Alfv́n eigenmodes in reversed shear plasmas in JT-60U negative-ion-based neutral beam injection discharges

Rapid frequency sweeping modes observed in reversed magnetic shear (RS) plasmas on the Japan Atomic Energy Research Institute Tokamak 60 Upgrade (JT-60U) [H. Ninomiya and the JT-60 Team, Phys. Fluids B 4, 2070 (1992)] have been identified as reversed-shear-induced Alfv́n eigenmodes (RSAEs), which are ideal magnetohydrodynamic Alfv́n eigenmodes (AEs) localized to the region of minimum safety factor, qmin, and are excited by negative-ion-based neutral beam injection. The chirping and subsequent saturation of the mode frequency are consistent with theoretical predictions for the transition from RSAEs to toroidal Alfv́n eigenmodes (TAEs). The previously observed rapid frequency sweeping modes in ion cyclotron wave heated plasmas in JT-60U can also be similarly explained. The observed AE amplitude is largest during the transition from RSAEs to TAEs, and fast ion loss is observed when the AE amplitude is largest at this transition. It is preferable to operate outside the transition range of qmin, e.g., 2.4< qmin <2.7 for the n=1 AE to avoid substantial fast ion loss in RS plasmas.