High ion temperatures and high beta in W7-AS

Experimental conditions concerning magnetic configurations, heating scenarios and density control for generating high ion temperatures and high beta in W7-AS are described and experimental results are compared with theoretical predictions. Ion temperatures of up to 1.5 keV, the highest in helical systems so far, have been achieved. Energy confinement times about twice as long as predicted by the International Stellarator Scaling are derived for these discharges. This improvement is clearly related to the steep temperature and density gradients in the outer part of the plasma. In the bulk pan, the experimental panicle and energy fluxes as well as the radial electric field, E-r, are well consistent with neoclassical calculations, but a discrepancy is found for E-r in the gradient region of the profiles, whereas at the outermost part the agreement is good again. The strongly negative E-r in the profile gradient region forms a transport barrier with respect to the ion channel. Both sufficient high ion heating and low anomalous particle fluxes at the outer radii, i.e. low recycling, are mandatory for establishing this strongly negative E-r. The effect of this poloidal plasma rotation on the edge turbulence and confinement may be quite similar as in tokamaks. The availability of 3.0 MW NBI input power gave access to higher beta values than in the previous experimental campaigns. Values of up to <beta > approximate to 1.82 have been obtained with maximum heating at magnetic fields between 0.8 and 1.25 T. The achieved beta values are in the range of the predicted stability and equilibrium beta limit. No hard stability limit has been observed so far.