Nonlinear effects in strong turbulence in magnetically confined plasmas

Strong electrostatic turbulence in magnetically confined plasmas is characterized by trapping or eddying the test particle trajectories produced by the E x B stochastic drift. Trapping is shown to produce non-standard statistics of trajectories: non-Gaussian distribution, memory effects and quasi-coherence. Two types of effects produced by trapping are analyzed. The first type concerns particle and energy transport and consists in very strong nonlinear modification of the diffusion coefficients. Anomalous diffusion regimes are obtained when the other components of the motion (particle collisions, plasma rotation, the motion along the confining magnetic field) do not destroy trajectory eddying. The second type of effects are evidenced by studying test modes on turbulent plasma. We show that trapping provides the physical mechanism for the inverse cascade observed in drift turbulence. (authors)