Speaker
Description
The Wendelstein 7-X (W7-X) stellarator experiment uses the island divertor concept for power and particle exhaust. In the most studied configurations, the scrape-off layer (SOL) includes a chain of five islands (m/n = 5/5) around the core. To characterize the SOL, different toroidally separated diagnostics view separate islands. The five-fold toroidal symmetry is typically used to combine measurements of separate quantities from different islands.
The gas puff imaging (GPI) diagnostic is a SOL turbulence diagnostic, imaging electron density/temperature fluctuations via H$_α$ emission of a localized gas puff at high spatio-temporal (mm/μs) resolution in W7-X [1]. It observes complex drift flow patterns in the islands [2] not yet included in any validated code. These flow patterns and SOL profiles are strongly dependent on the magnetic geometry of the island, and discharge parameters such as power and density.
GPI measurements show sudden shifts of light emission and fluctuation parameters, indicating corresponding changes in plasma parameters. These transitions are not reflected in global core plasma parameters. In addition, divertor heat flux measurements and further SOL diagnostics viewing different islands show that these sudden transitions often are isolated to just one of the five islands. This raises the question under which conditions the assumption of five-fold symmetry applies to processes and physical quantities in the SOL. In this contribution, we investigate the prevalence and asymmetry of these profile shifts, as well as their implications for the comparability of physical quantities in different islands.