Abstract
Zonal flows (ZFs) are excited transiently by geodesic acoustic modes (GAMs) in toroidal plasmas. When GAMs have radially inward and outward propagating components, these components interact to excite the zero frequency modes (ZFs) transiently, in addition to exciting higher harmonics of GAM. A fluid model is used to study this transient excitation. The Reynolds stress is evaluated from the action conservation equation, and a set of equations to analyze nonlinear couplings among GAMs and ZFs is derived. GAMs are driven by the drift wave (DW) turbulence, and ZFs are transiently driven by nonlinear self-interaction of GAMs. The dependences of the peak amplitudes of ZFs on coupling coefficients are explained theoretically. A new energy path from the DW turbulence to ZFs is found under a situation in which there is only energy transfer from the DW turbulence to GAMs.
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