Destabilization of toroidal Alfvén eigenmode during neutral beam injection heating on HL-2A

Toroidal Alfvén eigenmodes (TAEs) driven by fast ions during neutral beam injection heating have been observed on HL-2A. TAEs are found to be excited by the sideband resonance between fast ions and shear Alfvén waves. Their frequencies range from 90 kHz to 200 kHz, and toroidal mode numbers of the most unstable TAEs are n = 1-3. Nonlinear behaviours of TAEs, such as frequency chirping, pitch-fork splitting, and nonlinear mode-mode coupling, are also excited. In the down-chirping case, TAEs show typical ballooning mode structures and their amplitudes are around δB_r = 3.25 × 10^-4T at the edge of the plasma. The mode frequency is proportional to the square of time, i.e., f(t) ∼ t^1/2, which agrees well with the theoretical prediction. Pitch-fork splitting of TAEs has also been excited on HL-2A, and their growth rate is about γ_l ∼ 0.22 × 10⁴ s^-1. TAE coupling with a tearing mode results in the appearances of series of Alfvénic modes (AMs). The two AMs, which have the same absolute mode number but rotate in different diamagnetic drift directions, couple together and lead to the generation of a high frequency mode with a toroidal mode number of n = 0. The experimental results also indicate that nonlinear mode-mode coupling degenerates the confinement of fast ions.