Fast nonlinear scattering of runaway electron beams through resonant interactions with plasma waves

The resonant interaction between a runaway electron (RE) beam and a background reactor-grade background plasma is investigated through two-dimensional particle-in-cell (PIC) simulations, employing a simplified model of the system. The temporal evolutions of the electron momentum distribution function for two separate initial beam energies (1 and 10 MeV) are tracked, revealing the occurrence of plasma wave growth concomitant with pitch angle scattering or momentum distribution diffusion within the RE beam. Notably, we identify and confirm the dependence of the dominant resonance condition on the initial kinetic energy of the RE beam. Furthermore, we quantify the effect of particle-wave interactions on the RE momentum distribution diffusion by assessing the average kinetic energy flux from the runaway electron distribution function.