Edge plasma profile and particle transport study on the WENDELSTEIN7-AS stellarator

W7-AS has toroidally varying plasma cross sections (five field periods) and a complex magnetic surface topology at the edge which is more or less governed by 'natural' islands (at rationals 5/m of the rotational transform ι = n/m) of up to several centimeters radial and poloidal extension. The complexity is further increased by two movable, asymmetric local limiters introducing inhomogeneous connection lengths. Except a narrow ι-range close to 1/3, where magnetic surfaces extend to within the limiter shadow, a sophisticated plasma edge transport analysis requires three-dimensional treatment. In the case with ι just above 1/3 (closest to tokamak limiter configurations) reasonable results were obtained by a 1D (radial) approach basing on singular flux bundles. What we did now is, to explore the ground by Langmuir probes, in what manner the edge plasma parameter profiles are influenced by the resonant boundary structures in the higher-ι case (0.44 - 0.58, including the resonances at 5/11, 5/10 and 5/9). Further we searched for some ordering parameters allowing a comparison of estimated transport coefficients in the SOL at higher ι (at least in ranges free of stronger resonant perturbations) with results found at low ι. We focus on particle transport in this first attempt. (author) 5 refs., 3 figs