Kinetic modelling of the ECRH power deposition in W7-AS

Kinetic effects are important in low-density high-power ECRH discharges, and the electron distribution function can significantly deviate from Maxwellian. The ECRH power deposition is analysed for perpendicular on-axis heating in W7-AS, with different magnetic configurations characterized by either a minimum or a maximum of B on the plasma axis in the RF injection plane. The different heating scenarios are modelled by means of a new bounce- averaged Fokker-Planck code, well suited for the magnetic field geometry close to the plasma axis of W7-AS. The power deposition profile is estimated from the analysis of heat wave propagation stimulated by ECRH power modulation. In general, peaked deposition profiles as predicted from a ray-tracing code are obtained, but with an additional much broader contribution. The broadening of the thermal power deposition profile is assumed to be related to the radial transport by the rB drift of locally trapped suprathermal electrons. This is simulated by means of a simple convective Fokker-Planck model. The theoretical predictions are shown to be consistent with the experimental findings. Kinetic effects on the determination of the temperature both by Thomson scattering and by ECE diagnostics are briefly discussed.

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