Abstract
A simplified model to estimate nonlinear turbulent transport only by linear calculations is proposed, where the turbulent heat diffusivity in tokamak ion temperature gradient(ITG) driven turbulence is reproduced for a wide parameter range including near- and far-marginal ITG stability. The optimal nonlinear functional relation(NFR) between the turbulent diffusivity, the turbulence intensity mathcal{T}, and the zonal-flow intensity mathcal{Z} is determined by means of mathematical optimization methods. Then, an extended modeling for mathcal{T} and mathcal{Z} to incorporate the turbulence suppression effects and the temperature gradient dependence is carried out. The simplified transport model is expressed as a modified nonlinear function composed of the linear growth rate and the linear zonal-flow decay time. Good accuracy and wide applicability of the model are demonstrated, where the regression error of sigma _{textrm{model}} = 0.157 indicates improvement by a factor of about 1/4 in comparison to that in the previous works.
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