  A fast Alfv\'en wave with finite amplitude is shown to grow by a stimulated
emission process that we propose for exploitation in toroidal magnetically
confined fusion plasmas. Stimulated emission occurs while the wave propagates
inward through the outer mid-plane plasma, where a population inversion of the
energy distribution of fusion-born ions is observed to arise naturally. Fully
nonlinear first principles simulations, which self-consistently evolve
particles and fields under the Maxwell-Lorentz system, demonstrate this novel
"alpha-particle channelling" scenario for the first time.
