An experimental study is conducted on the onset and evolution characteristics of energetic particle-driven instabilities in Korea Superconducting Tokamak Advanced Research (KSTAR) with dominant tangential neutral beam injection (NBI). A scan of NBI beam energy shows the evanescence of the sawtooth crash and the concomitant onset of the strong passing particle-driven low-frequency fishbone instability. A quantitative analysis shows that the safety factor (q)-profile in the core region is clamped by a balance between the depletion of energetic passing particles by the fishbone instability and their external replenishment. Two synchronized chirping modes with distinct toroidal mode numbers (n = 1 and n = 5) supersede the fishbone instability after a self-organized q-profile is attained. An analysis shows that the n = 1 mode is likely to be a high-frequency beta-induced Alfvén eigenmode fishbone branch, while the n = 5 mode is an energetic particle mode (EPM). A dynamic system analysis of the synchronized EPM (S-EPM) shows that a stable S-EPM cycle can exist when the coupling between the two modes involved is insignificant. The potential impact of such EPMs on the establishment of a burning plasma scenario with a flat core q-profile is briefly discussed.
Beta-dependence of energetic particle-driven instabilities in spherical tokamaks