Beta induced Alfven Eigenmode Driven by Fast Ions in HL-2A plasma*

Researches on beta induced Alfven eigenmodes (BAEs) driven by fast ions have been carried out in HL-2A plasma. The mode frequencies are around 35-95kHz. The poloidal/toroidal mode numbers are m/n = 3/2 or m/n = 2/2 when the q_min is higher or lower than unit, respectively. They localize at the core region and their radial mode structures have been detected by the multi-channel microwave reflectometer. The BAEs are confirmed by the theoretical calculations given by the general fishbone-like dispersion relation (GFLDR) and Alfven mode code (AMC). Both the mode frequency and location are found to agree with each other between experimental measurements and theoretical calculations. There is a density threshold for the excitation of BAEs. The modes are excited more easily in low electron density discharges and suffer stronger ion Landau damping in high density cases. The BAEs can coexist with reversed shear Alfven eigenmodes (RSAEs), which performs the MHD spectroscopy well. Finally, the amplitude and growth rate of the frequency chirping BAEs will provide inputs for simulations of potential ion and alpha particle losses due to energetic particle driven modes.