Analysis of large-scale fluctuation structures in the scrape-off layer of the Wendelstein 7-AS stellarator

Floating potential and ion saturation current fluctuations in the scrape-of f layer of the Wendelstein 7-AS stellarator are investigated by using condi tional averaging. If floating potential fluctuations are taken as the refer ence signal for the conditional averaging procedure, large-scale single-vor tex structures are predominately found. If ion saturation current fluctuati ons are taken as the reference signal, double-vortex structures are detecte d. The phase shift between the two different structures is pi /2. The doubl e-vortices are interpreted as a superposition of counter rotating single-vo rtex potential structures. Both lead to ion saturation current fluctuations of the same sign due to ExB convection of plasma in parallel to the backgr ound plasma pressure gradient. The conditional averaging approach is direct ly compared to cross-correlation analysis. Both methods lead to similar phy sical conclusions. Lifetime and extent of structures are systematically ove restimated by correlation analysis. It is demonstrated that only fluctuatio ns with amplitudes larger than 0.5 sigma contribute to the detected large-s cale structures. (C) 2001 American Institute of Physics.