The Effect of Externally Applied and Self-Excited Waves on Relativistic Electrons*

Wave-particle interactions, by either externally launched or self-excited waves, have been shown in theory and simulation to have the potential to directly affect runaway electrons (REs) enabling utilization as a potential mitigation technique. Experiments were conducted which took advantage of the existing electron cyclotron heating (ECH) systems on the DIII-D tokamak. These experiments examined a novel RE mitigation strategy which relies on mode-conversion between the launched O-mode polarization and the slow-X mode where this new wave has the potential to resonate with the RE population unlike the standard O and X-mode. Application of the ECH during the RE plateau resulted in an increase of the plasma density, accompanied by increase of both the loop voltage and the synchrotron radiation from the REs. A distinct strong density peak is formed near the O-X conversion window when the conversion conditions are met: specifically, when the O-mode is launched at an optimal pre-calculated angle to the background magnetic field. This density peak indicates a successful generation of the slow-X mode. Kinetic modeling together with a 1D impurity ion and neutral density transport modeling allowed for improved understanding and predictive capabilities of RE plateau physics. Additionally, standard X-mode waves were launched into a low density RE regime where they were observed to flush out trace levels of REs. In addition to EC waves, self-excited whistler waves have been observed to execute a predator prey-like relationship with the RE population. This relationship was measured using visible fast camera data and results in a dithering of the wave amplitude as the particles are pitch-angle scattered, synchrotron damped, and reaccelerated. These results advance the physical basis of foundational wave-particle interactions and also offer promise as RE mitigation strategies.

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