Role of kinetic instability in runaway electron avalanche and elevated critical electric fields

Abstract: The effects of kinetic whistler-wave instabilities on the runaway-electron (RE) avalanche is investigated. With parameters from DIII-D experiments, we show that RE scattering from excited whistler waves can explain several poorly understood experimental results seen in a variety of tokamaks. We find an increase of the avalanche growth rate and threshold electric field, bringing the present model much closer to observations than previous results. The excitation of kinetic instabilities and the scattering of resonant electrons are calculated self-consistently using a quasilinear model. We also explain the observed fast growth of electron cyclotron emission (ECE) signals and excitation of very low-frequency whistler modes observed in the quiescent RE experiments at DIII-D [D. A. Spong et al., submitted to Phys. Rev. Lett.]. These results indicate that by controlling the background thermal plasma temperature, the plasma wave can be excited spontaneously in tokamak disruptions and the avalanche generation of runaway electrons may be suppressed.