An electron accelerated to near-light speed by the electric field during a tokamak disruption — capable of boring through meter-thick metal walls and one of the most dangerous threats to future fusion reactors.
ReviewedLast reviewed: 9 Aug 2026·Category: Glossary
Formation
During a disruption’s current quench, the rapid decay of the plasma current induces a large toroidal electric field. Electrons that are fast enough to outrun collisional drag are continuously accelerated by this field, gaining energy until they approach relativistic speeds (tens of MeV). These are runaway electrons.[1]
ITER threat: Runaway electron beams in ITER could carry up to 10–12 MA of current at energies of 10–30 MeV. If such a beam strikes the wall, it deposits its energy in a localized spot, potentially melting through centimetres of tungsten or steel in milliseconds. This is arguably the single most dangerous consequence of an unmitigated disruption.
Avalanche Multiplication
In large tokamaks, runaway electrons can multiply exponentially through “knock-on” collisions (Rosenbluth avalanche): a single runaway electron collides with a thermal electron, promoting it to runaway energy, and both continue accelerating. In ITER, the avalanche gain can amplify the runaway population by factors of 1010 or more.[2]
Mitigation
Runaway electron mitigation strategies include massive material injection (raising plasma density to increase collisional drag), resonant magnetic perturbations (deconfining runaway orbits), and controlled dissipation of the runaway beam. None has been fully validated at the ITER scale.[3]
Sources
Rosenbluth, M.N. and Putvinski, S.V. "Nuclear fusion with polarized fuel." Nuclear Fusion, 37, 1355, 1997.
Breizman, B.N. et al. "Physics of runaway electrons in tokamaks." Nuclear Fusion, 59, 083001, 2019.
Lehnen, M. et al. "Disruptions in ITER and strategies for their control and mitigation." Journal of Nuclear Materials, 463, 39–48, 2015.