Modeling the effect of MHD activity on runaway electron generation during SPARC disruptions

Magnetohydrodynamic (MHD) instabilities and runaway electrons (REs) interact in several ways, making it important to self-consistently model these interactions for accurate predictions of RE generation and the design of mitigation strategies, such as massive gas injection (MGI). Using M3D-C1—an extended MHD code with a RE fluid model—we investigate the effects of 3D nonlinear MHD activity, material injection, and 2D axisymmetric vertical displacement events (VDEs) on RE evolution during disruptions on SPARC—a high-field, high-current tokamak designed to achieve a fusion gain Q > 1. Several cases, comprising different combinations of neon (Ne) and deuterium (D₂) injection, are considered. Our results demonstrate key effects that arise from the self-consistent RE + MHD coupling, such as an initial increase in RE generation due to MHD instability growth, decreased saturation energies of the m / n = 1 / 1 mode driving sawteeth-like activity, RE losses in stochastic magnetic fields, and subsequent RE confinement and plateau formation due to re-healing of flux surfaces. Large RE plateaus (>5 MA) are obtained with Ne-only injection (2- 5 × 10²¹ atoms), while combined D₂ + Ne injection ( 2 × 10²¹ Ne atoms; 1.8 × 10²² D₂ molecules) produces a lower RE current (<2 MA). With D₂ + Ne injection, a post thermal quench ‘cold’ VDE terminates the RE beam, preventing a steady plateau. These simulations couple REs, 3D MHD instabilities, MGI, and axisymmetric VDEs for the first time in SPARC disruption simulations and represent a crucial step in understanding RE generation and mitigation in high-current devices like SPARC.