  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.
