Role of radiative heating and neutrals in surface heat flux during detachment in the DIII-D small angle slot divertor*

Data from surface eroding thermocouples (SETCs) in the DIII-D Small Angle Slot (SAS) divertor suggests that neutrals and radiative heating may contribute significantly to surface heat flux in the approach to detachment. Divertor detachment is one of the most promising ways to reduce heat flux at the divertor target through increased energy dissipation in the boundary region. It is widely observed in an open divertor that gas puffing is beneficial to power dissipation. However, the peak heat flux measured by the SETCs in the closed SAS divertor has been found to continuously increase at the beginning of a density ramp up induced by D₂ puffing while the T_e measured by Langmuir probes simultaneously decreases. The heat flux measured by SETCs near strike point in SAS divertor was higher than it inferred by Langmuir probes in the full detachment plasma while T_e was lower than 5 eV.  This evidence implies that radiated power and neutrals may be playing an important role in a closed divertor where the volume is relatively small and the neutral confinement is good compared to an open divertor, especially during detachment. A new set of recessed probes will be established to quantify these processes in a dedicated manner.