Effects of drifts on scrape-off layer transport in the W7-X stellarator

A key mission of the W7-X stellarator is developing the physics basis of the island divertor heat exhaust concept. To investigate the effects of drifts on transport in the island divertor scrape-off layer (SOL), plasmas with matched upstream parameters but oppositely directed magnetic fields, and therefore oppositely directed drifts, are compared. At low density (n_e < 2 × 10¹⁹ m^-3), C²⁺ flows measured by coherence imaging spectroscopy (CIS), which are a good proxy for the main ion flows, are near-unidirectional throughout the SOL and reverse direction upon field reversal. The divertor Langmuir probes (LP) measure a density asymmetry between upper and lower targets that also reverses with field direction. These observations are in qualitative agreement with a simple SOL particle and momentum transport model that includes the poloidal E × B drift. The model predicts the flow stagnation point to shift from the geometric center between targets to the X-point in the direction of the drift, thereby producing the observed unidirectional flow pattern. Both CIS and LP measurements indicate the drift velocity is ∼100 m/s at low density. As density increases, drift effects rapidly diminish in the island SOL but persist in the regions shadowed by divertor targets.