Investigation of double frequency fishbone in EAST with neutral beam injection

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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.
Funding Information
  • Strategic Priority Research Program of Chinese Academy of Sciences (XDB0790202)
  • National Natural Science Foundation of China (12275309)
  • National Key Research and Development Program of China (2019YFE03050002)
  • Science Foundation of Institute of Plasma Physics, Chinese Academy of Sciences (DSJJ-2022-04)

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