Design and operation of the first Runaway Electron Mitigation Coil (REMC)

A Runaway Electron Mitigation Coil (REMC) is a non-axisymmetric passive conducting structure designed to reduce the potential damage from runaway electrons (REs) by spoiling their confinement before they accelerate to dangerously high energies and currents during disruptions. Two in-vessel REMCs have been installed and plan to be operated this year in the High Beta Tokamak – Extended Pulse (HBT-EP) facility, marking the first implementation of such a purpose-built passive coil in any device. Two coils were built rather than one in order to explore coupling for different configurations, and also to mitigate experimental risk in case one of them suffers from a technical failure. Coils remain open-circuited during plasma startup, then one coil is switched closed prior to the disruption to be passively driven by the disruption loop voltage. Details of coil designs are presented, along with results from expected initial operations starting in summer/fall 2024. This study explores the effect of the coils on the progression of HBT-EP disruptions, including the influence on MHD behavior and halo current rotation as measured by magnetic sensors and current-collecting tiles. Upcoming plans for utilizing the coil are also presented, including extensive campaigns to measure disruption forces, RE energies, and RE deposition locations.