Investigation of turbulence rotation in the SOL and plasma edge of W7-X for different magnetic configurations

Abstract

The W7-X stellarator is optimized with respect to neoclassical transport. Therefore turbulent transport plays an important role. It is equipped with an inertial cooled graphite divertor which intersects the island chain at the plasma edge depending on the magnetic configuration. Additional control coils and the plasma current modify the iota profile at the plasma edge and shift the position of the island chain. To monitor the effects on the poloidal propagation velocity in the scrape-off layer (SOL) and the plasma edge, an O-mode Poloidal Correlation Reflectometer (PCR) is used which simultaneously monitors the propagation of low-k turbulence. Operating in the density range of 0.6 × 1019 m−3 to 2 × 1019 m−3 it covers a large part of the SOL and the plasma edge and allows for the experimental determination in the last closed flux surface (LCFS) and the associated shear layer in low to mid-density discharges. In this paper it is shown that the propagation in the shear layer and its vicinity is determined best, when based on an elliptical model. Different magnetic configurations with magnetic edge topology of five independent islands for ι = 1 and six linked islands for ι = 0.81 are investigated. Also the effects of the plasma current and additional control coils on the edge magnetic topology are studied. The coherence spectra of antenna pairs for different poloidal separations is investigated. Using a decomposition method for the measured coherence spectra the characterization of turbulence spectra is possible with respect to e.g. broad band turbulence and quasi coherent modes. A strong reduction of the broad band turbulence is observed in the vicinity of the LCFS which is evidence for the suppression of low-k turbulence at the shear layer.