Beta induced Alfven eigenmode driven by energetic ions on the HL-2A tokamak

Research on beta induced Alfven eigenmodes (BAEs) driven by energetic ions has been carried out on the HL-2A tokamak. The BAEs can be observed in the cases of q(min) being beyond or below unity. The mode frequencies are around 60-95 kHz. The radial mode structures have been detected by the multi-channel microwave reflectometer and soft x-ray arrays. The results suggest that the BAEs are highly localized at the normalized radius of rho = 0.07-0.26. Theoretical analyses based on the general fishbone-like dispersion relation and Alfven mode code support the experimental measurements in both mode frequencies and locations. The BAEs are found to be excited more easily in low electron density discharges. The coexistence of BAEs and reversed shear Alfven eigenmodes enables the estimation of q(min) directly from experiments.