Toroidal Alfvén eigenmodes (TAEs) associated with runaway electrons are observed in low density EAST Ohmic discharges (Zhu et al 2022 Phys. Plasmas29 062504), which motivate the present work to explore the possible destabilization mechanism using simplified hybrid MHD-kinetic simulations. We show that the barely circulating energetic electrons can satisfy the resonance condition with the TAE, mainly due to vanishing transit frequency near the phase space circulating/trapped separatrix. In addition, the nonlinear saturation study suggests that the phase space resonance structure plays an important role in the eventual saturation amplitude, where a low toroidal mode number is favored in this scenario.
Observation of toroidal Alfvén eigenmode excited by energetic electrons induced by static magnetic perturbations in the EAST tokamak