Madalina Vlad

Test modes on turbulent magnetized plasmas are studied taking into account the ion trapping that characterizes the E x B drift in the background turbulence. We show that trappyng provides the physical mechanism for the formation of large scale potential structures (inverse cascade) observed in drift turbulence. Trapping combined with the motion of the potential with the diamagnetic velocity determines ion flows in opposite directions, which reduce the growth rate and eventually damps the drift modes. It also determine transitory zonal flow modes in connection with compressibility effect due to the polarization drift in the background turbulence.

The transition from toroidal counter- to co- rotation in the core plasma has been observed at L to H transition in several tokamaks. Spontaneous reversal has also been observed in TCV beyond a threshold in the density. We develop a model based on the following phenomenology: (1) the increase of the gradient of the pressure triggers formation on a fast time scale of cells of convection (similar to Rayleigh-Benard (RB), but with a single sign of vorticity); (2) poloidal rotation is induced by the envelope of the peripheric velocity of the convection cells; via the baroclinic term the gradients of temperature and density sustain the poloidal rotation against the decay due to the parallel viscosity; (3) the fast increase of poloidal flow induces a high time derivative of the radial electric field; (4) the neoclassical polarization creates a series of parallel accelerations (kiks on each bounce) of the trapped ions, leading to an increase of the toroidal precession or to its reversal; the source of energy is the work done by the radial electric field. (5) the diffusion transfers on resistive scale the toroidal momentum from the trapped ions to the untrapped ones. The correlated interactions are examined and the estimated time scales are found to be compatible with the observations.