Transition from fishbone mode to β-induced Alfvén eigenmode on HL-2A tokamak

In the presence of energetic particles (EPs) from auxiliary heating and burning plasmas, fishbone instability and Alfvén modes can be excited and their transition can take place in certain overlapping regimes. Using the hybrid kinetic-magnetohydrodynamic model in the NIMROD code, we have identified such a transition between the fishbone instability and the β-induced Alfvén eigenmode (BAE) for the NBI heated plasmas on HL-2A. When the safety factor at magnetic axis is well below one, typical kink-fishbone transition occurs as the EP fraction increases. When q₀ is raised to approaching one, the fishbone mode is replaced with BAE for sufficient amount of EPs. When q₀ is slightly above one, the toroidicity-induced Alfvén eigenmode dominates at lower EP pressure, whereas BAE dominates at higher EP pressure.