runaway electrons are electrons accelerated to relativistic energies during a tokamak disruption, capable of forming a focused beam that melts and damages the vessel wall.
The mechanism is a loophole in how plasmas resist electric fields. Ordinarily, an electron pushed by a field is held back by collisions with the ions around it — but that collisional drag weakens as the electron moves faster. Above a critical speed, drag can no longer balance the push, and the electron "runs away," gaining energy without limit until it approaches the speed of light. The current quench of a 📝disruption creates ideal conditions: the collapsing 📝plasma current induces a large toroidal electric field through a suddenly cold 📝plasma. Worse, each runaway can knock other electrons into the runaway regime, so the population multiplies in an avalanche that can convert megaamperes of plasma current into a beam of electrons at energies of tens of megaelectronvolts.
When such a beam loses control and strikes the wall, it deposits its energy in a small area, melting plasma-facing surfaces — one of the most serious off-normal events a reactor-scale 📝Tokamak must design against. Countermeasures include injecting massive quantities of material to raise density and drain the beam, and passive coils that break up the beam's confinement before it fully forms.
📝SPARC carries a runaway electron mitigation coil — a passive coil inside the 📝vacuum vessel, published as part of the machine's open physics basis, designed to deconfine runaways before a damaging beam can form.