Insights from the DIII-D Small Angle Slot Divertor Program: Experiment-Modeling Comparisons and Conundrums*

Recent DIII-D experiment-modeling efforts featuring a series of Small Angle Slot (SAS) divertors suggest full detachment access and heat flux control to be more challenging than expected in favorable B_T, with potential implications for exhaust control in future fusion devices.  Comparisons of a flat-bottomed graphite SAS to a V-shape with a tungsten coating on the outer divertor face (SAS-VW), and detailed SOLPS-ITER modeling with E´B drifts do show reasonable agreement on detachment onset when using separatrix density as the independent parameter.  However, the modeling significantly underestimates the n_eSEP required for deep detachment.  Experimentally, both shapes exhibit nearly identical n_eSEP for detachment, for both drift directions, despite modeling predictions that the V-shape would enhance dissipation at lower n_eSEP and despite the achievement of significantly higher neutral pressures and compression in the SAS-VW slot. In-slot D₂ and impurity gas puffing is more effective than main chamber puffing in promoting detachment (by more efficiently increasing n_eSEP), also in accord with modeling. We will discuss both experimental and modeling limitations that might be influencing these results with the aim of improving model-driven divertor designs for next-generation devices utilizing similar V-shaped geometries.