Gyrokinetic modelling of the Alfvén mode activity in ASDEX Upgrade with an isotropic slowing-down fast-particle distribution

Abstract In the present paper, the evolution of the Alfvén modes (AMs) is studied in a realistic ASDEX Upgrade equilibrium by analyzing the results of simulations with the global, electromagnetic, gyrokinetic particle-in-cell code ORB5. The energetic particles (EPs) are modelled both via the newly implemented isotropic slowing-down and with Maxwellian distribution functions. The comparison of the numerical results shows that modelling the EPs with the equivalent Maxwellian rather than with the slowing-down, does not significantly affect the frequency of the driven AM, while its growth rate appears to be underestimated with a quantitative difference as large as almost 30%. Additionally the choice of the isotropic slowing-down allows a better description of the nonlinear modification of the dominant AM frequency, while an equivalent Maxwellian underestimates it. A good comparison with the experimental spectrogram is found.