Experimental evidence on inward momentum pinch on JET and comparison with theory and modelling

Dedicated experiments have been carried out in the JET tokamak in order to determine the diffusive and convective momentum transport separately. The power and torque, injected by the neutral beam heating system, are modulated to create a periodic perturbation in the toroidal rotation velocity. Transport analysis shows that the magnitude and profile shape of the momentum diffusivity are very similar to those of the ion heat diffusivity, resulting in the Prandtl number being around one throughout the whole plasma radius. Furthermore, a significantly large inward momentum convection term, up to 20 m/s, has been found. Its profile shape is roughly proportional to the momentum diffusivity. Both these results are consistent with recent developments in momentum transport theory and with linear gyro-kinetic simulations. These findings have significant implications on predictions for the magnitude and profile shape of the toroidal rotation velocity in future tokamaks. The existence of an inward momentum pinch will result in a centrally peaked toroidal velocity profile even in the absence of any central momentum source. (author)