Disruption dynamics during the first operation of a Runaway Electron Mitigation Coil (REMC) on a tokamak

Runaway electrons (REs) generated in high current tokamaks during disruptions pose a significant hazard for damaging first-wall components [1]. Preventing localized deposition of strong RE currents on first-wall components is critical to the commercial viability of reactor-scale tokamaks. Various active means of mitigating REs include injection of material [2,3] and triggering a large non-axisymmetric magnetic field [4]. Here, we detail the first passive mitigation scheme that does not require a control system or predictive capability, commonly called a Runaway Electron Mitigation Coil (REMC) [5]. The disruption loop voltage drives a large current in a non-axisymmetric coil which destroys plasma symmetry, causing stochastic magnetic fields when combined with the plasma response and reducing energetic particle confinement. A dedicated n = 1 REMC, similar to the geometry being implemented for the SPARC tokamak [6], now routinely operates in the HBT-EP tokamak [7]. Roughly 12% of the pre-disruption plasma current couples into the REMC during the current quench (CQ), excluding current driven by the ramping external coils. Consistent with plasma-wall contact from a large-scale locked n = 1 perturbation, halo currents redistribute during disruptions, though rotating features still dominate most of the CQ. Discharges with slide-away electrons are used to study energetic electron confinement. With the REMC active, we observe changes in measured hard x-ray emission, showing the REMC's influence on energetic electron impacts. Vacuum vessel force measurements, high-speed videography, and detailed electromagnetic calculations will also be discussed.

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[6] Tinguely, R.A. et al., Nucl. Fusion 61, 12 (2021) 124003

[7] Saperstein, A.R. et al., Phys. Plasmas 30, 4 (2023) 042506