Visualization of the O-X-B mode conversion process with a full-wave code

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

Full-wave simulations of the O-X-B mode conversion in a realistic experimental geometry in the RFX-mod device

A full-wave code has been used to model the O–X–B mode-conversion process in RFX-mod. Parameter scans were performed to find the optimum launching condition for the microwave beam. Vacuum walls play an important role in the overall conversion efficiency, which is a key parameter for the success of the experiment. This is nicely illustrated in simulations

Perturbing microwave beams by plasma density fluctuations

The propagation of microwaves across a turbulent plasma density layer is investigated with full-wave simulations. To properly represent a fusion edge-plasma, drift-wave turbulence is considered based on the Hasegawa-Wakatani model. Scattering and broadening of a microwave beam whose amplitude distribution is of Gaussian shape is studied in detail as a function of certain turbulence properties. Parameters leading to the strongest deterioration of the microwave beam are identified and implications for existing experiments are given

Full-wave modeling of the O-X mode conversion in the Pegasus toroidal experiment

The 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

Untersuchung von Mikrowellen-Heizszenarien in dem magnetisch eingeschlossenen Niedertemperaturplasma im Stellarator TJ-K

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.Die Erzeugung und Heizung von Plasmen mittels Mikrowellen stellt eine weit verbreitete Methode dar. Dieses gilt sowohl für Hochtemperatur-Fusionsplasmen als auch für Niedertemperaturplasmen. In Fusionsplasmen ist die Absorption der Mikrowelle gut verstanden: Die Welle koppelt resonant an die Zyklotronbewegung der Elektronen um die Magnetfeldlinien an. Die Effizienz dieser Heizung hängt stark von der Temperatur der Elektronen ab. In Niedertemperaturplasmen haben die Elektronen Temperaturen im Bereich von 1-10 eV. Bei diesen, im Vergleich zu Fusionsplasmen, niedrigen Temperaturen, spielt die Zyklotronresonanz nur für die Zündung eine Rolle. Für die Heizung des Plasmas müssen daher andere Prozesse genutzt werden. Eine Möglichkeit stellt dabei die Heizung durch Elektron-Bernstein-Wellen dar. Diese müssen durch Modenkonversionsprozesse im Plasma angeregt werden, da sie im Vakuum nicht propagieren können. Eine weitere Möglichkeit ist die Heizung an der oberen Hybridresonanz. Bei dem Stellarator TJ-K handelt es sich um ein Niedertemperaturplasmaexperiment, an welchem Mikrowellenheizung bei zwei verschiedenen Frequenzen durchgeführt werden kann: bei 2,45 GHz und in dem Bereich um 8 GHz. Die vorliegende Arbeit untersucht die möglichen Heizszenarien in TJ-K. Um die Wechselwirkung der Mikrowelle mit dem Plasma numerisch studieren zu können, wurde der Wellencode IPF-FDMC entwickelt. Damit konnte der Konversionsprozess einer elektromagnetischen Welle in die elektrostatische Elektron-Bernstein-Welle genau untersucht und die Konversionseffizienz für verschiedene, fusionsrelevante Experimente in Europa optimiert werden. Sowohl die experimentellen also auch die numerischen Untersuchungen zeigen, dass in TJ-K der Hauptanteil der Mikrowellenleistung an der oberen Hybridresonanz absorbiert wird. Zum Verständnis des hohen Absorptionskoeffizienten ist die reflektierende Wand des Vakuumgefäßes von entscheidender Bedeutung. In der aktuellen Konfiguration von TJ-K spielt die Heizung durch Bernstein-Wellen keine entscheidene Rolle. Im Verlaufe dieser Untersuchungen wurde ein neuer Operationsbereich entdeckt, in welchem es möglich ist, Plasmen effizient zu heizen, obwohl sich keine Resonanz für die eingestrahlte Mikrowelle im Plasma befindet

Fusionsforschung : eine Einführung

In diesem Vortrag wird ein Überblick und eine Einleitung in das Gebiet der Fusionsforschung gegeben

Benchmarking full-wave codes for studying the O-SX mode conversion in MAST Upgrade

Three full-wave codes for simulating microwave propagation and O-SX mode conversion in magnetized plasma are described and compared. Their feasibility to investigate mode conversion processes and obtain conversion efficiencies for parameters relevant for a potential MAST Upgrade 28 GHz electron Bernstein wave heating scenarios is explored