Observation of Low Frequency MHD Mode Driven by Energetic Particles in Large Helical Device Plasmas with Strong Interchange Mode Activities

Abstract

The beta-induced Alfvén eigenmode (BAE) like modes during strong interchange mode, whose mode numbers are m/n = 2/1, have been recently observed for the first time in Large Helical Device (LHD). The first harmonic frequencies of these oscillations range from 30 to 70 kHz, much lower than the toroidal-Alfvén-eigenmode (TAE) frequency, and are provided with the same order of the low-frequency gap induced by finite beta effects. The magnetic fluctuation spectrogram indicates that the BAEs often occur in pairs, and their mode-numbers are m/n = 2/1 and −2/−1. The analysis reveals that the modes propagate poloidally and toroidally in opposite directions, and form standing-wave structures in interchange-mode rest frame. The frequencies of the pair mode are associated with the Te/Ti ratio, and the frequency difference of the pair modes is determined by the frequency of interchange mode. The new finding shed light on the underlying physics mechanism for the excitation of the low frequency Alfvénic fluctuation.