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

Abstract

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 $P_{\mathrm{net}}$ and ionization. Measurements from an edge Thomson Scattering system are combined with those from a midplane-viewing Ly${}_{\alpha }$ camera to evaluate changes to $n_e$ and $T_e$ in response to changes to ionization rates, $S_{\mathrm{ion}}$. $n_e^{\mathrm{sep}}$ and $T_e^{\mathrm{ped}}$ are found to be most sensitive to changes to $S_{\mathrm{ion}}^{\mathrm{sep}}$, as opposed to $n_e^{\mathrm{ped}}$ and $T_e^{\mathrm{sep}}$. Dimensionless quantities, namely ${\alpha }_{\mathrm{MHD}}$ and ${\nu }^{\ast }$, are found to regulate attainable pedestal values. Select discharges at different values of $P_{\mathrm{net}}$ 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, $n_0$, along the separatrix, increasing $n_0$ 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.