Runaway electron deconfinement in SPARC and DIII-D by a passive 3D coil

TL;DR

This study models the use of a passive 3D coil to deconfine runaway electrons in SPARC and DIII-D tokamaks, highlighting the importance of coil geometry, transport assumptions, and q-profile evolution in runaway suppression.

Contribution

It introduces a comprehensive modeling approach combining nonlinear MHD, particle transport, and runaway evolution to evaluate passive coil effectiveness in tokamak disruption scenarios.

Findings

Only the n=1 coil effectively suppresses runaway growth.

Conservative transport assumptions lead to higher potential runaway energies.

Q-profile evolution critically influences coil performance and runaway behavior.

Abstract

The operation of a 3D coil--passively driven by the current quench loop voltage--for the deconfinement of runaway electrons is modeled for disruption scenarios in the SPARC and DIII-D tokamaks. Nonlinear MHD modeling is carried out with the NIMROD code including time-dependent magnetic field boundary conditions to simulate the effect of the coil. Further modeling in some cases uses the ASCOT5 code to calculate advection and diffusion coefficients for runaway electrons based on the NIMROD-calculated fields, and the DREAM code to compute the runaway evolution in the presence of these transport coefficients. Compared with similar modeling in Tinguely, et al [2021 Nucl. Fusion 61 124003], considerably more conservative assumptions are made with the ASCOT5 results, zeroing low levels of transport, particularly in regions in which closed flux surfaces have reformed. Of three coil geometries considered in SPARC, only the $n=1$ coil is found to have sufficient resonant components to suppress the runaway current growth. Without the new conservative transport assumptions, full suppression of the RE current is maintained when the TQ MHD is included in the simulation or when the RE current is limited to 250kA, but when transport in closed flux regions is fully suppressed, these scenarios allow RE beams on the order of 1-2MA to appear. Additional modeling is performed to consider the effects of the close ideal wall. In DIII-D, the current quench is modeled for both limited and diverted equilibrium shapes. In the limited shape, the onset of stochasticity is found to be insensitive to the coil current amplitude and governed largely by the evolution of the safety-factor profile. In both devices, prediction of the q-profile evolution is seen to be critical to predicting the later time effects of the coil.