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The transition from toroidal counter- to co- rotation in the core plasma hasbeen observed at L to H transition in several tokamaks. Spontaneous reversalhas also been observed in TCV beyond a threshold in the density. We develop amodel based on the following phenomenology: (1) the increase of the gradient ofthe 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 theconvection cells; via the baroclinic term the gradients of temperature anddensity sustain the poloidal rotation against the decay due to the parallelviscosity; (3) the fast increase of poloidal flow induces a high timederivative of the radial electric field; (4) the neoclassical polarizationcreates a series of parallel accelerations (kiks on each bounce) of the trappedions, leading to an increase of the toroidal precession or to its reversal; thesource of energy is the work done by the radial electric field. (5) thediffusion transfers on resistive scale the toroidal momentum from the trappedions to the untrapped ones. The correlated interactions are examined and theestimated time scales are found to be compatible with the observations.

A model for the reversal of the toroidal rotation in tokamak