Transformation of a Geodesic Acoustic Mode in the Presence of a Low-Frequency Zonal Flow in a Tokamak Plasma

A mechanism of nonlinear interaction between a geodesic acoustic mode and a low-frequency zonal flow has been proposed to explain the appearance of satellites of the geodesic acoustic mode observed in tokamak experiments. Interaction between eigenmodes with significantly different frequencies leads to the modulation of the amplitude of the geodesic acoustic mode at the frequency of zonal flow oscillations, which is manifested as two side harmonics near the frequency of the geodesic acoustic mode in the spectrum of oscillations. The frequencies of the indicated harmonics correspond to the sum and difference of the frequencies of the geodesic acoustic mode and zonal flow. The effect is triggered by a stationary toroidal plasma rotation.