First operation of a dedicated runaway electron mitigation coil (REMC)

Abstract We report observations from the first experimental tests of a dedicated runaway electron mitigation coil (REMC) installed inside a tokamak’s vacuum vessel and driven by the disruption loop voltage. An REMC may provide a ‘fail-safe’ avoidance of high-energy runaway electrons avalanching during disruptions. This REMC operates in the HBT-EP tokamak, with a coil geometry similar to the one planned for the SPARC tokamak and future devices. About 10% of the pre-disruption plasma current is present in the REMC by the middle of the current quench, reaching ∼14% at the peak of the REMC current. Coupling directly from I p is reduced to ∼8% and ∼10% respectively when removing contributions from ramping equilibrium coils. Measured coupling is well-modeled with the ThinCurr 3D thin-wall electromagnetic code that self-consistently computes eddy currents in the tokamak’s conducting structures. When the REMC is activated, we measure asymmetric variations in halo current that are consistent with the large n = 1 magnetic perturbation. Confinement of runaway electrons is studied using hard x-ray emission, though confinement results are inconclusive for this first campaign.