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 performan ce parameters to changes in the volume of a secondary divertor that tends t o form inside the vacuum chamber in high triangularity plasma shapes. The s ensitivity was examined by varying the vertical distance of the secondary X -point from the target plate, Z(X)(S), while holding the primary,X-point he ight fixed. The ion delB drift was in the direction of the primary divertor . Discharges with and without active primary divertor cryo-pumping were exa mined. The effective rate of rise of the core density at the L-H transition increased 80% as Z(X)(S) was reduced. At high density achieved by gas inje ction, the col.e line-averaged density at the H-L back transition decreased by 30% as Z(X)(S) was reduced. Both of these results indicate that perform ance may be affected when core plasma screening of neutrals in the secondar y divertor is reduced as Z(X)(S) decreases. The peak heat flux in the secon dary divertor decreased as Z(X)(S) was reduced due to increased flux expans ion until a threshold was leached at Z(X)(S) similar to 3 cm where it remai ned constant. This indicated that the high recycling character of the plasm a above the target in the secondary divertor was lost when the length of th e outer divertor leg was reduced below a critical value. (C) 2001 Elsevier Science B.V. All rights reserved.