Whistler Waves Driven by Runaway Electrons

In quiescent runaway electron plasmas, whistler waves with frequencies between 90-190 MHz are driven unstable in plasmas with appreciable hard x-ray and non-thermal electron cyclotron emission (ECE). Narrow (δf < 50 kHz) discrete modes are observed at erratically spaced frequencies, likely due to the bounding of the plasma [1]. The dependency of the frequency on field and density implies a wavenumber k ≈ 140 m^-1 with k_parallel much less than k. Reducing the gap between the plasma and the wall increases the number of detected modes. The high intensity gamma ray bremsstrahlung and synchrotron emission measurements suggest the waves are driven by electrons of several MeV through the anomalous Doppler resonance. The ECE signals often jump at whistler bursts, suggesting that the modes pitch-angle scatter the runaways via nonlinear predator-prey dynamics, implying that whistler waves can potentially be used to mitigate reactor damage from runaways.

[1] D.A. Spong et at., Phys. Rev. Lett. 120 (2018) 155002.