Electrons accelerated to near-light-speed during a tokamak disruption — carrying megajoules of energy and capable of melting through centimetres of steel, they are one of the most dangerous phenomena in fusion reactors.
In a plasma, the collisional drag force on an electron decreases as the electron’s velocity increases (because fast electrons spend less time near ions). Above a critical velocity, the electric field accelerating the electron exceeds the drag force, and the electron “runs away” — continuously accelerating to relativistic energies (tens of MeV). During a tokamak disruption, the rapid plasma cooling causes a spike in resistivity and a large induced electric field, creating conditions for massive runaway electron generation.[1]
The runaway electron problem is made worse by the avalanche (knock-on) mechanism: existing runaway electrons collide with thermal electrons, kicking them above the critical velocity and exponentially multiplying the runaway population. In ITER-scale plasmas, the avalanche gain could be 1010–1030, meaning even a tiny seed population grows to catastrophic levels.[2]
Strategies include: massive gas injection (neon, argon) to raise the critical electric field and increase drag; shattered pellet injection (SPI) to deposit material uniformly; and benign runaway beam termination through controlled wall contact or magnetic perturbations. ITER will use all three approaches.[3]