New transport experiments on JET indicate that ion stiffness mitigation in the core of a rotating plasma, as described by Mantica et al. [Phys. Rev. Lett. 102, 175002 (2009)] results from the combined effect of high rotational shear and low magnetic shear. The observations have important implications for the understanding of improved ion core confinement in advanced tokamak scenarios. Simulations using quasilinear fluid and gyrofluid models show features of stiffness mitigation, while nonlinear gyrokinetic simulations do not. The JET experiments indicate that advanced tokamak scenarios in future devices will require sufficient rotational shear and the capability of q profile manipulation.

The scan of Ion Cyclotron Resonant Heating power has been used to systematically study the pump out effect of central electron heating on impurities such as Ni and Mo in H mode low collisionality discharges in JET. The transport parameters of Ni and Mo have been measured by introducing a transient perturbation on their densities via the Laser Blow Off technique. Without ICRH, Ni and Mo density profiles are typically peaked. The application of ICRH, induces on Ni and Mo in the plasma center (at normalized poloidal flux r = 0.2) an outward drift approximately proportional to the amount of injected power. Above a threshold, of about 3MW of ICRH power in the specific case, the radial flow of Ni and Mo changes from inward to outward and the impurity profiles, extrapolated to stationary conditions, become hollow. At mid radius the impurity profiles become flat or only slightly hollow. In the plasma centre the variation of the pinch parameter v/D of Ni is particularly well correlated with the change of the ion temperature gradient, in qualitative agreement with the neoclassical theory. However, the experimental radial velocity is larger than the neoclassical one by up to one order of magnitude. Gyrokinetic simulations of the radial impurity fluxes induced by electrostatic turbulence do not foresee a flow reversal in the analyzed discharges.

We present an ultrafast neural network (NN) model, QLKNN, which predicts core tokamak transport heat and particle fluxes. QLKNN is a surrogate model based on a database of 300 million flux calculations of the quasilinear gyrokinetic transport model QuaLiKiz. The database covers a wide range of realistic tokamak core parameters. Physical features such as the existence of a critical gradient for the onset of turbulent transport were integrated into the neural network training methodology. We have coupled QLKNN to the tokamak modelling framework JINTRAC and rapid control-oriented tokamak transport solver RAPTOR. The coupled frameworks are demonstrated and validated through application to three JET shots covering a representative spread of H-mode operating space, predicting turbulent transport of energy and particles in the plasma core. JINTRAC-QLKNN and RAPTOR-QLKNN are able to accurately reproduce JINTRAC-QuaLiKiz T i,e and n e profiles, but 3 to 5 orders of magnitude faster. Simulations which take hours are reduced down to only a few tens of seconds. The discrepancy in the final source-driven predicted profiles between QLKNN and QuaLiKiz is on the order 1%-15%. Also the dynamic behaviour was well captured by QLKNN, with differences of only 4%-10% compared to JINTRAC-QuaLiKiz observed at mid-radius, for a study of density buildup following the L-H transition. Deployment of neural network surrogate models in multi-physics integrated tokamak modelling is a promising route towards enabling accurate and fast tokamak scenario optimization, Uncertainty Quantification, and control applications.

Detailed experimental studies of ion heat transport have been carried out in JET exploiting the upgrade of Active Charge Exchange Spectroscopy and the availability of multi-frequency ICRH with 3 He minority. The determination of ion threshold and stiffness offers unique opportunities for validation of the well-established theory of Ion Temperature Gradient driven modes. Ion stiffness is observed to decrease strongly in presence of toroidal rotation when the magnetic shear is sufficiently low. This effect is dominant with respect to the well-known w ExB threshold up-shift and plays a major role in enhancing core confinement in Hybrid regimes and Ion Internal Transport Barriers. The effects of T e /T i and s/q on ion threshold are found rather weak in the domain explored. Quasi-linear fluid/gyro-fluid and linear/non-linear gyro-kinetic simu lations have been carried out. Whilst threshold predictions show good match with experimental observations, some significant discrepancies are found on the stiffness behaviour.