Experiments on plasma detachment in the DIII-D V-shaped slot divertor*

Experiments in DIII-D demonstrate that the upstream plasma density to detach an un-pumped slot divertor is similar for a V-shaped and a flat-end slot, despite significantly higher neutral pressure in the V-shaped slot and in contrast to SOLPS-ITER predictions. However, the upstream density for detachment is reduced by using in-slot instead of main-chamber gas fueling or by placing the strike point on the inner slanted slot baffle instead of the slot end. In 2021, DIII-D modified its flat-end Small Angle Slot (SAS) divertor into a V-shaped slot (SAS-VW). H-mode experiments show that detachment onset occurs at the same upstream plasma density in SAS-VW as SAS, in each B_T direction, despite higher neutral pressure, neutral compression and particle fluxes in SAS-VW, and in contrast to SOLPS-ITER drift modeling. Using in-slot gas fueling instead of main chamber fueling lowers the upstream plasma density to detach by ~22% / ~7% for strike point on the vertex / inner slant, and with ion B×▽B drift into the slot, consistent with SAS data and SOLPS-ITER. Placing the strike point on the inner slanted baffle instead of the slot V-end also reduces the detachment density by ~12% / ~8% for ion B×▽B drift into / out of the slot, an effect consistent with SAS data but not captured by SOLPS-ITER.