Validity of quasi-linear transport model

In order to gain reliable predictions on turbulent fluxes in tokamak plasmas, physics based transport models have to be improved. Nonlinear gyrokinetic electromagnetic simulations for all species are still too costly in terms of computing time. On the other hand, interestingly, quasi-linear approximation seems to retain the relevant physics for fairly reproducing both experimental results and nonlinear gyrokinetic simulations. Quasi-linear fluxes are made of two parts, the quasi-linear weight and the squared electrostatic potential. The first is shown to follow well non-linear predictions; the second is based on both non-linear simulations and turbulence measurements. A new quasi-linear transport model, Qua-LiKiz, based on a fast linear gyrokinetic code has been recently developed. The validity of the quasi-linear approach is tested against nonlinear gyrokinetic simulations, using the Eulerian code GYRO and the semi- Lagrangian code GYSELA. A good agreement of the de-phasing between the potential and the transported quantities (particles, ion and electron energy) in coupled ITG-TEM turbulence is observed between quasi-linear and non-linear regimes. On the other hand, the quasi-linear flux weights for various ITG-TEM cases are affected by a slight, but constant, over prediction with respect to the nonlinear values. The chosen saturation level is discussed versus results from nonlinear GYRO/GYSELA simulations. The frequency and wave vector shapes obtained by reflectometry and laser backscattering measurements in Tore Supra are confronted to GYRO/GYSELA simulations. For example, the density fluctuations spectrum scaling as kθ-3.5 observed with laser backscattering experiments seems to be reproduced by collisionless GYRO simulations, while strong levels of collisionality lead the fluctuations to scale rather as kθ-4.7. The impact of both types of slopes is tested for the quasi-linear flux estimations. Finally, to further test the quasi-linear model for turbulence ranging from ITG to TEM dominated, the impact of Ti/Te is tested for parameters ranging from hot-ion mode in TFTR to dominant electron heating in Tore Supra. The ratios between ion heat flux, electron heat flux and particle flux for quasi-linear and non-linear simulations are shown to agree well for coupled ITG-TEM turbulence. (author)