Gas puff imaging measurements of plasma turbulence in the magnetic island region of the W7-X scrape-off-layer*

Turbulent transport processes in the magnetic island region of the scrape-off-layer (SOL) plasma on the W7-X stellarator are being studied using gas puff imaging (GPI) [J. Terry et al, arXiv:2405.09705 (2024)]. GPI is ideal for characterizing the SOL flow dynamics perpendicular to the B field, considered to be the dominant transport channel in the island SOL. GPI reveals the radial structure of oppositely directed poloidal flows around the O-point region in the "standard" magnetic configuration. These flows originate from the ExB poloidal drift flows since the flows reverse with field reversal. The magnitudes of the poloidal flows increases with the plasma current, which is thought to affect plasma potential via changes in the island size and connection length profile. Two shear layers are generally observed along the radially inner and outer boundaries of the confined island plasma, and this region expands with increasing toroidal current. Furthermore, the skewness values of the fluctuation amplitude distributions change from negative to positive across the inner shear layer, suggesting that the Kelvin-Helmholtz instability may play a role in turbulence generation in addition to the interchange instability. Cross-correlation analysis shows evidence of turbulent structures being sheared at a shear layer. An overview of the flow dynamics in various magnetic configurations will be presented.