[en] In this paper we have studied the contribution of reversed shear in reducing the anomalous transport in a Tokamak using both an analytical approach and simulations. We have used a special model for the drift wave fields. Comparison between particles trajectories for normal and negative shear is carried out. The diffusion coefficient of particles for the two cases; normal and reversed shear is evaluated. This work is organized as follows: in the first section, we review the theory of local magnetic shear using the Grad-Shafranov equation and deduce the analytical expression of the safety factor q. In Section 2 we recall recent experimental data in the formation of barrier transport in improved regimes of confinement. In section 3, we describe the motion of charged particles in the presence of drift waves. We write down the guiding-center orbits equation for a given model drift wave and replace theses equations with a local map which has the same Hamiltonian structure. Individual orbits obtained from the map can differ qualitatively from those obtained from the differential equations, but statistical maps tend to give correct quantitative descriptions. At the shearless point, this map has the form of the standard non-twist map. In section 4, we present a numerical simulation of the transport barrier and it's contribution in reducing anomalous transport in the Tokamak. (authors)

[en] In this paper we have studied the contribution of electrostatic and magnetic turbulence on particles anomalous transport in the tokamak. The diffusion coefficient is evaluated. Comparison between diffusion magnetic field lines and particles is carried out. We also treat the effects of the reversed shear barrier in reducing the particles anomalous transport in the tokamak. (orig.)