Excitation of the beta-induced Alfvén-acoustic eigenmode during sawtooth-like oscillation in EAST

The characteristics and excitation conditions of the beta-induced Alfvén-acoustic eigenmode (BAAE) have been investigated during sawtooth-like oscillation in the Experimental Advanced Superconducting Tokamak (EAST). The sawtooth-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 Alfvén eigenmodes (BAEs) and reversed-shear Alfvén eigenmodes (RSAEs), and the BAAE has similar characteristics to the pairing of BAEs-RSAEs. The radial structures of the BAAE and BAEs are deformed into analogous triangle shapes along the poloidal direction, and the BAAE is located more outward with broadened radial coverage. The frequencies of the BAAE and RSAEs sweep upward synchronously with decreasing qmin ( qmin < 1), and the BAAE can also provide valuable constraint conditions for the q-profile measurements. Furthermore, the mode structure of the 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 conditions of BAAE; namely, the BAAE can be excited when τo achieves the threshold condition. τo increases when the total injected power of neutral beam injection (NBI) and ion cyclotron resonance heating (ICRH) increases, where the NBI 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 qmin≤1 is also an important constraint condition for the excitation.