NIMROD + ThinCurr modeling of runaway electron suppression with a passively driven coil in DIII-D

The NIMROD 3D extended-MHD code is used in conjunction with the ThinCurr [1] electromagnetic modeling code to predict the enhanced loss of runaway electrons (REs) during a DIII-D current quench (CQ) due to a 3D passive coil. The RE mitigation coil (REMC) has been designed for installation on DIII-D [2] for validation of a concept first proposed by Boozer [3] to prevent RE plateau formation during tokamak disruptions, using the CQ loop voltage to passively drive the coil current. ThinCurr predicts the response of both the coil and surrounding conducting structures to a prescribed linear current ramp-down. The resulting 3D field spectrum is used as a time-dependent boundary condition in a NIMROD disruption simulation to calculate the nonlinear plasma response, where the relationship between the coil current amplitude and the plasma current is based on a parametric fit to the ThinCurr results. The NIMROD modeling includes drift-orbit calculations for a population of RE test-particles, and a large majority of initial test particles is found to be lost when the coil perturbations are included in the simulation, indicating that the coil will effectively prevent plateau formation for the modest avalanche gain factors in DIII-D.

[1] Battey, A.F., et al, Proc. 64th APS-DPP; Spokane, 2022.

[2] Weisberg, D. B., et al, (2021). Nucl. Fusion, 61(10), 106033.

[3] Boozer, A. H. (2011). Plasma Phys. Control. Fusion, 53(8), 084002.