Simulation studies on temperature profile stiffness in ITG turbulent transport of helical plasmas for flux-matching technique

Abstract

In the framework of the flux-matching method, which is a useful way for the validation of the gyrokinetic turbulence simulations, it is strongly demanded to evaluate the plasma profile sensitivity of the transport coefficients obtained in the employed simulation model within the profile gradient ranges estimated from the experimental observations. The sensitivity causes the plasma profile stiffness for wide ranges of the transport fluxes. In the nonlinear gyrokinetic simulations for the ion temperature gradient (ITG) turbulence in the Large Helical Device (LHD) [Takeiri et al., Nucl. Fusion 57, 102023 (2017)], it is found that the temperature gradients around the experimental nominal observations are slightly larger than the threshold of the instability, and the ion heat diffusivities are quite sensitive to the temperature gradient. The growth rates of the instability, the generations of the zonal flows, and the sensitivities of the transport coefficients to the temperature profiles depend on the radial locations, the employed simulation models, and the field configurations. Specifically, in the optimized LHD field configuration, the sensitivities are relaxed in the outer radial region due to the enhancement of the zonal flows and the reduction of the ITG instability. In order to estimate the range of the temperature gradients possible given the experimentally obtained data of the temperature with errorbars, the statistical technique, Akaike's Information Criterion [H. Akaike, in Proceedings of the 2nd International Symposium on Information Theory, edited by B. N. Petrov and F. Caski (Akadimiai Kiado, Budapest, 1973), pp. 267–281] is applied. Against the range of the temperature gradients, the flux-matching method to predict the temperature gradient in helical plasmas is demonstrated.