Nonlinear equilibria, stability and generation of zonal structures in toroidal plasmas

The long-lived saturated zonal flow (ZF) structures, spontaneously generated by drift-wave (DW) turbulence, are viewed as generators of nonlinear equilibria. Here, we derive a nonlinear evolution equation for the zonal response, valid for arbitrary wavelengths, whose time-asymptotic behaviour corresponds to the nonlinear equilibrium. Its corresponding stability, meanwhile, determines the nature of the ZF instability and the nonlinear up-shift of turbulent transport thresholds. On a shorter-time scale, the temporal evolution of the zonal response describes the DW–ZF generation and the regulation of the DW intensity by the ZFs. As an application, we present a kinetic stability analysis of zonal structures in the ion temperature gradient driven DW turbulence case as well as the corresponding collisionless zonal dissipation rates.