Excitation of Beta-induced Alfvén-Acoustic Eigenmode during sawteeth-like oscillation in EAST

The characteristics and excitation conditions of Beta-induced Alfven-Acoustic Eigenmode (BAAE) have been investigated during sawteeth-like oscillation in EAST. The sawteeth-like events are triggered periodically by the m = 2/n = 1 double-tearing reconnection crash (DTRC), and the oscillations with period τo 100 ms, where m and n are the poloidal and toroidal mode numbers respectively. The BAAE coexists 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 with the decreasing of qmin (qmin < 1), and the BAAE can also provide valuable constraint conditions for the q-profile measurements. Furthermore, the mode structure of BAAE is achieved experimentally with m = 4/n 4. The period τo of DTRC is an important factor, which can be adopted for evaluating the excitation condition of BAAE, namely the BAAE can be excited when the τo achieves the threshold condition. The τo increases when the total injected power of neutral beam injection (NBI) and ion cyclotron resonance heating (ICRH) increases, where the NBI with beam direction is perpendicular to the plasma current. The gradient of electron temperature at the vicinity of qmin increases with τo, and the portion of energetic ions in the central region increases as well. Therefore, the BAAE in EAST should be excited by the special distribution of trapped energetic ions through toroidal precessional resonances, and the qmin 1 is also an important constraint condition.