runaway electron mitigation coil (REMC) is a passive, non-axisymmetric coil mounted inside a tokamak's vacuum vessel that the disruption's own collapsing current drives, breaking up the magnetic surfaces that would otherwise confine a runaway electron beam.
The coil has no power supply, no controller and no trigger. It is wired as a closed loop linking the plasma's poloidal flux, so when the 📝plasma current collapses during a 📝disruption, the change in flux drives current through it by induction — the same 📝eddy current physics that loads the 📝vacuum vessel, turned into a defence. Because the winding is helical rather than axisymmetric, the field it produces carries a resonant three-dimensional component that destroys nested 📝magnetic flux surfaces and renders the field stochastic. Seed 📝runaway electrons are then lost to the wall — spread over area, at low energy — before the avalanche that would multiply them into a multi-megaampere beam has time to run. Passivity is the design argument: a system with nothing to arm cannot fail to arm, and a disruption gives a detection-and-response chain only milliseconds to work.
The concept was developed for 📝SPARC in two Nuclear Fusion papers. Tinguely and co-authors modelled the complete prevention of disruption-generated runaway beam formation with a passive 3D coil in 2021. Izzo and co-authors extended the analysis in 2022 across SPARC and 📝DIII-D using NIMROD, ASCOT5 and DREAM, finding that only the n = 1 coil geometry in SPARC showed sufficient resonant components for runaway suppression and that prediction of the 📝safety factor (q) profile's evolution is critical to the result. Battey and co-authors' 2024 thin-wall study then treated the engineering parameters — conductor resistivity, current quench duration, plasma vertical position — and recommended placing the conductors away from the machine midplane for robustness.
The REMC differs from a 📝resonant magnetic perturbation (RMP) coil in exactly the way that matters: an RMP coil is actively powered and used during normal operation to control 📝edge localized mode (ELM)s, while the REMC is unpowered and acts only during the event it is built for. It also differs from 📝massive gas injection, which suppresses runaways by raising density; the two are complementary layers rather than alternatives. The case for the coil remains entirely a modelling case — SPARC has not run, and no published result is an experiment.