  Alfven Eigenmodes are destabilized at the DIII-D pedestal during transient
beta drops in high poloidal beta discharges with internal transport barriers
(ITBs), driven by n=1 external kink modes, leading to energetic particle
losses. There are two different scenarios in the thermal beta recovery phase:
with bifurcation (two instability branches with different frequencies) or
without bifurcation (single instability branch). We use the reduced MHD
equations in a full 3D system, coupled with equations of density and parallel
velocity moments for the energetic particles as well as the geodesic acoustic
wave dynamics, to study the properties of the instabilities observed in the
DIII-D high poloidal beta discharges and identify the conditions to trigger the
bifurcation. The simulations suggest that instabilities with lower frequency in
the bifurcation case are ballooning modes driven at the plasma pedestal, while
the instability branch with higher frequencies are low n (n<4) Toroidal Alfven
Eigenmodes nearby the pedestal. The reverse shear region between the middle and
plasma periphery in the non-bifurcated case avoids the excitation of ballooning
modes at the pedestal, although Toroidal Alfven Eigenmodes and Reverse Shear
Alfven Eigenmodes are unstable in the reverse shear region. The n=1 and n=2
Alfven Eigenmode activity can be suppressed or minimized if the neutral beam
injector (NBI) intensity is lower than the experimental value (betaf < 0.03).
In addition, if the beam energy or neutral beam injector voltage is lower than
in the experiment (Vth,f / VA0 < 0.2), the resonance between beam and thermal
plasma is weaker. The n=3,4,5 and 6 AE activity can't be fully suppressed,
although the growth rate and frequency is smaller for an optimized neutral beam
injector operation regime. In conclusion, AE activity in high poloidal beta
discharges can be minimized for optimized NBI operation regimes.
