MEASUREMENTS AND MODELING OF ELECTROSTATIC FLUCTUATIONS IN THE SCRAPE-OFF LAYER OF ASDEX

In the edge plasma of the ASDEX tokamak, electrostatic fluctuations were observed with Lang muir probes and in H-alpha light with high poloidal and temporal resolution. These fluctuations contribute a significant fraction to the 'anomalous' radial particle transport in the scrape-off layer (SOL). The basic properties and the dependence of the fluctuation parameters on the discharge conditions are documented. A model for an instability mechanism specific to the SOL is introduced and the experimentally observed fluctuation parameters are compared with the predictions of the Linearized version of this model. For plasma temperatures above similar to 10eV in the SOL the observed parameter dependences of the fluctuations are well reproduced by the model. By mixing length arguments the radial transport and the resulting density and pressure gradients in the SOL are estimated from the model. Their dependence on plasma temperature and density qualitatively agrees with the behaviour observed in ohmic discharges on ASDEX.