Measurements of electron cyclotron emission during high power ECRH in the W7-AS stellarator

When applied to high power ECRH, the quasi-linear theory predicts the formation of a mildly suprathermal tail and a flattening in the perpendicular energy direction for the electron distribution function close to the resonance zone. In case of low field side perpendicular injection, the incident ECRH power is expected to be absorbed by ''thermal electrons'' and, contrary to the lower hybrid heating, no simple analytical model for the steady-state distribution function has been developed. Due to its complementarity with the absorption mechanism, the cyclotron emission can provide attractive tools to perform investigations on the impact of very high ECRH-power on the electron distribution function. With this aim, observation along viewing chords of ''constant-B'' are favourable as they permit, in principle, a detailed investigation of the high-energy part of the electron distribution function without the interference of re-absorption. Experiments of this kind have been carried out in the W7-AS stellarator, during high power ECRH in low density plasmas ((O)-mode, perpendicular injection from low field side). (author) 5 refs., 4 figs