04 Fakultät Energie-, Verfahrens- und Biotechnik
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/5
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Item Open Access Fusionsforschung : eine Einführung(2020) Köhn-Seemann, AlfIn diesem Vortrag wird ein Überblick und eine Einleitung in das Gebiet der Fusionsforschung gegeben.Item Open Access PFG-NMR studies of ATP diffusion in PEG-DA hydrogels and aqueous solutions of PEG-DA polymers(2018) Majer, Günter; Southan, AlexanderAdenosine triphosphate (ATP) is the major carrier of chemical energy in cells. The diffusion of ATP in hydrogels, which have a structural resemblance to the natural extracellular matrix, is therefore of great importance to understand many biological processes. In continuation of our recent studies of ATP diffusion in poly(ethylene glycol) diacrylate (PEG-DA) hydrogels by pulsed field gradient nuclear magnetic resonance (PFG-NMR), we present precise diffusion measurements of ATP in aqueous solutions of PEG-DA polymers, which are not cross-linked to a three-dimensional network. The dependence of the ATP diffusion on the polymer volume fraction in the hydrogels, φ, was found to be consistent with the predictions of a modified obstruction model or the free volume theory in combination with the sieving behavior of the polymer chains. The present measurements of ATP diffusion in aqueous solutions of the polymers revealed that the diffusion coefficient is determined by φ only, regardless of whether the polymers are cross-linked or not. These results seem to be inconsistent with the free volume model, according to which voids are formed by a statistical redistribution of surrounding molecules, which is expected to occur more frequently in the case of not cross-linked polymers. The present results indicate that ATP diffusion takes place only in the aqueous regions of the systems, with the volume fraction of the polymers, including a solvating water layer, being blocked for the ATP molecules. The solvating water layer increases the effective volume of the polymers by 66%. This modified obstruction model is most appropriate to correctly describe the ATP diffusion in PEG-DA hydrogels.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 Simulation of microwave beams with PROFUSION(2019) Plaum, BurkhardItem 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 Visualization of the O-X-B mode conversion process with a full-wave code(2008) Köhn, Alf; Holzhauer, Eberhard; Stroth, UlrichThe 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.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.