SOLPS-ITER study of the relative roles of fueling and plasma transport on setting the density pedestal in high SOL opacity H-modes on Alcator C-Mod

The formation of the density pedestal is found to be unaffected by high scrape-off layer neutral opacity in experiments performed on Alcator C-Mod, in H-mode regimes approaching ITER-like edge opacities[1]. To assess the roles of fueling vs transport, we use the SOLPS-ITER code to compute neutral density profiles for a high current (1.3MA) and reduced current (1.0MA) discharge. Simulated n_e and T_e profiles are matched to upstream empirical data by varying radial transport coefficient profiles, revealing midplane separatrix neutral densities an order-of-magnitude lower in the high current (~10^15 m^-3) vs lower current (~10^16 m^-3) discharge, in line with measurements of edge neutral densities (~10^17 m^-3) in slightly lower current (0.9MA) C-Mod discharges[1]. A ballooning transport model is implemented to approximate expected poloidal asymmetries[2], resulting in an order-of-magnitude difference between LFS and HFS neutral densities. Fitted neutral density e-folding lengths in the edge are found to saturate with increased plasma density, supporting empirical observations that the pedestal is not sensitive to neutral opacity, and is primarily set by plasma transport. [1] J. W. Hughes et al 2006 Physics of Plasmas 13 056103 [2] B. LaBombard et al 2004 Nucl. Fusion 44 1047