TURBULENCE IN THE SOL OF ASDEX AND W7-AS

Electrostatic fluctuations have been measured in the scrape-off layer of ASDEX and W7-AS by Langmuir probes and by observation of H-alpha li ght and light emitted from a fast Li atom beam with high spatial and t emporal resolution. It was demonstrated that these fluctuations contri bute a significant, if not dominant, fraction of the anomalous radial particle transport. The basic properties of the fluctuations are the s ame in both experiments. A model for an instability mechanism specific to the SOL is presented including density, temperature and electric p otential fluctuations. From this model mixing length estimates for the radial transport and resulting density and pressure gradients in the SOL are derived and compared to measurements in the mid-plane and in t he divertor of ASDEX. In spite of several simplifications in the model a quantitative agreement up to factors of 1-3 and a qualitative agree ment for variations of discharge parameters is achieved between the mo del predictions and the measurements. Data from poloidal multi-pin pro be arrays are decomposed into a sum of spatial-temporal ''events'' by means of a fitting procedure. Centres of selected events serve as refe rence points for the analysis of the dynamics in their surroundings. A veraging shows that positive and negative potential events appear most ly in pairs with the E x B drift in between directed radially outward.