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 $\nu^{*}$, 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.