Investigation of turbulence rotation and radial electric field in the island divertor and plasma edge at W7-X

Diverted plasmas in the stellarator W7-X are investigated using a hopping poloidal correlation reflectometer and a set of Langmuir probes to investigate the plasma edge and the scrape off layer at two different toroidal positions and poloidal cross sections, respectively. The properties of the scrape off layer are studied as function of two magnetic configurations and as function of plasma parameters. The experimentally determined radial position of the inversion point of the radial electric field is compared with calculations. For certain magnetic configurations a remnant island in the scrape off layer is observed and studied as function of the plasma current. Different methods to determine the radial electric field are applied which allow to estimate the phase velocity of the turbulence. The measured phase velocity is small and shows some evidence for the existence of resistive ballooning modes in the scrape off layer. Spectrum decomposition of coherence spectra is applied for a radial localization of modes and to determine their properties. An estimate of the radial correlation length of the broad band (BB) turbulence in the vicinity of the last closed flux surface is calculated from the decrease of BB turbulence amplitude as obtained from the coherence spectra. The obtained radial correlation length is found to be lambda(r) approximate to 15 mm.