Electrons that have been accelerated to relativistic velocities after plasma disruption in a tokamak. This is a dangerous phenomenon that can seriously damage the wall of the tokamak vessel. When the disruption, or sudden termination of the plasma discharge occurs, the electrons can gain such energy that they are no longer slowed down by collisions with other particles in the plasma and the electromagnetic field in the tokamak begins to act as an accelerator, giving them speeds approaching the speed of light. Upon hitting the tokamak wall, runaway electrons release their energy and can cause the chamber wall to melt, or in the worst case, to perforate. Ways to prevent or mitigate runaway electrons are being improved.