ECRH AND ECCD EXPERIMENTS IN AN EXTENDED POWER RANGE AT THE W7-AS STELLARATOR

An overview on physics studies on Electron Cyclotron Resonance Heating (ECRH) and ECCurrent Drive (ECCD) in an extended parameter range at W7-AS is presented. Experiments were performed with an upgraded ECRH power of up to 1.3 MW at 140 GHz. Electron temperatures of up to 5.7 keV were measured, which can only be explained by the beneficial effect of positive radial electric fields (‘electron root’). The experiments confirm, that the electric field is generated by ECRH driven particle losses in the specific stellarator magnetic field. ECCD experiments were performed at high input power (1.3 MW) resulting in EC-driven currents of up to 20 kA. The direction of the EC-driven current was varied in co- and counter-direction with respect to the bootstrap current in discharges with zero net-current. Three current contributions, i.e. the EC-driven current, the bootstrap current and the inductively driven current are calculated independently and modify the internal profile of the rotational transform significantly. A comparison with quasi-linear theory shows significant deviation in the co-current drive case, which may be attributed to strong MHD activity and/or violation of the quasilinear assumptions due to the high power density. 1