What are the leading parameters governing energy dissipation in the DIII-D SAS divertors?*

Recent DIII-D campaigns using a series of well diagnosed Small Angle Slot (SAS) divertors have confirmed experimentally that a combination of divertor closure and target shaping can be used to enhance cooling across the divertor target and increase dissipation, but with significant dependence on B_T direction. In these novel divertors, the roles of closure, target shaping, drifts, and scale lengths are all interconnected in optimizing dissipation. Previous SOLPS-ITER modeling indicated a small change of the original SAS geometry to a V shape (SAS-VW) should enhance dissipation when the ion B x grad B drift is directed into the divertor for better H-mode access. In contrast, SAS-VW experiments have shown little improvement in detachment access for a given upstream density, despite higher (2-7x) measured neutral pressures and compression for the new configuration. These effects persist as I_p and P_inj are increased. In-slot D₂ gas fueling is more effective (5-22%) in promoting detachment, in accord with modeling. In-slot impurity injection can yield 30% lower core Z_eff and 15% less confinement degradation after detachment. Modeling can also reproduce the improved detachment seen as the strike point moves inboard of the slot vertex.