Simulation study on collisionless loss of runaway electrons by magnetic perturbations in a tokamak

Simulations have demonstrated that magnetic islands having the widths expected in major disruptions cause collisionless loss of relativistic electrons and that the resultant loss rate is high enough to avoid or to suppress runaway generation. This is because, for the magnetic fluctuations in the disruptions, the loss of electron confinement due to the breakdown of toroidal momentum conservation overwhelms the runaway electron confinement due to the phase averaging effect of relativistic electrons. The simulation results provide strong support for the JT-60U experiments on fast plasma shutdown avoiding runaway generation. (author)