Impact of Ballooning-Like Cross-Field Transport on In-Out Divertor Power Asymmetries in SPARC Long-Legged Geometries

Several SOLPS-ITER L-mode Q>1 (f_G~0.1—0.5) simulations of SPARC single-null divertor configurations yield the majority of exhaust power reaching the inner target. This effect is connected in this work to the poloidal distribution of power crossing the separatrix (q_⊥), using SOLPS-ITER "ballooning" transport coefficients to scale local perpendicular diffusivities by 〈B〉/ B^α: larger α enhance low-field-side power fluxes, and move the stagnation point towards the outer midplane. Additionally, scanning upstream collisionality (α_t~0.1—1.0) in longer-legged geometries can reverse the in-out target power asymmetry, as reported in [J. Lore et al. Nucl. Fusion 2024]. Comparing simulations with and without currents enabled shows that thermoelectric currents enhance pre-existing asymmetries.

We interpret the results using a self-consistent "three-point model": inner and outer target, and a common stagnation point. The poloidal distribution of q_⊥ is a significant driver of in-out asymmetries, and adjusting it yields good agreement with SOLPS-ITER, capturing the stagnation point's dependency on collisionality and geometry. For outer legs comparable to the inner leg (~5—10 m), P_in/P_out<1 is observed in SPARC-relevant regimes. For longer outer legs (~25—50 m), q_⊥ profiles less peaked at the outer midplane lead to P_in/P_out>1. A more realistic implementation of ballooning-like transport may beneficially balance the in-out power asymmetries in SOLPS-ITER.