Revealing the strong interplay between divertor geometry and E×B drifts on divertor dissipation in the DIII-D Small Angle Slot (SAS) divertor

Experiments in DIII-D and SOLPS-ITER modeling with drifts find a strong interplay between drifts and divertor geometry in facilitating divertor dissipation. Supported by SOLPS-ITER modeling, experiments show that for ion B×▽B drift out of the divertor, ‘unfavorable-B_t’, cold plasma with T_e≤10 eV across the entire SAS divertor target can be achieved at relatively low upstream densities. In contrast, for favorable-B_t T_e remains >20 eV at the target until eventual onset of detachment at a comparatively high n/n_GW. The coupling of divertor geometry and drift flows can strongly affect the path towards divertor detachment. With the strike point on the inner slanted surface and unfavorable-B_t, detachment bifurcations were observed with T_e suddenly falling below 5eV. This differs from the open divertor where the T_e cliff was only observed for favorable-B_t. SOLPS-ITER shows that, at a higher density, both the radial and poloidal E×B flows reverse direction causing a rapid density accumulation right near the separatrix which eventually results in low T_e across entire divertor target plate. These results indicate that the interplay between geometry and drifts needs to be fully considered in future fusion reactor divertor designs.