Studies of the SOL impurity stagnation point in DIII-D using Coherence Imaging Spectroscopy with comparisons to UEDGE

Coherence imaging spectroscopy (CIS) is used to study main-chamber scrape-off-layer (SOL) carbon flows under a variety of divertor configurations in L and H-mode discharges on DIII-D. In lower-single-null (LSN) DIII-D L-mode discharges, regardless of the ion BΧ∇B (∇B) drift direction or the degree of detachment, the C²⁺ flow is found to stagnate at a single point near the crown of the main plasma, away from the divertor X-point. In contrast, in upper-single-null and matched discharges, the C²⁺ flow stagnates near the low-field-side (LFS) X-point. These experimental data are compared to UEDGE fluid simulations with drifts. For LSN, UEDGE predictions for the C²⁺ stagnation point roughly agree with experiment in both ∇B drift directions. UEDGE predicts the stagnation point moves from the crown to the outer midplane as the simulation density (n_e) increases towards detachment. In separate experimental studies, in LSN H-mode experiments the C²⁺ stagnation point moves from the LFS X-point to the plasma crown depending on the direction and magnitude of the plasma current (I_p). Impurity gas injection in some high-power H-modes is found to influence the C²⁺SOL velocity depending on the ∇B drift direction. Finally, high-power negative triangularity discharges show a dynamic C²⁺ SOL flows stagnation point that depends on n_e, injected neutral beam torque, I_p, and ∇B direction.