Trapped electron damping of geodesic acoustic mode

Global gyrokinetic particle simulation finds that the collisionless damping rate of the geodesic acoustic mode (GAM) in tokamak is greatly enhanced by trapped electrons in the high-$q$ region of tokamak (⁠$q$ is the safety factor). The electron damping has been identified to arise from the resonance of the GAM oscillation with the trapped electron bounce motion. The contribution of passing electrons to the GAM collisionless damping is much smaller than the trapped electrons. The residual level of the zonal flow is not sensitive to the trapped electron resonance.