Impact of solving a separate atom energy equation in UEDGE on predictions of a dissipation-focused divertor in DIII-D

UEDGE simulations of an upper-single null, dissipation-focused divertor (DFD) using both a common ion-atom energy equation and a separate atom energy equation, including plasma drift flows in the favorable direction, indicate a more stable radiation front along the baffled low-field side (LFS) leg when a separate atom energy equation is solved. The DFD pumping plenum is located 10-17 cm upstream of the LFS target along the outer baffle of the ∼50 cm LFS divertor leg to create a neutral gas cushion between the target and pump and attain sufficient radiative exhaust to sustain the high-power operation necessary to access super-H mode in DIII-D. The location of the radiation front is predicted to vary from the pump duct to the X-point as the gas injection is increased when a common ion-atom energy equation is solved in UEDGE. When solving a separate atom energy equation in UEDGE, the radiation front is predicted to remain 7-12 cm downstream of the X-point for the same gas-injection interval, with beneficial implications for maintaining high core confinement. Furthermore, solving a separate atom energy equation in UEDGE increases the predicted radiated power by up to 50% due to an increase in the divertor density compared to simulations solving a common ion-atom energy equation. The DFD, the second divertor in a series of modular divertors at DIII-D, explores the feasibility of a large-volume dissipative divertor without internal magnetic field coils and degradation of core plasma performance under detached operation.