  The aim of this study is to analyze the effect of the neutral beam injector
(NBI) operation regime on the saturation phase of the Alfven Eigenmodes (AEs)
in DIII-D plasma. The analysis is done using the linear and nonlinear versions
of the gyro-fluid code FAR3d. A set of parametric analyses are performed
modifying the nonlinear simulation EP \b{eta} (NBI injection power), EP energy
(NBI voltage) and the radial location of the EP density profile gradient (NBI
radial deposition). The analysis indicates a transition from the soft (local
plasma relaxation) to the hard MHD (global plasma relaxation) limit if the
simulation EP \b{eta} >= 0.02, leading to bursting MHD activity caused by
radial AEs overlapping. MHD bursts cause an enhancement of the EP transport
showing ballistic-like features as avalanche-like events. Simulations in the
soft MHD limit show an increment of the EP density gradient as the EP \b{eta}
increases. On the other hand, there is a gradient upper limit in the hard MHD
limit, consistent with the critical-gradient behavior. AEs induce shear flows
and zonal current leading to the deformation of the flux surfaces and the
safety factor profile, respectively, particularly strong for the simulation in
the hard MHD limit. Simulations in the hard MHD regime show a decrease of the
AE frequency in the saturation phase; this is caused by the destabilization of
a transitional mode between a 9/3-10/3 TAE and a 9/3 RSAE that may explain the
AE frequency down-sweeping observed in some DIII-D discharges. Reducing the EP
energy in the nonlinear simulations leads to a weakening of the plasma
perturbation. On the other hand, increasing the EP energy causes the opposite
effect. Nonlinear simulations of off-axis NBI profiles indicate a lower plasma
perturbation as the EP density gradient is located further away from the
magnetic axis.
