Evaluating the influence of particle sources and drifts on SOL flow and stagnation point in DIII-D L-mode discharges using Coherence Imaging Spectroscopy and UEDGE modeling*

In lower-single-null (LSN) DIII-D discharges, regardless of the ∇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, the C²⁺ flow stagnates near the low-field-side X-point.  Coherence imaging spectroscopy (CIS) is used to study main-chamber scrape-off-layer (SOL) carbon flows under a variety of divertor configurations in L-mode discharges on DIII-D. CIS on DIII-D utilizes an in-situ velocity calibration technique to absolutely calibrate the zero-velocity phase shift. This allows for accurate estimation of the location of the C²⁺ flow stagnation point in the SOL. The degree of divertor closure and background neutral pressure slightly modifies the stagnation point with flow near the divertor region being generally decreased with higher background neutrals. These experimental measurements are compared to UEDGE fluid simulations with drifts. In detached conditions, UEDGE roughly agrees with the measured C²⁺stagnation point. In contrast, in attached conditions, UEDGE underpredicts the C²⁺ flow in the main SOL when compared to measurements, and does not show a clear stagnation point.