Investigation of double frequency fishbone in EAST with neutral beam injection

Abstract In the EAST experiments with neutral beam injection and at a low electron density, fishbone instabilities characterized by mode numbers m / n = 1/1, 2/2 have been observed. The simultaneous growth of these modes, particularly m / n = 1/1 and m / n = 2/2, results in a decrease in neutron emission at the plasma core. As the electron density continues to decrease, the beta-induced Alfvén eigenmode (BAE) emerges, replacing the m / n = 2/2 fishbone just before a more severe sawtooth crash occurs. Numerical simulations with the global kinetic-magnetohydrodynamic (MHD) code M3D-K show that the m / n = 2/2 high frequency fishbone branch is linearly stable, but nonlinearly grows due to the coupling with the m / n = 1/1 low frequency fishbone branch. The m / n = 2/2 fishbone frequency is almost twice of the m / n = 1/1 fishbone, and both fishbone frequencies chirp down together. The mode frequencies and structures of the simulated fishbones are consistent with the experimental measurements. In addition, a BAE with m / n = 2/2 is observed in the late nonlinear stage of the simulation. Energetic particle nonlinearity is dominant for the m / n = 2/2 fishbone saturation and transition to BAE. In particular, the transition from m / n = 2/2 fishbone to BAE is found to be the consequence of fast ion redistribution due to the fishbone instability.