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 × ∇𝐵 out of the divertor. The 𝐶2+ 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 𝐶2+ poloidal emission is hollow, peaking near the divertor legs, in the open configuration. In contrast, in the closed configuration, the 𝐶2+ 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 mainchamber 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.
