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Alfven Eigenmodes are destabilized at the DIII-D pedestal during transientbeta drops in high poloidal beta discharges with internal transport barriers(ITBs), driven by n=1 external kink modes, leading to energetic particlelosses. There are two different scenarios in the thermal beta recovery phase:with bifurcation (two instability branches with different frequencies) orwithout bifurcation (single instability branch). We use the reduced MHDequations in a full 3D system, coupled with equations of density and parallelvelocity moments for the energetic particles as well as the geodesic acousticwave dynamics, to study the properties of the instabilities observed in theDIII-D high poloidal beta discharges and identify the conditions to trigger thebifurcation. The simulations suggest that instabilities with lower frequency inthe bifurcation case are ballooning modes driven at the plasma pedestal, whilethe instability branch with higher frequencies are low n (n<4) Toroidal AlfvenEigenmodes nearby the pedestal. The reverse shear region between the middle andplasma periphery in the non-bifurcated case avoids the excitation of ballooningmodes at the pedestal, although Toroidal Alfven Eigenmodes and Reverse ShearAlfven Eigenmodes are unstable in the reverse shear region. The n=1 and n=2Alfven Eigenmode activity can be suppressed or minimized if the neutral beaminjector (NBI) intensity is lower than the experimental value (betaf < 0.03).In addition, if the beam energy or neutral beam injector voltage is lower thanin the experiment (Vth,f / VA0 < 0.2), the resonance between beam and thermalplasma 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 beaminjector operation regime. In conclusion, AE activity in high poloidal betadischarges can be minimized for optimized NBI operation regimes.

Analysis of Alfven Eigenmode destabilization in DIII-D high poloidal $\beta$ discharges using a Landau closure model