High voltage insulation and operation of a runaway electron mitigation coil in SPARC

The SPARC runaway electron mitigation coil (REMC) is a conductive asymmetric coil placed inside the vacuum vessel for the purpose of limiting the build-up of runaway electron (RE) populations. During a disruption, a current is induced in the REMC to produce perturbing magnetic fields which increase the loss rate of REs from the plasma. To avoid interfering with plasma current ramp-up and ramp-down, the SPARC REMC circuit will contain a switch to actuate whether the circuit is open or closed. The first implementation of the REMC switch will be mechanical, with a response time of ~50 ms, which is too slow to respond in real time to a current quench of order 10 ms. The implications of this to REMC operations and commissioning will be discussed. In an open circuit state, a worst-case 3.2 ms linear current quench of the SPARC 8.7 MA Primary Reference Discharge will induce a large loop voltage on the REMC, up to 2.5 kV on coil, end-to-end. In order to insulate the REMC from the vacuum vessel, spray coated alumina is planned as an insulating material. Results from recent electrical and mechanical testing of spray coated alumina on samples with a geometry representative of the REMC as well as the effect of magnetic field on arcing in a representative seam geometry will be presented.