BOUNDARY-LAYER STUDY ON THE WENDELSTEIN 7-AS STELLARATOR

At low iota (similar to 0.34), W7-AS is operated with limiter; smooth flux surfaces extend into the limiter shadow. At higher iota, the plasma surface is separatrix dominated. The resonant 5/m island chain gives rise to corrugated surfaces. This paper deals with two difficulties on the way to a complete edge transport modelling in this complex 3D geometry. (1) At low iota, strong deviations from exponential edge density decay (shoulders) are observed. Measurements with reversed B-field indicate that they are caused by superimposed, stationary E(theta) X B particle drifts. (2) The open edge topology of the corrugated high iota case cannot be represented by standard magnetic coordinates. As a first step towards 3D modelling, the open structures were parametrized on the basis of the flux conserving, open magnetic surface concept. As a first application, wall recycling fluxes for a low density ECRH discharge have been derived by mapping Langmuir probe data onto the wall along the open surfaces and used to simulate the 3D neutral gas distribution with the EIRENE code.