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

TL;DR

This paper models the complex interactions between MHD activity and runaway electrons during disruptions in the SPARC tokamak, revealing how different material injections influence RE generation, confinement, and mitigation strategies.

Contribution

It introduces the first self-consistent simulation coupling REs, 3-D MHD instabilities, MGI, and VDEs in SPARC disruption scenarios, advancing understanding of RE dynamics.

Findings

Large RE plateaus (>5 MA) with Ne-only injection.

Lower RE currents (<2 MA) with D2 + Ne injection.

Post-quench VDEs can terminate RE beams.

Abstract

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 3-D nonlinear MHD activity, material injection, and 2-D 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 ($\text{D}_2$) 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 ($2-5 \times 10^{21}$ atoms), while combined $\text{D}_2$ + Ne injection ($2 \times 10^{21}$ Ne atoms; $1.8 \times 10^{22} \, \text{D}_2$ molecules) produces a lower RE current (<2 MA). With $\text{D}_2$ + Ne injection, a post thermal quench "cold" VDE terminates the RE beam, preventing a steady plateau. These simulations couple REs, 3-D 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.