Generation of poloidal and toroidal rotation and their impact on turbulent heat transport

Self-generated rotation in turbulent plasmas plays an important role in determining the overall level of anomalous transport. In particular, zonal flows (ZFs), self-generated large-scale poloidal ExB flows, are known to efficiently regulate the turbulence level. Evidence of anomalous toroidal momentum has also been reported in the plasma core of several tokamaks, without any external momentum sources. Theoretical work emphasizes the role of velocity nonlinearities, or Reynolds stresses, as the source of these flows. Quantitative analysis relies mainly on numerical simulations of specific plasma turbulence models. In this contribution, we report on the observation and characterization of poloidal and toroidal rotation obtained by means of numerical simulations of a 3D fluid model describing flux-driven ITG turbulence. Specifically, the following points will be addressed: 1/ The dependence of zonal flows and plasma turbulence characteristics as a function of the (collisional) poloidal flow damping. Besides the now well-known transport reduction effect that occurs at lower damping and higher flow, a noteworthy aspect of the ZF dynamics is that, as the damping is reduced, the ZFs become progressively slower, with a longer correlation time, while the corresponding correlation time of the plasma turbulence become shorter, due to the shearing effects. This observation opens up an opportunity of an analytic treatment of the ZF dynamics that exploits scale separation. 2/ The investigation of the mechanism of generation of toroidal rotation from a poloidal flow shear layer [3-4]. Theoretical calculations of the parallel momentum generation, based on the quasilinear approximation, are quantitatively checked against the numerical observations in a model of ITG turbulence where the poloidal shear flow layer is imposed by an injection of poloidal momentum. (authors)