Observation and Simulation of n=1 Reversed Shear Alfvén Eigenmode on the HL-2 A Tokamak

A branch of high-frequency Alfvénic modes is observed on the HL-2A tokamak.The electromagnetic mode can be driven unstably in the plasma with an off-axis neutral beam heating.Its mode frequency keeps almost unchanged or presents a slow-sweeping behavior,depending on the detail current evolution.The poloidal and toroidal mode numbers are m/n=1/1.The mode has a quite short duration(≤20ms)and usually appears 5-10 ms after the neutral beam being injected into the plasma.Hybrid simulations based on M3D-K have also been carried out.The result suggests that co-passing energetic particles are responsible for the mode excitation.The simulated mode structures are localized nearby location of minimum safety factor(qmin)and agree with the structures obtained through tomography of soft x-ray arrays.Further,the modes are localized in the continuum gap and their frequencies increase with variation of qmin in a wide range.Last but not least,the characteristic of unchanged frequency on experiment is also reproduced by the nonlinear simulation with a fixed safety factor.All those evidences indicate that the n=1 high-frequency mode may belong to a reversed shear Alfvén eigenmode.