Multi-branch m/n=1/1 energetic particle modes driven by fast ions in a tokamak with weakly reversed magnetic shear

The linear, non-perturbative MHD-kinetic hybrid code MARS-K [Liu et al., Phys. Plasmas 15, 112503 (2008)] is employed to investigate the stability of the energetic particle modes (EPMs) driven by trapped fast ions in a HL-2A discharge with weakly reversed central magnetic shear. Fast ions with anisotropic pitch angle distribution are found to drive multi-branch n=1 (n is the toroidal mode number) EPMs, which satisfy the Sturmian rule. The computed eigenmode structure of one sub-unstable branch matches well with that observed in the experiment, implying the possibility of existence of sub-unstable EPM on top of the classical fishbone mode. Parametric studies also confirm the robustness of the multi-branch instabilities against variations of the core safety factor profile, the equilibrium pressure, the plasma resistivity as well as the toroidal flow speed.