Sheared poloidal plasma flows in the island divertor scrape-off layer of Wendelstein 7-X

This is the authors' abstract. We don't add key points for this paper.

Gas puff imaging (GPI) has been routinely operated since the installation of the water-cooled high heat flux divertor in the Wendelstein 7-X (W7-X) stellarator. GPI provides the first systematic, two-dimensional experimental characterization of perpendicular plasma flows in the magnetic island divertor scrape-off layer (SOL) on W7-X. Using spatiotemporal analysis of turbulent fluctuations, we find that, across magnetic configurations and plasma scenarios, poloidal flows ubiquitously dominate over radial flows, with typical poloidal velocities of order km s − 1 . In contrast to tokamaks, radially propagating blob filaments are not observed, consistent with the predominantly normal fluctuation statistics measured in the island SOL. Net radial motion is detected only in the vicinity of an X -point of the magnetic island, which is accessed in a low-iota magnetic configuration by the present GPI field-of-view. In general, the poloidal flow profiles in the island SOL exhibit a complex structure, with multiple counter-propagating flow channels as narrow as 1 cm. The experimental parameters that order the observed poloidal flow structure near island O -points are investigated. Both the magnitude of the poloidal velocity and the number of shear layers, as well as their radial location, depend sensitively on the size and position of the magnetic islands, as well as on the toroidal current, plasma density, and toroidal field direction. These results demonstrate that cross-field transport in the W7-X island divertor SOL is correlated primarily with topology-driven E × B sheared flows, rather than by radial filamentary transport, and provide the first experimental documentation of the complex, shear-layer-dominated flow dynamics in the island divertor over a wide range of plasma conditions.