Destabilization of ellipticity-induced Alfven eigenmodes during ICRF heating and Stabilization by negative-ion-based neutral beam injection-in JT-60U

High-frequency modes in the 525-550 KHz range with toroidal mode numbers n = 3-7 were excited after the sawtooth crash during ion cyclotron range of f requency (ICRF) heating. Based on the stability analysis using the NOVA-K c ode, these modes were found to be consistent with ellipticity-induced Alfve n eigenmodes (EAEs) excited in the EAE gap located at the q = 1 surface. A change in the safety factor after the sawtooth crash was essential for the excitation of EAEs in the core region of the plasma. The high-frequency mod es were stabilized by the neutral beam injection at 350 keV using negative- ion-based neutral beam (NNB) injectors. Stability analysis using the NOVA-K code has shown that NNB-injected ions enhance the damping of the EAEs and tend to stabilize them.