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

Abstract

Floating potential and ion saturation current fluctuations in the scrape-off layer of the Wendelstein 7-AS stellarator are investigated by using conditional averaging. If floating potential fluctuations are taken as the reference signal for the conditional averaging procedure, large-scale single-vortex structures are predominately found. If ion saturation current fluctuations are taken as the reference signal, double-vortex structures are detected. The phase shift between the two different structures is π/2. The double-vortices are interpreted as a superposition of counter rotating single-vortex potential structures. Both lead to ion saturation current fluctuations of the same sign due to E×B convection of plasma in parallel to the background plasma pressure gradient. The conditional averaging approach is directly compared to cross-correlation analysis. Both methods lead to similar physical conclusions. Lifetime and extent of structures are systematically overestimated by correlation analysis. It is demonstrated that only fluctuations with amplitudes larger than 0.5σ contribute to the detected large-scale structures.