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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.
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.
Open Access
Entwicklung und spektroskopische Untersuchung eines Mikrowellen-Plasmabrenners für die Schichtabscheidung aus Pulvern
(2012) Kopecki, Jochen; Stroth, Ulrich (Prof. Dr.)
Nachhaltige Energiekonzepte, wie die Verwendung von regenerativen Energien, spielen in Zeiten von immer größer werdendem Energiebedarf eine entscheidende Rolle. Insbesondere im Bereich der Photovoltaik hat Deutschland seit jeher eine Vorreiterrolle eingenommen. Damit ist Deutschland der zentrale Standort für Photovoltaikforschung. In der vorliegenden Arbeit wird eine Möglichkeit zur plasmagestützten Abscheidung von amorphem Silizium aus Pulver entwickelt und untersucht, um eine kostengünstige Alternative zu den bestehenden Verfahren zu finden. Die Plasmaquelle hierfür basiert auf einem Mikrowellenresonator, welcher mit Hilfe von numerischen Simulationen mit CST Microwave Studio dimensioniert wurde und für den Betrieb bei Atmosphärendruck ausgelegt ist. Mit diesem Plasmabrenner ist es möglich ein linear ausgedehntes, freistehendes Plasma zu erzeugen, in dem Si-Partikel verdampft werden können. Die Partikel werden hierfür über einen Trägergasstrom eingeblasen und müssen während ihrer Verweilzeit im Plasma die benötigte Energie zum Verdampfen aufnehmen. Dieser Prozess wird im Detail untersucht, da sich über die Plasmalänge und den Gasfluss die maximal verwendbare Partikelgröße einstellt und für die Abscheidung von qualitativ hochwertigem amorphen Silizium der Schichtbildner vollständig gasförmig sein muss. Die Plasmaparameter T_gas, T_e und n_e, welche ebenfalls ausschlaggebend für den Beschichtungsprozess sind und in die verwendeten Verdampfungsmodelle eingehen, werden mittels optischer Emissionsspektroskopie in Abhängigkeit der Gaszusammensetzung ermittelt. Hierfür werden sowohl die relativen Intensitäten sowie die Linienprofile der Atomlinien der Balmerserie gemessen und ausgewertet. Die abgeschiedenen Schichten wurden mittels REM-, FTIR- und XPS-Analyse sowie der Photospektroskopie analysiert, wobei eine Verunreinigung der Schichten mit Sauerstoff ermittelt wurde, welche negative Auswirkungen auf die Leitfähigkeit der Schicht hat. Dennoch ist die Morphologie sowie die optische Qualtität der Schichten vergleichbar mit den Ergebnissen gängiger PECVD-Verfahren.
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.
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.
Open Access
Visualization of the O-X-B mode conversion process with a full-wave code
(2008) Köhn, Alf; Holzhauer, Eberhard; Stroth, Ulrich
The O-X-B mode conversion is a process to couple electromagnetic waves into an overdense plasma. At the vicinity of the cutoff, the wave is converted into a Bernstein wave, which is very well absorbed in the plasma without further density cutoff. Therefore, these waves are a promising tool to heat high-density plasmas. The conversion process has been investigated in great detail using a full-wave code, and for the first time, the time-dependent formation of the Bernstein waves has been visualized by using the data obtained with this simulation.
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.
Open Access
Investigation of three-dimensional turbulent structures in the torsatron TJ-K
(2007) Mahdizadeh, Navid; Stroth, Ulrich (Prof. Dr.)
In fusion plasmas, the turbulent transport is responsible for the major fraction of particle and heat losses, which degrade the confinement quality in a fusion reactor. Therefore, the investigation of the turbulence dynamics is of great importance. In the core of fusion plasmas, instabilities of interchange-type drive the turbulence, whereas in the plasma edge, the driving mechanism of turbulence is not yet clear. However, numerical simulations of plasma turbulence show that the drift wave should be the dominant instability in this region. It is well known that the edge plasma influences strongly the global confinement properties. Furthermore, the turbulence in the edge sets up a boundary condition for core and scrape of layer transport, which has still not been understood. Hence, the understanding of the physics of the plasma edge is an important issue of fusion research. The key element of the drift wave is the parallel electron dynamics. It can couple the drift wave to the shear-Alfvén wave and determines the degree of instability and the level of transport. On the other hand, the turbulence dynamics parallel to the magnetic field is strongly coupled to the dynamics perpendicular to it. Hence, detailed understanding of drift-wave turbulence needs both information on the perpendicular and the parallel dynamics. For the experimental investigation of plasma turbulence, diagnostics with high temporal and spatial resolution are required. Langmuir probes are well suited for such measurements. In dense and hot fusion plasmas, however, the use of Langmuir probes is limited to the scrape-off layer, which is the region outside the last closed flux surface. The toroidal low-temperature plasma in TJ-K is dimensionally similar to the one in the edge of fusion plasmas. In contrast to fusion plasmas, the whole plasma volume in TJ-K is accessible to Langmuir probes. This allows the use of probe arrays with a large number of tips and high temporal and spatial resolution. Such highly resolved spatial and temporal measurements cannot be carried out in fusion plasmas. A further important advantage of TJ-K is that the comparison of experimental and simulated data is possible in the plasma confinement region. To this end, simulations for the parameters of the TJ-K plasma were carried out using the turbulence code GEM3. In this work, for the first time, the three-dimensional nature of drift waves has been verified experimentally inside the confinement region of the toroidal plasma in TJ-K. The perpendicular dynamics of turbulence has been studied with the focus on the poloidal wavenumber spectra and the scaling of the turbulent structure with the drift scale. To this end, a 64 tip Langmuir probe array has been used, which is poloidally positioned on a flux surface. For the first time, the parallel dynamics of turbulence has been investigated in the core of a toroidally confined plasma. In contrast to previous experiments, multi-probe measurements were carried out to get simultaneous information on the shape and the propagation direction of the turbulent structures. The results for the parallel wave number and the parallel propagation velocity have been compared with results from the simulation code GEM3. It is demonstrated that the propagation in the direction parallel to the magnetic field is affected by Alfvén dynamics. Together, these results strongly confirm previous investigations, which have demonstrated the importance of drift-wave turbulence in TJ-K and therefore also in fusion edge plasma.
Open Access
Microwave heating of plasmas with the new 14 GHz system at the stellarator TJ-K
(2013) Loiten, Michael
The aim of this thesis has been to investigate the plasmas generated by the newly installed 14 GHz microwave heating system at TJ-K in the equilibrium state. The new heating system has been installed in order to operate TJ-K at a wider range of controllable parameters. Several diagnostics have been used to investigate the plasma: An interferometer was used to obtain the line averaged density. A radially movable device with three Langmuir probes was used to obtain the radial profiles of the electron density and the electron temperature. An optical diode was used to obtain the radiation mainly in the visible range, whereas a bolometer with eight channels was used in order to obtain the poloidal radiation profiles. In addition, the neutral gas pressure, the magnetic field (based on the current running through the coils), and the injected and reflected microwave power was measured. Magnetic and pressure scans in the new regime have been performed, meaning that the scanned parameter has been varied on a shot to shot basis, whereas the other parameters have been kept constant. In addition to increase the parameter space, the magnetic field has been varied in order to vary the power deposition in the plasmas. The pressure has been varied in order to approach regimes where neoclassical effects become important. When lowering the collisionality, collisional regimes where neoclassical effects dominates can be reached. Lower collisional regimes were found for low pressures in hydrogen. However, operation at these collisional regimes is not readily available as it was found that the plasmas become increasingly unstable when closing in on these regimes. With this heating system one can operate at higher magnetic fields, and thus increase the confinement of the plasma. It has been found that plasmas in this regime have higher densities than the previously installed heating systems. This makes the new heating system a good candidate in studying over-dense plasmas.
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.

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