The beta-induced Alfvén eigenmode (BAE) excited by large m / n = 2/1 magnetic islands has been recently observed on HL-2A. The frequencies of these oscillations range from 15 to 35 kHz, and is 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. The analysis reveals that the modes propagate poloidally and toroidally in opposite directions, and form standing-wave structures in the island rest frame. The frequencies of the pair mode are associated with the Te/Ti ratio, Alfvén velocity and island width, and the frequency difference of the pair modes is determined by the frequency of tearing mode. Note to point out that the BAE frequency is influenced by the Te/Ti ratio. The island width has been evaluated quantitatively, and the island width threshold of BAE excitation has been found by using statistical analysis method. The threshold w c is about 3.4 cm on HL-2A, and the BAE can be driven while the width island exceeds the threshold w c .
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Chen et al. (2010) studied this question.
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