The role of neutrals for power dissipation in the Small Angle Slot divertor (SAS) in the DIII-D tokamak*

SOLPS modeling of experiments with the new SAS divertor shows more than a factor of 2 reduction in Te at the strike point compared to a matched open divertor case. This is due to the fact that SAS targets redirect neutrals towards the separatrix, which thereby enhances the neutral buildup by roughly a factor of 4, thus increasing energy losses and facilitating detachment. Moving the strike point from the inboard side to the deep corner of the slot decreases Te and heat flux by a factor of 3 at the strike point but leads to higher values in the far SOL indicating that a too strongly focused neutral distribution may sacrifice far SOL cooling for increased strike point cooling. Increasing the X-point height with a fixed strike point location leads to a decreased molecular density by more than a factor of 2 increasing Te and heat flux. These results, in qualitative agreement with the experimental data, underlie the importance of controlling recycling neutrals for divertor power dissipation.