KINETIC MODELING OF THE ECRH POWER DEPOSITION IN W7-AS

Kinetic effects are important in low-density high-power ECRH discharge s, and the electron distribution function can significantly deviate fr om Maxwellian. The ECRH power deposition is analysed for perpendicular on-axis heating in W7-AS, with different magnetic configurations char acterized by either a minimum or a maximum of B on the plasma axis in the RF injection plane. The different heating scenarios are modelled b y means of a new bounce-averaged Fokker-Planck code, well suited for t he magnetic field geometry close to the plasma axis of W7-AS. The powe r deposition profile is estimated from the analysis of heat wave propa gation stimulated by ECRH power modulation. In general, peaked deposit ion profiles as predicted from a my-tracing code are obtained, but wit h an additional much broader contribution. The broadening of the therm al power deposition profile is assumed to be related to the radial tra nsport by the del B drift of locally trapped suprathermal electrons. T his is simulated by means of a simple convective Fokker-Planck model. The theoretical predictions are shown to be consistent with the experi mental findings. Kinetic effects on the determination of the temperatu re both by Thomson scattering and by ECE diagnostics are briefly discu ssed.