Nonlinear generation of zonal flows by drift waves

A new nonlinear mechanism for the excitation of zonal flows by a finite amp litude monochromatic drift wave is presented. It does not require any initi al symmetry breaking. and thus provides an efficient triggering mechanism f or the L-H transition in tokamaks. The nonlinear process, suggested here, i s the self-focusing of a drift-wave pump, governed by the nonlinear interac tion with the second harmonic reactive quasimodes, which determines the ste ady-state profile of the shear flow. The analytical results are confirmed b y means of numerical computations for a tokamak plasma configuration. (C) 2 001 Elsevier Science B.V. All rights reserved.