Effects of flow shear on the correlation length of drift wave turbulence

Experiments on JET have shown that stiffness of ion energy transport above a threshold gradient is strongly reduced in the plasma core due to flow shear. Initially transport models had difficulties to reproduce this feature. Later TGLF2 has reproduced the trend, particularly in the region of low normalized heat flux, i.e. near threshold3. Also Gyro4 simulations have been made but these are still uncertain. The experimental observations also indicated in more detail that it is the combination of high flow shear and small magnetic shear that leads to stiffness reduction. This naturally limits the region of stiffness mitigation to the interior of tokamaks3. Since flow shear stabilizes drift wave transport by damping out primarily long wavelength perturbations, it is obvious that it influences the correlation length. Thus it was natural to generalize our previous work on making the correlation length for drift waves parameter dependent to include also the effect of flow shear. This has now been implemented and is found to reproduce the experimental feature that stiffness is reduced for a combination of large flow shear and small magnetic shear. The reason is that for large magnetic shear the radial correlation length is determined primarily by magnetic shear, whilst for low magnetic shear it is determined by flowshear. The first results of this modified correlation length model show good quantitative agreement with experiment.