Density fluctuation phenomena in the scrape-off layer and edge plasma of the Wendelstein 7-AS stellarator

The fluctuation of electron density in the scrape-off layer (SOL) and edge plasma regions of the Wendelstein 7-AS stellarator [H. Renner , Plasma Phys Controlled Fusion 31, 1579 (1989)] is investigated by the beam emission sp ectroscopy technique, observing fluctuations in the Li(2p) line emission in tensity of a 50 keV accelerated Li beam. A recently developed numerical tec hnique enables correct reconstruction of correlation functions (and as a co nsequence, fluctuation amplitudes as well) of the electron density fluctuat ions from correlations of beam light fluctuations. Depending on the plasma electron density, the radial range of the measurement is either limited to r/a > 0.9 (at the highest densities) or can extend to about r/a > 0.3 at th e lowest densities. As the technique is nonperturbing to the plasma, severa l hundred shots were analyzed and a catalog of the observed phenomena was e stablished. The results always show a change in the properties of the fluct uations at approximately the last closed flux surface. This observation mig ht indicate differences in the turbulence drive in the SOL and the edge pla sma. However, one cannot exclude that the same turbulence phenomenon result s in different density fluctuations in the two regions due to the different magnetic topologies. The relevance of the observed phenomena to anomalous transport is discussed. (C) 1999 American Institute of Physics. [S1070-664X (99)00711-9].