The spatial structure of edge fluctuations in the Wendelstein 7-AS stellarator

In the scrape-off layer and outermost edge of the Wendelstein 7-AS (W7-AS) stellarator, fluctuations of, mostly, ion saturation current I-sat and floating potential Phi(fl) have been measured with high poloidal, radial (both 2-3 mm) and temporal (0.5 mus) resolution. Parallel to the magnetic field, measurements with two probe tips on the same magnetic field line were performed. The basic spatio-temporal structure of the fluctuations is presented in terms of correlation functions and of wavenumber-frequency (kf) spectra. They are characterized by the parameters of a fit function to the correlation function, which describe the spatio-temporal structure. The behaviour of these fluctuation parameters under variations of the discharge parameters is investigated. A high correlation of the fluctuations parallel to the magnetic field is documented, and we analyse, which frequency components contribute to this high correlation. In our radially and poloidally resolving measurements, we find that the fluctuation structures are inclined in the poloidal-radial plane at our position of measurement. The consequences of such inclined structures for the interpretation of measurements with one-dimensional arrays are discussed, with an emphasis on the risk of an erroneous interpretation of purely radially resolving measurements. There is strong evidence that this inclination is due to the local magnetic shear in W7-AS in conjunction with the high correlation of the fluctuations parallel to the magnetic field. A further contribution to the inclination may originate from the radial shear of the poloidal velocity of the fluctuations in the plasma edge. The different consequences of magnetic shear and velocity shear for an inclination of the fluctuations in the poloidal-radial plane are discussed.