In order to understand the mechanisms of anomalous transport due to micro-instabilities and transport barrier formation related to generation of sheared plasma flows, transport regimes of ion-temperature gradient (ITG)-driven turbulence are studied using a low degree-of-freedom model composed of 18 ODEs [1]. When the system is close to the threshold of ITG instability, an L-H-like transition and periodic oscillations are observed for the kinetic energy and convective flux fluctuations. As the ion temperature gradient is increased further, the system bifurcates to the turbulent regimes. In the strongly turbulent regime, ELM-like intermittent bursts (so-called avalanches) are observed, unlike the previously proposed low degree-of-freedom model based on 11 ODEs [2]. This intermittency is caused by the competition of the following 3 factors; (1) generation of sheared flows due to nonlinear couplings between higher harmonics, and suppression of ITG turbulence by the sheared flows, (2) gradual reduction of the sheared flows due to viscosity, and (3) rapid re-growth of ITG modes due to the reduction of the stabilizing effect by the sheared flows. In this presentation, we shall also compare simulation results of our 18 ODE model with those of full PDE simulations for the Horton-Choi-Tang model [3]. For the linear phase, behaviour of the system in full PDE simulations agrees well with that in the 18 ODE model. Details of nonlinear behaviour of the toroidal ITG mode will also be given.

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