  This study comprehensively investigates fast ion-driven drift instability,
extending the theory in [B. J. Kang and T. S. Hahm, Phys. Plasmas 26, 042501
(2019)]. The eigenmode equation, including the resonant contribution of passing
fast ions, is derived and solved using the shooting method. Passing fast ions
significantly affect the instability in weak negative shear or moderate
positive shear plasmas. Eigenstate transitions to non-ground states occur more
readily in weak magnetic shear, high safety factor, and long wavelength
perturbations. Linear gyrokinetic simulations using the GKV code verify the
theory, showing good agreement with shooting method results. The estimated
quasilinear transport indicates that the net energy flux can be inward, without
contradicting the second law of thermodynamics. These findings have important
implications for heating efficiency and plasma confinement in the heating
process, such as Ion Cyclotron Resonance Heating (ICRH) in future fusion
devices.
