Speaker: Dr. Chang Liu, PPPL (Princeton Plasma Physics Laboratory)
Abstract: High-energy runaway electron beams can be generated in fusion experiments during various discharge phases. Control and mitigation of runaway electron beams and reducing their energy impact on the plasma-facing walls is critical to the operation of ITER and other tokamaks. Interestingly, the high energy of these electrons can also provoke plasma instabilities, creating pathways for the electrons to dissipate energy and diffuse in space. In this presentation, I will discuss some of the common instabilities driven by runaway electrons observed in experiments and the efforts to model them. High-frequency plasma waves, such as whistler and extraordinary electron (EXEL) waves, can be stimulated by the anisotropic distribution of runaway electrons and resonating with their cyclotron motion. This resonance can lead to rapid pitch-angle scattering, manifested as an abrupt increase in electron cyclotron emission (ECE) signals. In addition, these instabilities can affect the runaway electrons' avalanche growth and raise the critical electric field for the transition from growth to decay of runaway electron population. Lower frequency plasma waves can also be excited if they resonate with the precession motion of the trapped runaway electrons. The excitation of plasma instabilities in the Alfvén frequency range has been observed in DIII-D, AUG and JET disruption experiments, and has been shown to contribute to the loss of runaway electrons during current quench. The interaction between these Alfvén oscillations in the background plasma and runaway electrons can be explored through kinetic-MHD simulations, including calculations of linear growth, nonlinear saturation, and the consequent transport of runaway electrons.