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 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 Investigation of microwave heating scenarios in the magnetically confined low-temperature plasma of the stellarator TJ-K(2010) Köhn, Alf; Stroth, Ulrich (Prof. Dr.)The generation and heating of plasmas by means of microwaves is a widely-used method. This is the case for high-temperature fusion plasmas as well as for low-temperature plasmas. In fusion plasmas, the absorption of the microwave is well understood: The wave couples resonantly to the cyclotron motion of electrons around the magnetic field lines. The efficiency of the heating depends strongly on the temperature of the electrons. In low-temperature plasmas, the electrons have temperatures in the range of 1-10 eV. At these temperatures, which are low compared to those in fusion plasmas, the cyclotron resonance only plays a role for the plasma breakdown. Hence, other mechanisms must be used for plasma heating. One possibility is heating by electron Bernstein waves. They must be excited by mode conversion processes in the plasma, because they cannot propagate in vacuum. Another candidate is heating at the upper-hybrid resonance. The stellarator TJ-K is a low-temperature experiment at which microwave heating can be carried out at two different frequencies: at 2.45 GHz and in the range around 8 GHz. The thesis presented here, investigates the possible heating scenarios in TJ-K. To numerically study the interaction of the microwave with the plasma, the full-wave code IPF-FDMC was developed. With this code, the efficiency of the conversion process of an electromagnetic wave into the electrostatic electron Bernstein wave could be investigated in detail for different fusion-relevant experiments in Europe. Both the experimental and the numerical results show that, in TJ-K, most of the microwave power is absorbed at the upper-hybrid resonance. To understand the high absorption coefficient, the reflecting vacuum vessel walls are of vital importance. In the present experimental configuration of TJ-K, heating by Bernstein waves does not play an important role. In the course of these investigations, a new operational regime was discovered in which it is possible to efficiently heat plasmas, albeit there is no resonance for the injected microwave in the plasma.Item Open Access Full-wave modeling of the O-X mode conversion in the Pegasus toroidal experiment(2011) Köhn, Alf; Jacquot, Jonathan; Bongard, Michael W.; Gallian, Sara; Hinson, Edward T.; Volpe, FrancescoThe ordinary-extraordinary (O-X) mode conversion is modeled with the aid of a 2D full-wave code in the Pegasus toroidal experiment as a function of the launch angles. It is shown how the shape of the plasma density profile in front of the antenna can significantly influence the mode conversion efficiency and, thus, the generation of electron Bernstein waves (EBWs). It is therefore desirable to control the density profile in front of the antenna for successful operation of an EBW heating and current drive system. On the other hand, the conversion efficiency is shown to be resilient to vertical displacements of the plasma as large as ±10 cm.