Toroidal and Poloidal Momentum Transport Studies in Tokamaks
This paper reports on recent studies of toroidal and poloidal momentum transport in tokamaks. The ratio of the global energy confinement time to the momentum confinement is found to be close to tE/tf = 1 among several tokamaks. On the other hand, local transport analysis in the core plasma shows a larger scatter in the ratio of the local effective momentum diffusivity to the ion heat diffusivity cf,eff/ci,eff among different tokamaks, for example the value of effective Prandtl number being typically around cf,eff/ci,eff 0.2 on JET. Perturbative NBI modulation experiments on JET have indicated, however, that the Prandtl number (calculated only with diffusive terms) cf /ci is around 1, and in addition, a significant inward momentum pinch is needed to explain the amplitude and phase behaviour of the momentum perturbation. The experimental results, i.e. the high Prandtl number and pinch, are in good qualitative and to some extent also in quantitative agreement with linear gyro-kinetic simulations. Concerning the poloidal velocity, the experimental measurements on JET show that the carbon poloidal velocity can be an order of magnitude above the neo-classical estimate within the ITB. This significantly affects the calculated radial electric field and therefore, the E¥B flow shear used for example in transport simulations. Several fluid turbulence codes have been used to identify the mechanism driving the poloidal velocity to such high values. CUTIE and TRB turbulence codes predict the existence of an anomalous poloidal velocity, peaking in the vicinity of the ITB and being dominantly caused by flow due to the Reynold's stress. It is important to note that both codes treat the equilibrium in a simplified way and this affects the geodesic curvature effects and Geodesic Acoustic Modes (GAMs). The neo-classical equilibrium is calculated more accurately in the GEM code and interestingly, the simulations suggest that the spin-up of poloidal velocity is a consequence of the plasma profiles steepening when the ITB grows, with poloidal velocity tight to the 2D neo classical equilibrium and following in particular the growth of the toroidal velocity within the ITB.