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

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 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 &gt; 1. Several cases, comprising different combinations of neon (Ne) and deuterium ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mtext>D</mml:mtext> <mml:mn>2</mml:mn> </mml:msub> </mml:mrow> </mml:math> ) 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>m</mml:mi> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:mi>n</mml:mi> <mml:mo>=</mml:mo> <mml:mn>1</mml:mn> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:mn>1</mml:mn> </mml:mrow> </mml:math> 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 (&gt;5 MA) are obtained with Ne-only injection (2– <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>5</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mn>21</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> atoms), while combined <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mtext>D</mml:mtext> <mml:mn>2</mml:mn> </mml:msub> </mml:mrow> </mml:math> + Ne injection ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>2</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mn>21</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> Ne atoms; <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>1.8</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mn>22</mml:mn> </mml:mrow> </mml:msup> <mml:mstyle scriptlevel="0"/> <mml:msub> <mml:mtext>D</mml:mtext> <mml:mn>2</mml:mn> </mml:msub> </mml:mrow> </mml:math> molecules) produces a lower RE current (&lt;2 MA). With <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mtext>D</mml:mtext> <mml:mn>2</mml:mn> </mml:msub> </mml:mrow> </mml:math> + 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.