Non-perturbative statistical theory of intermittency in ITG drift wave turbulence with zonal flows

The probability distribution functions (PDFs) of momentum flux and zonal flow formation in ion-temperature-gradient (ITG) turbulence are investigated in two different models. The first is a general five-field model (n(i), phi, T-i, T-e, v(i parallel to)) where a reductive perturbation method is used to derive dynamical equations for drift waves and a zonal flow. The second is a reduced two-field model (phi, T-i) that has an exact non-linear solution (bipolar vortex soliton). In both models the exponential tails of the zonal flow PDFs are found with the same scaling (PDF similar to exp{-cZF phi(3)(ZF)}), but with different coefficients c(ZF). The PDFs of momentum flux is, however, found to be qualitatively different with the scaling (PDF similar to exp{-c(M)R(s)}), where s = 2 and s = 3/2 in the five and two-field models, respectively.