1348 / 2019-11-14 15:10:56

Excitation of Beta-induced Alfven-Acoustic Eigenmode during Off-axis Sawteeth in EAST

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The characteristics and excitation conditions of Beta-induced Alfven-Acoustic Eigenmode (BAAE) have been investigated during off-axis sawteeth (OAS) oscillation in EAST. The BAAE is coexisted with Beta-induced Alfven Eigenmode (BAEs) and Reversed Shear Alfven Eigenmodes (RSAEs), and the BAAE has similar characteristics as the pair of BAEs-RSAEs. The radial structures of BAAE and BAEs are deformed into the analogous triangle shapes along poloidal direction, and the BAAE is located more outward with broadened radial coverage. The frequencies of BAAE and RSAEs are sweeping upward synchronously for the decreasing of q_min (q_min<1), and the BAAE can also provide valuable constraint conditions for the q-profile measurements.
The period τ_saw of OAS increases when the total injected powers of neutral beam injection (NBI) and ion cyclotron resonance heating (ICRH) are increased, where the NBI with beam direction perpendicular to plasma current. The portion of energetic ions in central region increases with increases of τ_saw, and the BAAE can be excited when the τ_saw achieves the threshold condition, and the gradient of electron temperature in central region also increases with the increases of τ_saw. Therefore, the BAAE should be excited by the special distribution of energetic ions through toroidal precessional resonances.