Thermal transport from a phenomenological description of ion-temperature-gradient-driven turbulence

In this work, a phenomenological model of the thermal transport caused by ion-temperature-gradient-driven (ITG) turbulence is introduced. By combining the semi-quantitative knowledge of ITG turbulence provided by the general theory and by the numerical simulations with working hypothesis suggested by the experimental data, new formulas for the turbulence correlation length and for the ion and electron heat conductivities are proposed. A remarkable property of this model (unlike previous microturbulence-based models) is that the correlation length and the conductivities increase with minor radius, as observed in the experiments. A wide class of models possessing this property is indeed consistent with dimensional analysis of the general ITG equations. Transport simulations with the JETTO code based on the new model are then carried out after calibration of two multiplicative constants on a particular JET shot. The results are compared with L-mode experimental profiles of a number of machines drawn from the ITER data base and with the prediction of more conventional Bohm and gyro-Bohm models. The overall performance of the model in the outer half radius is good. Qualitatively, it predicts overall gyro-Bohm scaling for the energy confinement time, but, as a consequence of the radial behavior of the transport coefficients, the profiles turn out similar to those predicted by Bohm models