Stochastic Instability of Relativistic Runaway Electrons Due to Lower Hybrid Waves

Runaway electrons and lower hybrid waves in tokamak discharges are coupled because of the relativistic decrease of the cyclotron frequency. Two interaction regimes must be considered. For the low-energy part of the relativistic runaway spectrum, neighbouring low harmonic nonlinear cyclotron resonances may overlap. For the high-energy part, the influence of the higher-order resonances are more conveniently studied with the help of a map describing nearly perpendicular Landau coupling. In both cases it is shown that the stochasticity threshold can be reached in typical discharges, leading to quasi-linear diffusion.