TURBULENT FLUCTUATIONS AND RADIAL TRANSPORT IN THE SCRAPE-OFF LAYER OF THE ASDEX TOKAMAK

Electrostatic fluctuations have been measured in the scrape-off layer of ASDEX by Langmuir probes and by observation of H-alpha light with h igh poloidal and temporal resolution. It was demonstrated that these f luctuations contribute a significant, if not dominant, fraction of the ''anomalous'' radial particle transport. A model for an instability m echanism specific to the SOL is presented including density, temperatu re and electric potential 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 m id-plane and in the divertor of ASDEX. In spite of several simplificat ions in the model a quantitative agreement up to factors of 1-3 and a qualitative agreement for variations of discharge parameters is achiev ed between the model predictions and the measurements.