Electromagnetic modeling of the HBT-EP runaway electron mitigation coil

Electromagnetic modeling is presented from the design of a Runaway Electron Mitigation Coil (REMC) in the HBT-EP tokamak – the first coil of this kind. ThinCurr, a thin-wall, 3D electromagnetic modeling code, predicts currents and fields in the coupled REMC, plasma, vacuum vessel, and ohmic heating coil system [Battey NF 2024]. These predictions will be compared to experimental data as available to validate numerical models, starting with initial tests performed under vacuum conditions. The magnetic field of the REMC driven by high-power amplifiers will be measured by Mirnov coil arrays. The signals are predicted to exhibit complex time dependencies due to varying 3D eddy current decay time scales. Then, the REMC current will be measured as it is inductively driven by the ohmic heating (OH) coils. More than 3 kA is expected to be driven by a maximum OH coil ramp. Following these tests, plasmas will be allowed to disrupt and inductively drive the REMC. A maximum of 15% of the plasma current is expected to be converted. An analogous workflow has been used for the upcoming DIII-D [Weisberg NF 2021] and SPARC [Tinguely NF 2021] REMCs with similar mid-disruption normalized field perturbations for each device. The HBT-EP REMC research program seeks to experimentally validate the workflow predicting these fields and currents.