Gas puff imaging of filament dynamics in the scrape-off layer of Wendelstein 7-X

In order to study turbulent transport in the scrape-off layer in and around the magnetic islands that are key to the divertor concept employed on the W7-X stellarator, a gas puff imaging (GPI) diagnostic [1] has been operated for the first time in its most recent OP2.1 campaign. GPI observes line emission from excitation of a collimated cloud of hydrogen or helium gas in a 45x78 mm area using an 8x16 array of avalanche photodiodes at 2 MHz. Recordings show coherent structures 1-3 cm in diameter in the direction perpendicular to the magnetic field, typically propagating at poloidal speeds of 1-5 km/s. GPI sees several oppositely directed flows near the O-point of an island, all of which reverse when the toroidal magnetic field is reversed, indicating that this motion is likely due to ExB drifts. The 2D electric potential structure implied by these flows is qualitatively supported by probe measurements. The filament poloidal velocity is found to be sensitive to the size and radial position of the islands. The complex radial motion observed in some cases is compared to the usually dominant poloidal motion.

[1] J.L. Terry et al., Bulletin of the American Physical Society, NP11.00043 (2022)