Edge Fueling and Neutral Density Studies of the Alcator C-Mod Tokamak Using the SOLPS-ITER Code

Understanding edge neutral dynamics in high-field tokamaks has strong consequences for both fueling and plasma profile predictions. We validate the ability of SOLPS-ITER, a 2D fluid plasma/kinetic Monte Carlo neutral code, to accurately model upstream neutral density profiles of L-mode, I-mode, and H-mode discharges in the Alcator C-Mod tokamak. Via iterative tuning of perpendicular transport coefficient profiles alone, we achieve modeled Lyman-alpha emission, neutral density, and plasma profiles within empirical measurement uncertainties for all discharges, providing confidence in the baseline SOLPS model to accurately simulate neutrals in the upstream plasma edge. SOLPS is then used to assess the relative role of edge fueling versus transport in high density Alcator C-Mod discharges approaching fusion-relevant opaqueness conditions. We model a set of high density H-mode discharges with gas puffs of varying magnitude applied to probe the response of the density pedestal to increased edge fueling. Analysis of the simulated radial neutral e-folding length reveals that in high opacity conditions, neutrals tend to get trapped in the PFR, suggesting that upstream fueling dominates x-point fueling, an important consideration for future fusion devices operating in high opacity regimes.