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

Abstract

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 δBr = 3.25 × 10−4T at the edge of the plasma. The mode frequency is proportional to the square of time, i.e., f(t) ∼ t1∕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 γl ∼ 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.

Nonlinear dynamics of toroidal Alfvén eigenmodes in the presence of tearing modes

“…It is interestingly found that, for the small resistivity, the fre quency chirping shows updown symmetry structure which is consistent with previous TAE simulation results [24,26], while the frequency chirping phenomenon becomes weaker due to strong tearing mode activity in cases with large resistivity. This simulation result is also consistent with experiment results on HL2A [6,7]. They found that TAEs show a typical pitchfork splitting in the frequency for weak tearing mode activity while there is no frequency chirping of TAE modes due to nonlinear modemode coupling in the presence of strong low frequency tearing modes [7].…”

“…The axissymmetry magnetic activities with n = 0 have been observed during neutral beam injection (NBI) on HL2A and interpreted that these modes are generated by the nonlinear mode coupling between high frequency Alfvén eigenmodes and lowfrequency magnetohydrodynamics (MHD) modes [6]. Besides, for the strong AE mode and TM mode cou pling case, TAEs show a weak chirping feature while pitch fork TAEs are observed associated with weak tearing mode activities [7]. In MAST plasmas, they showed evidence of fre quency splitting of CAE(compressional Alfvén eigenmodes) activity with modulation by a strong low frequency n = 1 tearing mode.…”

“…The nonlinear mode couplings between Alfvén waves and magnetic islands (due to tearing mode instability) are observed in tokamak experiments, such as FTU [4], MAST [5], HL2A [6][7][8] and JTEXT [9]. In FTU ohmic plasmas without EPs, the observed interaction between high frequency (HF) modes and tearing modes [4] was interpreted as betainduced Alfvén eigenmodes (BAEs) that are nonlinearly excited via three wave coupling with a sufficiently large magnetic island [10].…”

Observation and Simulation of n = 1 Reversed Shear Alfvén Eigenmode on the HL-2A Tokamak

“…Usually, the BAE frequency increases with increasing ion temperature [35] while the TAE frequency shows a reversed relation with the electron density. [36] Great changes have been found in both BAE and TAE frequencies, whereas the mode frequencies of 𝑛 = 1 Alfvénic modes vary slightly during multiple discharge environment. Here, the great error bars in TAE frequencies mainly come from the change of electron den-sity or frequency chirping behavior related to the phasespace dynamics of energetic particles.…”

Progress of microwave diagnostics development on the HL-2A tokamak

“…The heterodyne technique is used in the interferometer system, leading to high resolution for density fluctuation measurement (temporal resolution: <1 μs). This system provides a critical experimental evidence in the study of nonlinear behaviors of the toroidal Alfven eigenmode during neutral beam injection [38,39].…”