Electron cyclotron current drive and wave absorption experiments in the W VII-AS stellarator

Experiments on non-inductive current drive by electromagnetic waves in the vicinity of the first and second harmonic of the electron cyclotron frequency (ECCD) were performed at the W VII-AS stellarator with up to 1 MW RF power at 70 GHz in long pulse operation (< 1.5 s). The single pass absorption as an important input quantity for current drive investigations was directly measured for first harmonic O-mode and second harmonic O- and X-mode operation, respectively, in a wide plasma parameter range. A good agreement with a 3-D ray tracing model was found. ECCD was investigated by (a) a toroidal launch angle variation of the microwave beams while the total net current was kept close to zero (Ip < 0.2 kA) by feedback control of the OH transformer. The change of the required loop voltage with respect to perpendicular launch (no ECCD) was measured as a function of the launch angle; (b) the adjustment of the launch angle of the microwave beams to balance the bootstrap current without making use of the OH transformer (counter current drive). Here, the EC wave driven current is measured in units of the bootstrap current; (c) a perturbation experiment at up/down shifted frequencies, where 0.2 MW were launched at a fixed toroidal angle and the EC resonance layer was shifted out of the confinement region (B0 variation). The loop voltage change required for compensation of the EC driven current was measured as a function of the magnetic induction. The parameter dependence on the launch angle, the electron temperature, the plasma density and the microwave power for all types of ECCD experiments is in good agreement with a linear theoretical model, which takes into account quasi-linear and trapped particle effects. The results are confirmed by Fokker-Planck calculations. (author). 8 refs, 4 figs