Radiation Dependence on Divertor Leg Length in Detachment on DIII-D

Experiments performed on DIII-D demonstrate that extending the outer divertor leg allows for expanding the volume for impurity radiation as required for dissipation in detachment. This is roughly in line with convective transport estimates although deep detachment can lead toward radiative collapse to the X-point. Convective estimates relate the spatial scale for dominant radiating impurities (e.g., C or N on DIII-D) for conditions with 6 MW injected power suggest approximately 20 cm of divertor leg length for C or N. The radiated power is found to extend along the outer leg at approximately this scale in detachment in small core-volume DIII-D plasmas at 1MA and 2T with an elongated outer leg as measured by CIII emission and total radiated power. With increasing density, the strong peak in CIII radiation at the target pulls away with some target radiation remaining transiently due to ELMs. The radiating volume can ultimately collapse to the X-point pushing the divertor into deep detachment. SOLPS-ITER simulations similarly show the extended radiation pattern along the outer leg at detachment onset and into the detached state with it being pushed deeper toward the X-point at higher density. Future devices may require little to no core degradation when coupled to a detached divertor, thus determining these mechanisms is crucial.