  Fast ions have recently been found to significantly impact and partially
suppress plasma turbulence both in experimental and numerical studies in a
number of scenarios. Understanding the underlying physics and identifying the
range of their beneficial effect is an essential task for future fusion
reactors, where highly energetic ions are generated through fusion reactions
and external heating schemes. However, in many of the gyrokinetic codes fast
ions are, for simplicity, treated as equivalent-Maxwellian-distributed particle
species, although it is well known that to rigorously model highly
non-thermalised particles, a non-Maxwellian background distribution function is
needed. To study the impact of this assumption, the gyrokinetic code GENE has
recently been extended to support arbitrary background distribution functions
which might be either analytic, e.g. slowing down and bi-Maxwellian, or
obtained from numerical fast ion models. A particular JET plasma with strong
fast-ion related turbulence suppression is revised with these new code
capabilities both with linear and nonlinear gyrokinetic simulations. It appears
that the fast ion stabilization tends to be less strong but still substantial
with more realistic distributions, and this improves the quantitative power
balance agreement with experiments.
