Recent results with 140 GHz ECRH at the W7-AS stellarator

Net current free plasma build-up and heating is achieved at W7-AS with up to 0.8 MW ECRH at 70 GHz (< 3 s) with Bres = 2.5 T and ne,crit = 6.2x1019 m-3 (1st harmonic O-mode). A 140 GHz prototype gyrotron with 0.5 MW power (< 1.1 s) was set into operation recently. First experiments with 2nd harmonic X-mode launch focused on high density operation up to 1.1x1020 m-3, which could be achieved up to now with NBI only. H-mode transitions, which are well known from tokamaks were observed in W7-AS. The operational window for the H-mode is discussed. The basic features of the Stellarator H-mode were reported in previous papers. Here we concentrate on the influence of gas puffing on the H-transitions and heat wave experiments. Combined heating experiments with NBI were performed, which is inherently related to high density operation. Typical high-power NBI-heated plasmas in W7-AS are non-stationary, because beam particle fuelling and recycling causes a steady density rise. Under combined heating conditions, however, the density can be controlled despite the beam fuelling and a strong impact on the impurity confinement was found. (author) 6 refs., 4 figs