The effect of divertor closure on the impurity stagnation point in detached L-mode discharges on DIII-D

This paper presents experimental measurements of the location of the impurity flow stagnation point in the scrape-off-layer (SOL) of a tokamak plasma. Coherence imaging of carbon-2+ emission (465 nm) is used to track the main-chamber impurity velocity of DIII-D L-mode plasmas with B × ∇ B out of the divertor. The C 2 + flow stagnates near the top or crown of the plasma when an open divertor (no baffling) is used. In contrast, with matched conditions and using a divertor with baffling, the C 2 + flow stagnates near the outer divertor leg (X-point). The C 2 + poloidal emission is hollow, peaking near the divertor legs, in the open configuration. In contrast, in the closed configuration, the C 2 + emission is flat through most of the main-chamber SOL. Changing divertor dissipation from attached to detached conditions had only a minor effect on the main-chamber midplane impurity velocity. Numerical simulations using the multi-fluid edge transport code UEDGE including cross-field drifts show qualitative agreement with the open divertor experimental result. • Measurements of impurity flow stagnation point, carbon in this study, in DIII-D tokamak scrape-off-layer. • Coherence imaging used to track main-chamber C 2 + SOL flow. • Presents observation that DIII-D divertor baffling configuration changes where C 2 + flow stagnates.