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

Abstract

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 $B\times \nabla B$ 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.