Predicting 3D heat loads from non-axisymmetric plasmas for SPARC tokamak using the HEAT code

SPARC is a high-field fusion device being built by Commonwealth Fusion Systems, designed to operate at magnetic fields up to 12T and achieve net energy gain (Q > 1) with deuterium-tritium fuel. The high steady-state heat fluxes anticipated in SPARC and expected at high poloidal magnetic field, demand precise understanding of resulting heat flux distributions in the presence of 3D fields to optimize power exhaust. Non-axisymmetric magnetic field perturbations can generate localized heat flux peaks that can lead to increased energy fluxes to the divertor and, in turn, enables benchmarking the accuracy of 3D field perturbations through heat flux measurements. Understanding how 3D magnetic fields modify heat flux patterns is thus crucial for ensuring the longevity and heat-handling capability of the PFCs. In the present work a new module developed in the HEAT code to predict heat loads from non-axisymmetric plasmas upon 3D PFCs is applied to investigate the impact of 3D fields on the heat fluxes at the divertor plates in SPARC. We here consider the fields produced from the 6 coils toroidal array at the outboard midplane with n=1,2 toroidal periodicity. The magnetic perturbations are computed using the resistive MHD code M3D-C1, with field line tracing computed by MAFOT. Heat fluxes are then calculated via a 3D heat flux layer model, an extension of the Eich profile, distinguishing between the scrape-off layer, magnetic lobes, and private flux region using a set of 0D input parameters. Compared to the axisymmetric case, 3D perturbations significantly alter the heat flux profile, leading to localized peak enhancements. The peak intensity varies nonlinearly with perturbation strength due to changes in the wetted area from the magnetic footprint. Thermal simulations show that stationary perturbations can drive PFCs to unacceptable temperatures in the absence of radiative cooling, whereas slowly rotating fields substantially reduce thermal loads.