Particle control via cryopumping and its impact on the edge plasma profiles of Alcator C-Mod

At the high n_e proposed for high-field fusion reactors, it is uncertain whether ionization, as opposed to plasma transport, will be most influential in determining n_e at the pedestal and separatrix. A database of Alcator C-Mod discharges is analyzed to evaluate the impact of source modification via cryopumping. The database contains similarly-shaped H-modes at fixed I_P=0.8 MA and B_t=5.4 T, spanning a large range in P_net and ionization. Measurements from an edge Thomson scattering system are combined with those from a midplane-viewing Ly_α camera to evaluate changes to n_e and T_e in response to changes to ionization rates, S_ion. n_e^sep and T_e^ped are found to be most sensitive to changes to S_ion^sep, as opposed to n_e^ped and T_e^sep. Dimensionless quantities, namely α_MHD and ν^∗, are found to regulate attainable pedestal values. Select discharges at different values of P_net and in different pumping configurations are analyzed further using SOLPS-ITER. It is determined that changes to plasma transport coefficients are required to self-consistently model both plasma and neutral edge dynamics. Pumping is found to modify the poloidal distribution of atomic neutral density, n₀, along the separatrix, increasing n₀ at the active X-point. Opaqueness to neutrals from high n_e in the divertor is found to play a role in mediating neutral penetration lengths and hence, the poloidal distribution of neutrals along the separatrix. Pumped discharges thus require a larger particle diffusion coefficient than that inferred purely from 1D experimental profiles at the outer midplane.