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Description
Synergistic heating using Ion Cyclotron Range of Frequencies (ICRF) waves and Neutral Beam Injection (NBI) can produce fast ions (FIs) with energies significantly higher than those of the beam ions. The effects of NBI-ICRF synergy (NIS) FIs on NBI-fast-ion-driven fishbone instabilities are investigated using the M3D-K code. The FIs distribution function contains a slowing-down distribution and a drifting bi-Maxwellian distribution, which represent the NBI FIs and the NIS FIs, respectively.
When the equilibrium modification induced by NIS FIs is neglected, the NIS FIs exerts a stabilizing influence on the NBI-driven fishbone mode. This stabilizing effect is enhanced with the increasing perpendicular temperature $T_\perp$ and beta $\beta_{NIS}$ of the NIS FIs. A high-frequency fishbone branch is driven when $\beta_{NIS}$ exceeds an onset threshold. Besides, a fully stable region may occur for sufficiently large $T_\perp$, in which case the fishbone is suppressed before the high-frequency fishbone is driven. These results indicate a fishbone-free regime under NBI-ICRF synergy conditions. When the equilibrium modification by NIS FIs is taken into account, the net effect becomes stabilizing for larger $T_\perp$ and destabilizing for smaller $T_\perp$.
The generalized energy principle indicates that the stabilizing contribution mainly originates from the non-resonant kinetic response of the NIS FIs, which depends on $T_\perp$. In contrast, the adiabatic contribution, corresponding to the equilibrium modification, is destabilizing and is independent of $T_\perp$. Therefore, the overall impact of NIS FIs is determined by the competition between the adiabatic and kinetic effects.