In-Out Power and Temperature Asymmetries in SPARC Single-Null SOLPS-ITER Modeling*

Power and temperature asymmetries are observed in SPARC single-null (SN) divertor geometries through SOLPS-ITER simulations, showing a majority (~ 60%) of exhaust power flowing to the inner target plate, with peak heat flux densities greater by up to an order of magnitude. Maintaining fixed upstream density (f_G ~ 0.13–0.22) and power (P_SOL ~ 4–15 MW), the influence of divertor target geometry, leg length ratio, and SOL collisionality on the location of the stagnation point (L_in/L_out) is observed, and compared with the analytical conduction-limited scaling P_in/P_out ~ L_out/L_in. These findings suggest power exhaust challenges in SPARC and future reactor-class devices when attempting scenarios with long outer legs in SN.

To interpret the SOLPS-ITER results, a semi-analytical “three-point model” code is developed, connecting inner and outer sheaths to a common upstream stagnation point. The inclusion of flux limitation corrections in the Spitzer-Härm heat conduction model is found to play a significant role in correctly reproducing SOLPS-ITER trends, over a scan with SOL collisionalities between 25 and 1. Additionally, the importance of self-consistently accounting for thermoelectric currents in the SOL is emphasized, impacting the sheath potential and energy transmission coefficient.