stroth, Ulrich (Prof. Dr.)In Fusionsplasmen ist die Turbulenz und der damit inhergehende turbulente Transport für den größten Anteil der Teilchen- und Energieverluste verantwortlich. Durch die annähernd freie Bewegung der Ladungsträger parallel zum Magnetfeld kann die Turbulenz in magnetisierten Plasmen, rotierenden Flüssigkeiten im geophysikalischen Kontext entsprechend, als zweidimensional betrachtet werden. In zweidimensionaler Turbulenz bilden sich durch Wirbelvermischung größere Wirbelstrukturen aus. Es wird davon ausgegangen, dass die Wirbel untereinander wechselwirken und sich gegenseitig durchmischen und so schrittweise immer größere Wirbel bilden. Da dieser Prozess stufenweise abläuft wird dieser als Kaskade bezeichnet. Große Wirbelsysteme können für die Fusionsforschung von entscheidender Bedeutung sein, da sie nicht gleichmäßige radiale elektrische Felder aufbauen können, die eine Schlüsselgröße von internen Transport-Barrieren sind. Die nichtlineare Wechselwirkung zwischen Wirbeln verschiedener Skalen wird im Detail untersucht. Die Untersuchung erlaubt Rückschlüsse auf den Entstehungsmechanismus von großskaligen Wirbelstrukturen in magnetisierten Plasmen.Item Open Access Charakterisierung der elektromagnetischen Turbulenz im Torsatron TJ-K(2007) Rahbarnia, Kian; Stroth, Ulrich (Prof. Dr.)Es wird die elektromagnetische Turbulenz im Torsatron TJ-K gemessen und analysiert.Item Open Access Wetting, de-icing and anti-icing behavior of microstructured and plasma-coated polyurethane films(2019) Grimmer, Philipp E. S.; Hirth, Thomas (Prof. Dr. rer. nat.)Ice build-up on surfaces, for example on wings of airplanes or on rotor blades of wind turbines, impairs the functionality of transportation vehicles or technical systems and reduces their safety. Therefore, functional anti-ice surfaces are being researched and developed, which shall enable an easy removal or reduce the amount of ice on the surfaces at risk. The starting hypothesis for this work is that superhydrophobic polyurethane (PU) films with microstructure base diameters of 35 µm or more reduce the wetting by water, show a low ice adhesion for easy removal of ice and reduce or delay icing. Superhydrophobic PU films for passive anti- and de-icing were created by hot embossing and plasma enhanced chemical vapor deposition (PECVD). The hot embossing process as well as the plasma coating and etching processes were analyzed for the dependence of the surface characteristics on different process parameters. The functionalized PU films were characterized for their surface topography, surface chemistry, stability against erosion, wettability, ice adhesion and icing behavior. For comparison, the ice adhesion and icing behavior were examined on relevant technical materials (aluminum, titanium, copper, glass, epoxy resin of carbon fiber reinforced polymer and other fluoropolymers) and on some commercial anti-ice coatings. The PU films were chemically analyzed by IR spectroscopy. As the first process step for functionalization, microstructures of cylindrical, elliptical or linear shape were imprinted in PU films by a hot embossing technique with different ns-pulsed laser-drilled stamps and characterized by several microscopy methods. The microstructures had heights of 15 µm to 140 µm, diameters or widths of 35 µm to 300 µm and distances (pitch values) of 50 µm to 500 µm. The embossing process was analyzed and optimized in terms of the process parameters temperature, pressure, time, PU film release temperature and reproducibility of the microstructures. In a second functionalization step (PECVD) the microstructured surfaces were coated with thin, hydrophobic plasma polymers using different fluorocarbon precursors (CHF3, C3F6 and C4F8) or hexamethyldisiloxane (HMDSO). Different process parameters for plasma coating and etching (Ar or O2 plasmas) were used in order to create various nanoscale roughness values. Electron spectroscopy for chemical analysis (ESCA), spectroscopic ellipsometry and atomic force microscopy (AFM) were used for analysis of the chemical composition, the thickness and the nanoroughness of the plasma polymers. The functionalizations, especially the plasma coatings, were completely worn off by a UV/water weathering test (1000 h, X1a CAM 180 Test, SAE J-2527), but showed sufficient stability against sand erosion (DIN 52348), in a long-term outdoor test for 13.5 months and against fivefold repeated pull-off of ice. The silicone-like plasma coatings were more stable than the fluorocarbon plasma coatings. The wetting behavior of water was determined by static, advancing and receding contact angle measurements. Static contact angle measurements with diiodomethane (DIM) were made for determination of the surface free energies of the relevant surfaces. Advancing contact angles of over 150° and very low contact angle hysteresis values below 10° were reached on some of the cylindrically and elliptically structured PU samples with microstructure base diameters in the range of 35 µm to 50 µm. The measured water advancing contact angles did not reach the theoretical values of the Cassie-Baxter state. Starting from a mixed wetting state near Cassie-Baxter in case of the superhydrophobic PU surfaces, they approached the Wenzel state with an increasing pitch/diameter (P/d) factor. Fluorescence laser scanning microscopy images were taken of some microstructured, uncoated or plasma coated samples during the wetting by a water drop containing a fluorescent dye. These images show the Wenzel state or a mixed wetting state by visualization of the interface between the water droplet and the surface. A new icing test chamber and a test setup were developed for characterization of the ice adhesion and the icing behavior. The tensile ice adhesion was measured at -20 °C by pull-off of ice cylinders (highly purified water, (<0.056 µS/cm, diameter of 4 mm, similar to the diameter of large raindrops) and compared to the theoretical values and the wetting behavior. The technical material surfaces measured for comparison showed a high ice adhesion, which led to cohesive fractures especially on the metal surfaces, whereas some of the commercial anti-ice coatings showed lower ice adhesion values. The flat, plasma coated PU surfaces showed adhesive fractures with a reduced ice adhesion compared to the technical material surfaces and uncoated PU and revealed a good correlation of the ice adhesion with the wetting behavior of water (work of adhesion). On the other hand, the microstructured PU surfaces showed a greatly increased ice adhesion in comparison to the flat PU and technical material surfaces which was enhanced even further by the plasma coatings and did not correlate with the wetting behavior. The reason for this is the wetting transition from the Cassie-Baxter to the Wenzel state during the cooling or freezing process, leading to an increased ice-surface contact area and mechanical interlocking of the ice with the micro- and nanostructures. The freezing of water drops was examined in thermodynamic equilibrium (static experiment) and under quasi-steady conditions (dynamic experiment). In the static experiment, 15 µl water drops (corresponding to medium to large raindrops) at room temperature were dispensed onto a cold surface at a constant temperature of -20 °C. The freezing delay times, the crystallization times and the total freezing times were measured and compared to calculated expected values. On the flat samples, the freezing delay times could be extended by the plasma treatments. On the microstructured samples, the freezing (nucleation) could sometimes be delayed even further, but not always reproducible because of an unstable Cassie-Baxter state. In the dynamic experiment, 25 µl water drops (corresponding to large raindrops) were cooled down in quasi-steady conditions with the surface and the surrounding atmosphere by a constant, low cooling rate of 1 K/min while the water drop temperature was measured by an IR camera for determination of the surface-specific nucleation temperature and crystallization time. A lower nucleation temperature could be measured on the flat, plasma coated PU surfaces compared to uncoated PU and the hydrophilic glass and metal surfaces. The superhydrophobic PU surfaces did not show a further reduction of the nucleation temperature because of an unstable Cassie-Baxter state. The resulting measured nucleation temperatures were compared to the expected values calculated with an enhanced nucleation theory including a quasi-liquid interfacial layer of the ice nucleus and a Poisson process. Overall, it is shown that hot embossing and PECVD are useful processes for creating superhydrophobic PU surfaces with regard to a roll-to-roll process. The flat, plasma coated PU films show a reduced ice adhesion and lowered nucleation temperature compared to the relevant technical material surfaces. The microstructured, plasma coated PU films are far more water repellent than the flat, plasma coated PU surfaces or the other technical materials. However, the microstructures with base diameters of 35 µm or more and the nanoroughness of the plasma coatings cannot stabilize the Cassie-Baxter state of a freezing water drop enough for a low ice adhesion or a significant decrease of the nucleation temperature. These superhydrophobic PU films are therefore not more icephobic than the flat, plasma coated PU films. In the outlook, the reduction of the geometrical parameters of the microstructures (diameter D, distance P) and nanostructures (curvature radius R) of the surface functionalizations for lower ice adhesion values and nucleation temperatures is proposed.Item Open Access Diagnostik und Modellierung eines Mikrowellen-Plasmabrenners bei Atmosphärendruck(2017) Gaiser, Sandra; Hirth, Thomas (Prof. Dr.)Mikrowellen-Plasmaprozesse bei Atmosphärendruck bieten eine Vielzahl von Anwendungsmöglichkeiten. Dazu gehören das Plasmaspritzen zur Beschichtung, die Behandlung von Oberflächen für die Reinigung oder Aktivierung sowie der Abbau schädlicher Abgase. Für die Entwicklung und Optimierung dieser Verfahren sind sowohl experimentelle Untersuchungen als auch eine theoretische Betrachtung von Bedeutung. Diese Arbeit beschäftigt sich deshalb neben der Diagnostik vor allem mit der Modellierung und numerischen Simulation eines bei Atmosphärendruck betriebenen Mikrowellen-Plasmabrenners. Dazu wird die Simulationssoftware Comsol Multiphysics verwendet. Das Ziel ist es, mittels einzelner Modelle die unterschiedlichen physikalischen Vorgänge zu beschreiben und das Brennersystem zu optimieren. Die Simulationen werden schließlich schrittweise miteinander verknüpft, um so ein möglichst selbstkonsistentes Modell der Plasmaquelle zu erhalten. Die Simulationsergebnisse werden zudem mit experimentellen Daten verglichen. Zunächst werden die Verteilung des Mikrowellenfeldes im Plasmabrenner sowie die Resonanzfrequenzen der Resonatoranordnung berechnet, was die Grundlage für eine zuverlässige Zündung und den Betrieb des Plasmas bildet. Anschließend wird ein Modell der kalten Gasströmung erstellt. In dieses wird schließlich eine Wärmequelle implementiert, um den Einfluss des heißen Plasmas auf die Strömung zu untersuchen. Die Gasströmung soll dahingehend optimiert werden, dass sie das Plasma einschließt, um so eine Beschädigung des Gas führenden Quarzrohres zu vermeiden. In einer weiteren Simulation wird das Plasma mit Hilfe des Drude-Modells beschrieben. Hierbei werden dem Plasma eine Permittivität und eine Leitfähigkeit zugewiesen. Eine Erweiterung erfolgt durch das Fluid-Modell, das Bilanzgleichungen für die Elektronendichte sowie Reaktionsmechanismen für ein Argon-Plasma enthält. Die Simulationsergebnisse werden durch den Vergleich mit experimentellen Ergebnissen verifiziert. Dazu wird zum einen die räumliche Lage des Plasmas mit Hilfe von Kameraaufnahmen qualitativ untersucht. Zum anderen stehen Messwerte aus der optischen Emissionsspektroskopie zur Verfügung.Item Open Access Electron cyclotron emission investigations at the stellarator TJ-K(2020) Sichardt, Gabriel; Hirth, Thomas (Prof. Dr.)Microwave diagnostics are widely used in fusion-oriented plasma research. Especially, electron cyclotron emission (ECE) measurements are routinely employed for reliable investigations of radial temperature profiles. Furthermore, an ECE diagnostic can be used to measure electron densities or detect superthermal electrons. Due to its non-invasive character, it is well suited for application to extreme conditions like in fusion plasmas since neither the plasma is perturbed nor the diagnostic harmed. Despite decades of development, ECE diagnostics are still subject of current research and development. Especially the correct interpretation of measurements at plasmas with low densities and temperatures, which are in contrast to fusion plasmas optically thin, is challenging. The stellarator experiment TJ-K in Stuttgart is operated with such thin plasmas allowing for the use of Langmuir probes for temperature measurements and thus as a benchmark for a new ECE diagnostic system. This work is about the development, optimization, construction and application of an ECE diagnostic for TJ-K. Modeling, simulation and experiment are combined to understand the processes at the specific experiment and to adapt the setup to these conditions. The first part of this thesis describes the development and test of the diagnostic. To this end, the transport and propagation of electron cyclotron radiation is simulated in the three-dimensionally modeled plasma of TJ-K. From the results, an optimization approach is derived: with a suitably positioned and optimally curved mirror for defined reflections, a tunable resonator system is built that improves the localization of the measurements significantly. After identification of the measurement signals as ECE opposed to thermal bremsstrahlung, the measurement system is calibrated with the hot-cold method. Although only about 0.2 % of the black body intensity is emitted from the optically thin plasma the temperatures obtained from the ECE diagnostic could be verified by Langmuir probe measurements. In the second part, numerical investigations of electron trajectories in the 3D magnetic field of TJ-K are employed to study their dependence on the kinetic particle energy. The trajectories form drift orbits which depend on the speed and orientation of the electron compared to the magnetic field. To what extent electrons on larger drift orbits collide with the vessel wall and thus contribute to toroidal net currents is investigated using simulations with different velocity distributions. It becomes apparent that especially electron populations additional to the thermal distribution at higher energies like for instance 1 keV, superthermal electrons, can result in large toroidal net currents. Already thermal electrons with typical energies of 10 eV provide numerically toroidal net currents that are comparable to the experimentally observed currents. The installed ECE diagnostic allows for temporally resolved measurements of local radiation temperatures for correlation with toroidal net currents.Item Open Access Dynamics and structure analysis of coherent turbulent structures at the boundary of toroidally confined plasmas(2013) Fuchert, Golo; Stroth, Ulrich (Prof. Dr.)Die sichere und finanzierbare Deckung des steigenden Energiebedarfs ist eine der größten Herausforderungen unseres Jahrhunderts. Kernfusionskraftwerke nach dem Prinzip des magnetischen Einschlusses können möglicherweise einen entscheidenden Beitrag leisten. Derzeit verhindern Energieverluste des Fusionsplasmas durch Turbulenz einen effizienten Betrieb und erhöhen die Erosion der Innenwand des Fusionsreaktors. Nahe der Wand, in der sogenannten Abschälschicht, wird der Transport dominiert von Blobs oder Filamenten: lokalisierte Strukturen erhöhten Drucks, die Energie und Teilchen in Richtung der Wand transportieren. Der Transport hängt unter anderem ab von der Größe, Geschwindigkeit und Entstehungsrate der Blobs. Für einfache Geometrien des einschließenden Magnetfelds sagt ein analytisches Modell die Größe und Geschwindigkeit der Blobs voraus, nicht aber die Entstehungsrate. Experimentelle Beobachtungen deuten auf eine Beteiligung der Randschichtturbulenz in der Nähe der letzten geschlossenen Flussfläche (dem Beginn der Abschälschicht) bei der Blobentstehung hin, was sich in der Entstehungsrate widerspiegeln sollte. Diese Arbeit beantwortet vorrangig zwei Fragen: Beschreiben die einfachen Modelle die Blobeigenschaften auch in Magnetfeldgeometrien tatsächlicher Fusionsexperimente und welchen Einfluss hat die Randschichtturbulenz auf diese Eigenschaften? Mit einer Hochgeschwindigkeitskamera wurden Größe, Geschwindigkeit und Entstehungsrate der Blobs im Stellarator TJ-K und dem Tokamak ASDEX Upgrade untersucht. Während eine grundsätzliche Übereinstimmung mit den Vorhersagen besteht, konnte zum ersten Mal gezeigt werden, dass die Randschichtturbulenz die untersuchten Eigenschaften beeinflusst. Die Messungen beinhalten den ersten systematischen Vergleich der Strukturgrößen inner- und außerhalb der letzten geschlossenen Flussfläche. Darüber hinaus wird mit Sondenmessungen die dreidimensionale Struktur der Blobs in einem Stellarator vermessen und gezeigt, dass die Blobs mehr als 50 % des lokalen und mehr als 20 % des totalen Transports in der Abschälschicht ausmachen. Messungen eines Stroms entlang der Filamente bestätigen, dass das analytischen Modell die relevanten physikalischen Prozesse behinhaltet. In ASDEX Upgrade werden Blobeigenschaften bestimmt und in zwei Einschlussregimen, der sogenannten L- und H-Mode, verglichen. Wie schon in TJ-K zeigt sich eine weitgehende Übereinstimmung mit den analytischen Vorhersagen. Größenmessungen deuten einen Einfluss der hohen Ionentemperatur auf die Blobdynamik hin. Außerdem wird eine überraschend geringe Variation der Blobeigenschaften zwischen L- und H-Mode beobachtet.