Fast ion effects on gyrokinetic turbulence

Experimental and dedicated gyrokinetic studies have reported strong fast ion effects on ion-scale plasma turbulence. Depending on the origin of the energetic tails - e.g., generated through auxiliary heating systems such as neutral beam injection or ion cyclotron resonance heating or in view of ITER from fusion reactions - these effects are found to possibly provide substantial overall confinement improvements. In particular, a wave-fast ion resonant stabilising mechanism has been acknowledged quite recently [Di Siena et al Nucl. Fusion 58 054002 2018]. Corresponding studies with the gyrokinetic code GENE and a reduced analytic model will be summarized in the first part of this contribution. The second part will deal with removing an important deficiency that is present in most of such studies, which employ equivalent Maxwellian fast ion distributions. Anisotropic velocity structures which might modify the aforementioned resonance effects and which can be particularly relevant in driving energetic particle induced modes are not captured. In the contribution at hand, the impact of fast ions is studied for the first time for realistic physics inputs taken from actual discharges with an extended version of the gyrokinetic code GENE [Di Siena et al Phys. Plasmas 5 042304 2018], which allows to relax the assumption on the energetic ion background distribution functions for flux-tube and radially-global simulations. Numerical backgrounds, computed with state-of-the-art heating codes are employed to model highly non-thermalised particles for ASDEX Upgrade and JET discharges with substantial fast ion fraction. By comparing GENE heat/particle fluxes and fast-ion driven frequency spectra with experiment, a much better agreement is observed. Based on these findings, a brief outlook on how these intriguing fast ion effects may affect ITER plasmas will be given.