Modeling particle removal in the baffled upper DIII-D divertor using the UEDGE advanced fluid neutral model

Particle removal at the divertor target and unpumped private-flux region (PFR) is necessary to match Langmuir-probe measured n_e-, T_e-, and J_sat profiles at the baffled, upper low-field side (LFS) divertor plate in UEDGE simulations of DIII-D discharges (unfavorable B-field direction for H-mode access, upper cryo-pump active, I_p=1.3 MA, n_e/n_GW=0.4, P_NBI=1 MW, and 2100 s plasma exposure since boronization). The UEDGE advanced fluid neutral model assumes particle removal via albedo-like pumping processes, removing a fraction of the neutral flux incident on boundary surfaces. To match the measured LFS target profiles under attached divertor conditions, 90% removal of particles incident at both the boundary surfaces corresponding to the pump duct and the radial PFR boundary opposite of the pump duct is necessary in the UEDGE simulations. Under detached conditions, an additional 10% background particle removal rate across the LFS target is required to match probe measurement, indicative of the albedo-like pumping model applied not accurately capturing important kinetic pumping effects, such as reflections of from the PFR baffle into the pumping plenum. The neutral transport and pumping rate of neutrals in the baffled, upper divertor is modeled using the kinetic Monte-Carlo code DEGAS2, and the effective target pumping rates compared to UEDGE predictions. Understanding and predicting particle transport and removal is critical to model and design divertors capable of sustaining the target heat loads.