  Magnetohydrodynamic instabilities like Toroidal Alfv\'en Eigenmodes or
core-localized modes such as Beta Induced Alfv\'en Eigenmodes and Reversed
Shear Alfv\'en Eigenmodes driven by fast particles can lead to significant
redistribution and losses in fusion devices. This is observed in many ASDEX
Upgrade discharges. The present work aims to understand the underlying
resonance mechanisms, especially in the presence of multiple modes with
different frequencies. Resonant mode coupling mechanisms are investigated using
the drift kinetic HAGIS code [Pinches 1998]. Simulations were performed for
different plasma equilibria, in particular for different q profiles, employing
the availability of improved experimental data. A study was carried out,
investigating double-resonant mode coupling with respect to various overlapping
scenarios. It was found that, depending on the radial mode distance,
double-resonance is able to enhance growth rates as well as mode amplitudes
significantly. Small radial mode distances, however can also lead to strong
nonlinear mode stabilization of a linear dominant mode. With the extended
version of HAGIS, losses were simulated and directly compared with experimental
loss measurements. The losses' phase space distribution as well as their
ejection signal is consistent with experimental data. Furthermore, it allowed
to characterize them as prompt, resonant or stochastic. It was found that
especially in multiple mode scenarios (with different mode frequencies),
abundant incoherent losses occur in the lower energy range, due to a broad
phase-space stochastization. The incoherent higher energetic losses are
"prompt", i.e. their initial energy is too large for confined orbits.
