Experimental and modeling studies of scrape-off-layer impurity velocity stagnation points in negative triangularity plasmas on DIII-D

Comparison between DIII-D measurements of impurity ion flow stagnation point (Carbon-2+ emitting at Te ~ 7- 10 eV) and modeling with drifts are made in positive (PT) and negative triangularity (NT) discharges in both ion directions. NT discharges provided high quality measurements of the C2+ SOL flows without edge localized modes and at high injected powers. Measurements of plasma flow stagnation points in the divertor and main-chamber SOL are needed to validate predictions of complex SOL physics models implemented in the SOLPS and UEDGE edge fluid codes. In NT discharges (Ip = 0.6-1 MA, ne/ng = 0.5-1.5, Pinj = 3-9 MW), with drift into the divertor, the main chamber C2+stagnation point is located on the high field side (HFS) and moves poloidally from the inner X-point up to the crown as the density is increased towards detachment. Changes of the main-chamber impurity stagnation point with density but before detachment are not observed in PT discharges. At detachment, the HFS C2+ radiation front sits stably above the X-point, below the flow stagnation point. These NT results are compared to UEDGE simulations with drifts, which are typically able to predict the location of the impurity stagnation point in PT L-mode discharges.