  The observation that fast ions stabilize ion-temperature-gradient-driven
microturbulence has profound implications for future fusion reactors. It is
also important in optimizing the performance of present-day devices. In this
work, we examine in detail the phenomenology of fast ion stabilization and
present a reduced model which describes this effect. This model is derived from
the high-energy limit of the gyrokinetic equation and extends the existing
"dilution" model to account for nontrivial fast ion kinetics. Our model
provides a physically-transparent explanation for the observed stabilization
and makes several key qualitative predictions. Firstly, that different classes
of fast ions, depending on their radial density or temperature variation, have
different stabilizing properties. Secondly, that zonal flows are an important
ingredient in this effect precisely because the fast ion zonal response is
negligible. Finally, that in the limit of highly-energetic fast ions, their
response approaches that of the "dilution" model; in particular, alpha
particles are expected to have little, if any, stabilizing effect on plasma
turbulence. We support these conclusions through detailed linear and nonlinear
gyrokinetic simulations.
