Identifying SPARC divertor operation space and understanding divertor asymmetry through SOLPS-ITER simulations

Density scans using the SOLPS-ITER code reveal asymmetric divertor conditions in the SPARC V2 8 T (2/3 of full field) H-mode scenario with P_SOL ~ 10 MW [1]. These conditions, characterized by sharp, cliff-like transitions in target parameters relative to particle source or upstream density, are consistent across different main particle sources. Time-dependent SOLPS-ITER simulations with OMP gas ramp up and down were conducted to map the phase space of the SPARC V2y geometry. The phase space, defined by upstream density and target electron temperature, exhibits strong non-linearity and hysteresis. The asymmetry transition event happens in both directions, with localized high density spots exchanging between two targets through the PFR. Despite the strong target electron temperature asymmetry, the thermo-electric current contribution to heat flux is negligible with heat fluxes primarily driven by electron conduction and supported by convection in the detached condition. The narrow PFR geometry is suspected to be the main driver of the non-linearity in SPARC's divertor operation space as similar behaviors were observed in KSTAR divertor upgrade. Symmetric, linear regimes can be found by injecting neon, suggesting feasible divertor operation space can be achieved using impurity seeding.





[1] Creely A J et al. Overview of the SPARC tokamak. J. Plasma Phys. 86 865860502 (2020)