quench is the sudden loss of superconductivity in a magnet, an event that converts the magnet's stored magnetic energy into heat.
The word's everyday senses — extinguishing a fire, satisfying a thirst, cooling hot steel in water — all carry the idea of abrupt termination, and the magnet sense inherits it. A 📝superconductor carries current without resistance only while it stays below its 📝critical temperature, current, and magnetic field. If any local spot in a winding crosses one of those limits, that spot turns resistive, its 📝ohmic heating warms its neighbors, and the normal zone spreads: within seconds the entire magnet is an ordinary resistor absorbing the energy it once stored without loss. In a fusion-scale magnet that stored energy is measured in gigajoules — enough, if dumped into one small region, to melt conductor and destroy the coil.
A quench should not be confused with a 📝disruption, its plasma-side near-neighbor: a disruption is the sudden loss of 📝plasma stability that dumps the plasma's energy into the machine, while a quench is the magnet's own failure mode. The two are easy to conflate because disruption physics borrows the same word for its own phases: a disruption proceeds through a thermal quench, in which the plasma sheds its stored thermal energy in under a millisecond, and then a current quench, in which the 📝plasma current decays away. Neither of those has anything to do with superconductivity — they name the collapse of a plasma, not of a magnet — and a reader who meets "thermal quench" or "current quench" should follow the disruption entry rather than this one. The distinction matters most for 📝High-Temperature Superconducting (HTS) Magnets, which run with large thermal margins that make quenches rarer but also slower to propagate — and therefore harder to detect before damage is done. Managing that risk is the province of 📝quench protection and of winding techniques such as the 📝no-insulation coil.
The best public data on how a fusion-scale HTS magnet actually quenches comes from a quench that was caused on purpose: 📝The SPARC Toroidal Field Model Coil Program (Paper) reports driving the coil into an open-circuit quench at 31.5 kiloamps to test whether it could protect itself passively. Two detection methods developed for the same program — fiber-optic sensing and torsional acoustic waves — are covered in 📝The TFMC Engineering Papers.