Validation of deuterium neutral density measurements in NSTX-U using the Monte Carlo neutral transport code DEGAS 2

In NSTX-U, deuterium neutral density $n_D$ profiles are inferred at the outboard midplane from deuterium Balmer-$\alpha$ (D-$\alpha$) emission measured with a filtered camera (Edge Neutral Density Diagnostic, ENDD). $n_D$ measurements obtained from D-$\alpha$ emissivity are validated using the Monte Carlo neutral transport code DEGAS 2. Contributions to emissivity due to electron impact excitation and molecular processes are estimated with DEGAS 2 towards assessing the direct use of ENDD for $n_D$ estimates. Experimental measurements and DEGAS 2 simulations were compared over a database of L-mode discharges, showing good agreement in D-$\alpha$ emissivity profiles. D-$\alpha$ emissivity generally peaked at larger radii in simulations, while far-SOL emission was under predicted. A scan in far SOL plasma parameters in DEGAS 2 showed that a small increase in $T_e$, $n_e$ improved the agreement with experimental profiles, pointing to the possible importance of intermittent transport. One-way coupling of DEGAS 2 with UEDGE fluid simulations is underway to study fueling and neutral penetration in NSTX-U discharges.