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Description
The kinetic-magnetohydrodynamic hybrid code, MEGA, is employed to study the effect of energetic particle (EP) distribution on reversed shear Alfvén eigenmodes (RSAEs). Focusing on the most unstable (n=2) RSAE in an EAST discharge, the EP distribution is represented as a linear sum of multiple Gaussian functions, following Gorelenkov [Nucl. Fusion 45, 226 (2005)] and Liu [Phys. Plasmas 21, 056105 (2014)]. This modification concentrates more EPs along the NBI direction, consistent with realistic observation in the experiment, and yields the phase-space distribution closer to NUBEAM output, thus better explaining the enhanced RSAE instability when the beam switches from tangential to perpendicular injection. The linear growth rate is larger with co-injection than counter-injection, which is especially sensitive to the peak of the EP distribution in pitch angle space close to the dominant resonance band. For this reason, the saturation mechanism is radial decoupling for co-injection. The frequency and amplitude have drastic fluctuations since mode structure varies significantly. For counter-injection, the saturation mechanism is resonance detuning due to low linear growth rate so that the mode structure largely preserves its shape. The energy transfer from each component is comparatively lower, therefore does not induce significant amplitude fluctuations. In addition, counter-injection case has higher saturated level of zonal current, related to the RSAE evolution, and more EPs are confined due to the weaker interaction between mode and particle.