Performance of high triangularity plasmas as the volume of the secondary divertor is varied in DIII-D

This paper examines the sensitivity of edge pedestal and divertor performance parameters to changes in the volume of a secondary divertor that tends to form inside the vacuum chamber in high triangularity plasma shapes. The sensitivity was examined by varying the vertical distance of the secondary X-point from the target plate, ZSX, while holding the primary X-point height fixed. The ion ∇B drift was in the direction of the primary divertor. Discharges with and without active primary divertor cryo-pumping were examined. The effective rate of rise of the core density at the L–H transition increased 80% as ZSX was reduced. At high density achieved by gas injection, the core line-averaged density at the H–L back transition decreased by 30% as ZSX was reduced. Both of these results indicate that performance may be affected when core plasma screening of neutrals in the secondary divertor is reduced as ZSX decreases. The peak heat flux in the secondary divertor decreased as ZSX was reduced due to increased flux expansion until a threshold was reached at ZSX∼3 cm where it remained constant. This indicated that the high recycling character of the plasma above the target in the secondary divertor was lost when the length of the outer divertor leg was reduced below a critical value.