Fully Kinetic Simulations of Wave-particle Interactions with Runaway Electrons in Tokamak towards Nonlinear Saturation

Runaway electrons at relativistic energies can cause severe damage on the plasma-facing wall in a tokamak. They can drive plasma kinetic instabilities, if not for the strong collisional damping by a cold background plasma such as after thermal quench. In the cases where the background plasma is warm and/or of low density, the runaway-driven kinetic instabilities can be fully excited. Here we present the first self-consistent fully kinetic simulations with runaway-driven waves towards nonlinear saturation in a warm plasma (T>100 eV), whose parameters correspond to the tokamak start-up process. We find that the interactions of self-driven waves with runaways convert a portion of the electric current from being carried by high energy electrons to low and moderate energy electrons, through two pairs of wave driving-and-damping resonances. In the first pair, the runaways drive whistler waves through anomalous Doppler resonance which smears out the pitch angles of the high energy runaways to reduce the high energy current, while these whistler waves landau damp at moderate momentums (p~0.5me c) to drive current. But the whistler-landau-damping momentums are sometimes not sufficiently low to approach the thermal bulk of ~100 eV electrons to effectively drive current. In the second pair, the runaways drive waves around the upper hybrid frequency through anomalous Doppler resonance as well as even higher harmonic resonances, while these waves can damp though normal Doppler resonance with the low energy thermal bulk electrons to drive current. This accelerates some electrons from the thermal bulk to reach the momentums of the whistler landau damping to further drive current – which is not possible with the whistler landau damping alone. These dynamics are essential to understand and model wave-runaways interaction in order to effectively mitigate the runaways.