New Nonlinear Mechanism for the Generation of Zonal Flows by Drift Waves

Abstract

A new mechanism for the excitation of zonal flows by a finite amplitude monochromatic drift wave is presented. It does not require any initial symmetry breaking, and thus provides an efficient triggering for the L–H transition in tokamaks. The nonlinear process consists in the self focusing of a drift-wave pump, governed by the nonlinear interaction with the second harmonic reactive quasimodes, which determines the steady-state profile of the shear flow. In a tokamak geometry, this mechanism is relevant in the regime when the variation of the effective parallel wavenumber of drift-waves, due to toroidal effects, turbulent decorrelation etc., prevents the exact beating of the drift waves along the entire torus, and yields a component of adiabatic electrons associated with the zero-frequency nonlinear mode.The numerical results indicate that, in a tokomak geometry, the magnetic shear does not affect the basic configuration of zonal flows, which remain restricted to the critical region in which the second harmonic is evanescent. The configuration of the zonal flows appears to be robust, while the distribution of the short-scale (both in x and y) quasimodes is very sensitive to the small changes of the phase of the pump wave, and cannot be effectively controlled by the asymptotic boundary condition. As they are also rather slowly converging with large ky, the behavior of the short-scale quasimodes indicates also the possibilities of the intermittency and chaos.