Stimulated Emission of Fast Alfv\'{e}n Waves within Toroidally Confined Low Beta Fusion Plasmas

A fast Alfv\'{e}n wave of initially low energy is shown to be greatly amplified by a new stimulated emission process [J. W. S. Cook, R. O. Dendy, and S. C. Chapman, Phys. Rev. Lett. 118, 185001 (2017)]. This extracts energy from a population inversion of fusion-born ions, which is observed to arise naturally in the outer mid-plane of large tokamak plasmas. The inward propagation of a fast Alfv\'{e}n wave through~the outboard edge of a tokamak plasma, in the presence of this fast ion population, is modeled using~full orbit kinetic~particle-in-cell simulations~using the nonlinear self-consistent Maxwell-Lorentz equations.~Within the constraints~of periodic boundary conditions, and initially uniform density and magnetic field, these simulations demonstrate this novel alpha-particle channelling scenario for the first time.