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.
This paper explores the phenomenon of fishbone instabilities observed in the EAST tokamak during neutral beam injection experiments. The authors report the simultaneous growth of m/n = 1/1 and m/n = 2/2 fishbone modes, leading to a decrease in neutron emission at the plasma core. As the electron density decreases, a beta-induced Alfvén eigenmode (BAE) emerges, replacing the m/n = 2/2 fishbone before a sawtooth crash. Numerical simulations show that the m/n = 2/2 fishbone is linearly stable but nonlinearly grows due to coupling with the m/n = 1/1 fishbone, and both modes exhibit frequency chirping.