Nonlinear simulations of GAEs in NSTX-U

A set of nonlinear simulations has been performed to study the nonlinear evolution of unstable global Alfven eigenmodes (GAEs) in the NSTX-U. Results of the single toroidal mode number simulations are compared with a full nonlinear simulation (all toroidal harmonics included). It is shown that single-n results are close to full nonlinear simulation only for the most unstable mode, in which case the saturation amplitudes and changes in the fast ion distribution are comparable. In contrast, peak amplitudes of subdominant modes in all-n simulations are smaller by a factor of 5-10 compared to single-n runs due to flattening of the beam ion distribution by the fastest growing mode. In single-n simulations, the conservation of two integrals of motion of a particle in a cyclotron resonance with the mode is demonstrated, resulting in a one-dimensional evolution of the particle distribution. Nonlinear simulations show a significant redistribution of the resonant fast ions, especially in the pitch parameter. Thus, the changes in the resonant particles parallel and perpendicular energies can be several times larger than the mode energy. This implies that even a relatively small amplitude mode can significantly modify the beam distribution in the resonant region.