Gyrokinetic modelling of the Alfv\'en mode activity in ASDEX Upgrade with an isotropic slowing-down fast-particle distribution

In the present paper, the evolution of the Alfv\'en modes is studied in a realistic ASDEX Upgrade equilibrium by analysing the results of simulations with the global, electromagnetic, gyrokinetic particle-in-cell code ORB5. The energetic particles 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 energetic particles with the equivalent Maxwellian rather than with the slowing-down, does not affect the frequency of the driven Alfv\'en mode, 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 Alfv\'en mode frequency, while an equivalent Maxwellian underestimates it. A good comparison with the experimental spectrogram is found.