Electron cyclotron resonance heating and confinement in the W VII-A stellarator

Abstract

Plasma generation and heating of OH-current free plasmas by strong ECR-wave irradiation were studied on the Garching W VII-A stellarator for three kinds of wave launching: direct irradiation of the gyrotron modes (mainly TE02 mode corresponding to a 50% O-mode and 50% X-mode mixture) from the low field side, and advanced wave launching (linearly polarized TE11 and HE11 mode) in O-mode orientation (E//Bo, Bo, k perpendicular to Bo). The nonabsorbed fraction was reflected into the plasma in the X-mode (E perpendicular to Bo) by a focusing polarization twist reflector mounted to the inner torus wall. The experiments were performed at 28 GHz (1 Tesla resonance B-field) with 200 kW RF power and a pulse duration of <or approximately=40 ms. In all modes of operation clean stellarator plasmas could be generated from a neutral gas background (d2, H2) and could be heated to reasonable parameters. In the first case electron temperatures around 700 eV were achieved, whereas the central temperature could nearly be doubled (Te approximately 1200 eV) with the small aperture polarized waves. However, in both cases the heating efficiency was almost the same (<or approximately= 40% and 50%, respectively). The reflected X-mode fraction does not contribute to bulk plasma heating via Bernstein wave conversion and absorption as expected. The reason seems to be local absorption of the arising Bernstein waves due to a macroscopically turbulent structure around the conversion zone (upper hybrid layer). Correlated with X-mode irradiation direct ion heating was observed (500 eV ion tail), possibly due to wave decay into lower hybrid waves. Studies of the transport and confinement properties of the l=2 stellarator in the plateau and LMFP regime were performed by variation of the characteristic parameters (rotational transform t, plasma density, RF power, power deposition profile, etc.).