Survey of magnetohydrodynamic instabilities in the advanced stellarator Wendelstein 7-AS

Magnetohydrodynamic (MHD) instabilities in the Wendelstein 7-AS stellarator (W7-AS) [G. Grieger , Phys. Fluids B 4, 2081 (1992)] are characterized exp erimentally in various plasma parameter regimes and heating scenarios. The observations are compared with theoretical predictions for particular cases . In the high-beta range ([beta] less than or equal to 2%) no clear evidenc e of a stability beta -limit could be found yet. In the lower beta regime f ast particle driven global Alfven modes are the most important instabilitie s during neutral beam injection (NBI). Besides of coherent modes with almos t no effect on the plasma performance additional Alfven modes appear at hig her frequencies up to 400 kHz, which show nonlinear phenomena-like bursting , frequency chirping, and MHD induced energy and fast particle losses. The activity of edge localized modes (ELMs) is investigated in NBI heated disch arges. The issue of current driven instabilities and their potential stabil ization by a stellarator field has been investigated with regard to the des ign of compact hybrid stellarator systems. (C) 2001 American Institute of P hysics.