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EBW power deposition and current drive in WEGA—comparison of simulation with experiment

Josef Preinhaelter1, Heinrich P Laqua2, Jakub Urban1, Linda Vahala3 and George Vahala4

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Detailed computational studies of electrostatic electron Bernstein waves (EBWs) propagation in the WEGA stellarator are performed and compared with experimental results. Using the WEGA antenna, the two O-/X-mode radiation lobes are modelled by sets of rays whose intensities are proportional to the measured radiation pattern. After projecting these rays onto the plasma periphery, the O–X-EBW mode conversion efficiency around the upper hybrid resonance is determined from a full wave adaptive mesh solver of the cold plasma equations. From the roots of the electrostatic EBW dispersion relation, ray tracing is performed to determine the power absorption on the first or second cyclotron harmonic as well as current drive assuming the Fisch–Boozer mechanism. Good agreement is achieved between our EBW simulations on specific WEGA equilibria and the experimental results from the antenna launch of 2.45 GHz waves. The experimentally observed off-axis power deposition and the outward shift dependence of the absorption maxima on increasing magnetic field can only be explained by the existence of a hot electron component in the WEGA plasma. It is this hot electron component that permits wave absorption at the second harmonic near the plasma boundary. Moreover, the simulations not only reproduce the current density reversal at the plasma centre for low magnetic fields but also the destruction of this current density reversal for larger magnetic fields.


PACS

52.35.Hr Electromagnetic waves (e.g., electron-cyclotron, Whistler, Bernstein, upper hybrid, lower hybrid)

52.40.Db Electromagnetic (nonlaser) radiation interactions with plasma

52.55.Wq Current drive; helicity injection

52.65.-y Plasma simulation

52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)

52.55.Hc Stellarators, torsatrons, heliacs, bumpy tori, and other toroidal confinement devices

Subjects

Plasma physics

Dates

Issue 12 (December 2009)

Received 6 May 2009, in final form 7 October 2009

Published 4 November 2009



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