Evaluation of SPARC divertor conditions in H-mode operation using SOLPS-ITER

Abstract

The predicted divertor conditions for the SPARC tokamak are calculated using SOLPS-ITER for a range of scrape-off-layer (SOL) heat flux widths $\lamq$, input powers, and particle fuelling locations. Under H-mode scenario conditions with an upstream separatrix density of 1\e20 m$^{-3}$, the most conservative range of $\lamq$ extrapolations ($\approx$ 0.15 mm) results in extremely high unmitigated particle and energy fluxes to the divertor, both under full field (12.2 T) and power ($P_{SOL}$ = 29 MW) conditions, and 2/3 field with $P_{SOL}$ = 10 MW. Increasing the cross-field SOL diffusivities by 2-10x reduces the magnitude of the mitigation challenge, however strategies such as impurity seeding or strike-point-sweeping will likely still be required. 

A combination of steady-state and time-dependent SOLPS-ITER simulations are used to map out phase space diagrams of upstream and divertor conditions. At low upstream density the inner and outer divertor conditions are highly asymmetric, with a large temperature difference and significant heat fluxes driven by parallel currents. The solution has sharp bifurcations with a region of hysteresis, depending on whether the initial state is at a low or high density. This behavior is observed even when the fuelling location, cross-field diffusivity, and impurity level is changed, although the density window with asymmetry is reduced.

The addition of neon impurity seeding reduces the divertor heat fluxes, but also causes a drop in the upstream electron density with fixed particle throughput. This drop can be counteracted by increased main ion throughput, however too much neon results in a back transition into the asymmetric divertor regimes suggesting a need for control of both main ion and impurity seeding levels to achieve a desired divertor state.