Overview on the radial electric field, plasma rotation and transport in the stellarator W7-AS

In the advanced stellarator W7-AS the radial electric field Er is measured by active charge exchange recombination spectroscopy CXRS. In parallel, it is calculated by using the neoclassical DKES code. A comparison of calculated and measured solutions reveals in how far the neoclassical model is valid for the description of the radial particle transport and the formation of Er. In general good consistency is found, even for the outer radii where the neoclassical fluxes become much smaller than the experimental ones. In this paper the interplay between the particular Er roots and transport is considered. For strongly positive Er a reduction of χe is observed in the vicinity of the magnetic axis. The typically negative ion-root in the gradient region strongly influences the local ∇ Ti, thus determining the maximum attainable Ti(0). (author)