On Nonlinear Geodesic Acoustic Modes in Tokamak Plasmas

It is shown that, in tokamak plasmas, finite drift/banana-orbit width (FOW) effects play crucial roles in the nonlinear evolution of Kinetic/ Geodesic Acoustic Modes (KGAM/GAM) [1, 2]. In particular, it is found that, in contrast to the negligible second-harmonic generation, KGAM/GAM can generate appreciable zero-frequency zonal flow (ZFZF) [3] due to the FOW effects. On the other hand, it is shown that, ZFZF has negligible effects on the dynamics of GAM/KGAM. The implications of these findings for nonlinear dynamic evolutions of zonal structures spontaneously generated by drift wave turbulences are discussed.\\[4pt] [1] N. Winsor, J. L. Johnson and J. M. Dawson, Phys. Fluids {\bf 11}, (1968) 2448. \newline [2] F. Zonca and L. Chen, Europhys. Lett. {\bf 83}, (2008) 35001. \newline [3] M. Rosenbluth and F. Hinton, Phys. Rev. Lett. {\bf{80}}, 724 (1998).