transition from a state with stationary zonal flows to a state with oscillatory zonal flows. In all parameter scalings the coherent mode coupling model agrees well with a model derived using the WKE model for the same background physics.

The paper is organized as follows. In section II the toroidal and slab ITG driven modes are presented and in section III the equations for the zonal flow are discussed. Section IV, contains results and discussion and finally there is a summary in section V.

Toroidal and slab ion-temperature-gradient driven modes
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The model for the ITG driven modes consists of the ion continuity, ion temperature and the parallel ion momentum equations [@a29]- [@a44]. The system of equations is closed by using quasi neutrality with Boltzmann distributed electrons. The effects of magnetic shear and finite beta are neglected in the present study. It has been found earlier that the effect of parallel ion motion on the ITG mode growth rate is rather weak [@a28]. It has recently been found that magnetic shear may modify the zonal flow generation significantly for the Trapped Electron Modes (TEM), however, the effects of magnetic shear on the ITG mode are modest and are also out 