Nonlinear excitation of energetic particle-induced geodesic acoustic mode via resonance overlap in CFQS

Energetic particle (EP) driven instabilities are investigated in the Chinese First Quasi-axisymmetric Stellarator using the hybrid kinetic-magnetohydrodynamic code MEGA. EPs are described as the anisotropic slowing-down distribution with pitch angle variable peak value Λ0. Simulations reveal that for or 0.65, two energetic-particle-modes (EPMs) with close growth rates coexist during the linear phase, and poloidal/toroidal mode numbers are and , respectively. For , the EPM is the dominant linear instability, and in the nonlinear phase, an EP-induced geodesic acoustic mode (EGAM) is excited and ultimately becomes the dominant instability. The nonlinear excitation mechanism of the EGAM is identified as resonance overlap, based on the analysis of the EP perturbed distribution function δf. During the linear growth of the EPM, hole-clump structure of δf forms within its resonance region in energy E and pitch angle variable Λ space. As the EPM saturates and its frequency chirps, the dominant clump structure in space migrates towards and gradually overlaps with the EGAM resonance region. This resonance overlap triggers the nonlinear excitation of the EGAM. This is the first demonstration of resonance overlap inducing mode nonlinear excitation in space and in a stellarator.