Stochastic Instability of Runaway Electrons in Tokamaks

The motion of runaway electrons in Tokamak field configuration is found to be unstable with respect to a stochastic instability due to nonlinear cyclotron resonances with the toroidal magnetic field ripple. We demonstrate that some neighbouring poloidal harmonics of these relativistic resonance overlap. The resulting instability provides an efficient mechanism to convert the parallel kinetic energy driven by the loop voltage into perpendicular one. When combined with cyclotron losses this conversion acts as an effective radiative slowing-down process causing an energy limit in agreement with recent observations.