Interaction between Coherent Whistler Waves and Energetic Particles in the Magnetosphere and its Exploitation for Runaway Electron Suppression in Tokamaks

Recent spacecraft observations indicate that whistler mode chorus subelements are extremely coherent, i.e., are monochromatic and phase-aligned. In such cases, popular diffusion mechanisms such as the quasi-linear mechanism that requires a broadband wave spectrum are invalid, and coherent wave-particle interactions must be examined. It is shown that the vector relativistic equation of motion of a particle under a circularly polarized electromagnetic wave can be reduced to a scalar conservative equation of motion of a resonance mismatch parameter. This equation of motion involves a conserved pseudo-energy in a pseudo-potential, whose shape if double-welled can lead to a rapid, extreme scattering, which is often observed in the magnetosphere. This mechanism can then be leveraged to construct a "momentum firewall" for hazardous runaway electrons in tokamaks, wherein the accelerating electrons are prevented from gaining parallel momentum upon hitting the firewall and are instantaneously scattered in momentum space. This method is further verified by self-consistent particle-in-cell simulations.