The effect of 3-D MHD activity on runaway electron generation during SPARC disruptions

Magnetohydrodynamic (MHD) activity often accompanies runaway electron (RE) generation during disruptions in tokamaks. Using M3D-C1, an extended MHD code that couples a RE fluid model to the MHD equations, we investigate effects of 3-D nonlinear MHD activity on RE evolution on SPARC – a compact, high-field, high-current tokamak designed to achieve Q > 2 in D-T plasmas. We present various cases, including unmitigated midplane disruptions, mitigated disruptions with different levels of Neon only-MGI, and combined D2+Ne MGI. Strong electric fields generated during the early stages of MHD instability growth leads to an enhancement of RE generation, not captured in 2-D axisymmetric simulations or models without the RE + MHD coupling. The growth of the RE current during the current quench also leads to a suppression of MHD activity. MGI-induced MHD activity generates completely stochastic field lines, facilitating RE losses via enhanced transport between the core and the first wall. Re-healing of flux surfaces, however, enables confinement and subsequent growth of RE. At low levels of Neon-only MGI, the fast rehealing of flux surfaces prevents significant RE losses. Finally, we present results that explore the seeding of MHD instabilities via an n = 1 runaway electron mitigation coil (REMC). Our results demonstrate that the magnitude, longevity, and the spatial localization of MHD instabilities play an important role in deconfining REs during disruptions.