By employing both nonlinear gyrokinetic simulation and analytical theory, we have investigated the effects of zonal (electromagnetic) fields on the energetic particle's (EPs) drive of reversed-shear Alfvén eigenmodes (AEs) in tokamak plasmas. Contrary to the conventional expectation, simulations with zonal fields that are turned on and off in the EP dynamics while keeping the full nonlinear dynamics of the thermal plasma indicate that zonal fields further enhance the instability drive and thus lead to a higher saturation level. These puzzling simulation results can be understood analytically in terms of the general fishbone-like dispersion relation with the correspondingly different EP phase-space structures induced by the zonal fields. Analytical expressions for the zonal fields that are beat driven by the reversed-shear AEs are also derived, and shown to be in good agreement with the simulation results.
This paper investigates the impact of zonal electromagnetic fields on the instability of reversed-shear Alfvén eigenmodes driven by energetic particles in tokamak plasmas. Contrary to expectations, simulations show that zonal fields enhance the instability drive, leading to higher saturation levels. The authors provide an analytical explanation for this unexpected result, deriving expressions for the zonal fields and their interaction with the reversed-shear Alfvén eigenmodes.