Alfvén eigenmodes driven by Alfvénic beam ions in JT-60U

Abstract

Instabilities with frequency chirping in the frequency range ofAlfvén eigenmodes have been found in the domain 0.1% < ⟨βh⟩ < 1% and vb||/vA ∼1 with high energy neutral beam injection in JT-60U. One instability with a frequency inside the Alfvén continuum spectrum appears and its frequency increases slowly to the toroidicityinduced Alfvén eigenmode (TAE) gap on the timescale of an equilibrium change ( ≈ 200 ms).Other instabilities appear with a frequency inside the TAE gap and theirfrequencies change very quickly by 10-20 kHz in 1-5 ms.During the period when these fast frequency sweeping (fast FS) modes occur, abrupt large amplitude events (ALEs) often appear with a drop of neutron emission rateand an increase in fast neutral particle fluxes.The loss of energetic ions increases with a peakfluctuation amplitude of B̃θ/Bθ. An energy dependence of the loss ions is observed and suggests a resonant interaction between energetic ions and the mode.