Inverse Compton scattering of the ITG turbulence by energetic ions

Linearly stable toroidal Alfven eigenmodes (TAEs) can be non-linearly excited by the ion temperature gradient mode turbulence having frequencies an order of magnitude lower than TAE. An excitation mechanism is the inverse Compton scattering by energetic ions, i.e., the inverse non-linear Landau damping accompanied by a frequency increase in the scattered waves. This effect can be responsible for excitation of the stable TAE by turbulence observed in numerical simulations [Di Siena et al., Nucl. Fusion 59, 124001 (2019)]. Such non-linear coupling to stable TAE via energetic ions provides an efficient energy sink for turbulence and can explain strong reduction of the thermal ion heat flux in the presence of fast ions observed in gyro-kinetic modeling [Citrin et al., Phys. Rev. Lett. 111, 155001 (2013)].