Investigating the dependence of a Long-Leg, Dissipative Low-Field Side Divertor on High-Field Side Divertor Leg Length in DIII-D Using UEDGE Simulations Including Drift Flows

UEDGE simulations of upper-single null plasmas with a 50 cm baffled low-field side (LFS) divertor leg, including drift flows in the favorable direction, predict a 25% decrease in LFS target separatrix electron temperature (T_e,LFS-t,sep) and a 5% increase in LFS midplane separatrix electron density when the high-field side (HFS) divertor leg length is reduced from 20 cm to 15 cm. The simulations indicate E_θ×B radial drift flows through the private flux region are responsible for the observed T_e,LFS-t,sep-dependence on HFS divertor leg length. Simulations in the unfavorable drift direction predict a negligible impact on LFS divertor conditions as the HFS divertor leg length is reduced from 20 cm to 15 cm. In the unfavorable drift direction, the drift flows are directed towards the pump, increasing the particle removal rate. Consequently, T_e,LFS-t,sep increases by 15% and 50% compared to the favorable drift direction for 20 cm and 15 cm HFS divertor leg lengths, respectively, for the same boundary conditions. The long legged, baffled divertor of the DIII-D Staged Modular Divertor program will explore the feasibility of a large-volume dissipative divertor, by increasing the LFS divertor volume without internal magnetic field coils, without degradation of core plasma performance.