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Due to its high magnetic field SPARC is expected to have a narrow heat-flux width (λq~0.5mm) in the similar range to ITER, making power exhaust a central challenge. Previous studies explored SPARC’s operational space through density and impurity scans [1, 2], but the diffusivity values (D, χ) used to reproduce λq are not uniquely constrained. Recent experiments reported upstream decay lengths broadening, extending the multi-machine power width scaling by up to a factor of ~2.5 near the density limit. This work examines how much of this broadening can be explained by parallel transport toward the divertor, and to what extent increased radial turbulent transport contributes [3].
For SPARC conditions in SOLPS, λq broadening appears with increasing density even at fixed D and χ, driven by divertor conditions (similar to ITER [4,5]), reaching factors of 2-3, consistent with experiments [3,6]. Additional broadening arises from thermo-electric current contributions, which become significant under SPARC’s high density, and narrow decay lengths condition [2].
We investigate the impact of D and χ (D/χ = 10, 1, 1/3; χ = 0.03-1) across two SPARC scenarios, full-field (12.2 T, 29 MW) and 2/3-field (8 T, 10 MW) [1]. Separatrix density scans from 1/4 to 1/2 Greenwald reproduce the reported lam-T (and thus lam-q) broadening, though not the much larger widening of the density channel. Notably, scans at fixed D and χ capture a substantial portion of the turbulence correlated broadening observed on AUG, supporting the continued use of constant transport coefficients for reproducing first-order broa\dening trends.
An additional requirement for SPARC is to avoid abrupt changes in the in/out power balance. Analysis of parallel flow asymmetry shows that lower D and χ increase power to the divertor while reducing radial losses. The D/χ = 10 cases show the strongest in/out power imbalance, triggering the HICO → HOCI transition [2], while increasing density or choosing different D/χ combinations mitigates this asymmetry. These results identify an operational regime that avoids strongly asymmetric divertor conditions, requiring a minimum collisionality of αt ~ 0.1-0.5 and ~1/4 Greenwald fraction at the separatrix.
[1] Kuang A. Q. et al., Journal of Plasma Phys 86.5 (2020) 865860505.
[2] Lore J. D. et al., Nucl Fusion 64.12 (2024) 126054.
[3] Eich T. et al., Nucl Fusion 60.5 (2020) 056016.
[4] Park J.-S. et al., Nucl Fusion 64.3 (2024) 036002.
[5] Rivals N. et al., Nucl Fusion 65.2 (2025) 026038.
[6] Brown, Goldston, NME (2021) 101002