Transport study on the boundary plasma of the Wendelstein 7-AS stellarator

For a limiter defined SOL, as given by small values of the rotational transform, the edge topology of W7-AS is characterized by large bundles distributed regularly over the poloidal angle. These bundles are poloidally decoupled to a good approximation, as indicated by 2D resolved Langmuir probe data. Therefore, for ι≤0.4, a 1D radial plasma model is a reasonable first step to provide a qualitative description of the transport in the W7-AS boundary layer. The toroidal variation of the flux bundle geometry is taken into account by averaging the transport equations along the corresponding field lines. The effects of the neutral gas sources and the sensitivity of the diffusion coefficient to the unknown Ti profile are discussed. A ι scan of the diffusion coefficient shows a minimum at ι≅0.35, where smooth magnetic surfaces exist throughout the SOL. This suggests that topological effects related to perturbations at the 'natural' 5/m resonances may be responsible for the observed ι dependence of the diffusion coefficient. (orig.)