Particle diffusion in turbulent fields: transition from quasilinear to nonlinear stage

Charged particles in random electric fields are known to diffuse over the phase velocity resonant layer in velocity space, whereas charged particles placed in a sheared magnetic field diffuse over the mode rational surfaces due to E*B random fluctuations. The author has studied in detail the transition of particle diffusion in turbulent fields from the quasilinear (constant diffusion rate) to the nonlinear (time dependent rate) regime by numerical experiments with test particles. The author has observed that the particle diffusion rate deviates from the quasilinear value and depends on the time at which the amplitude of the random field becomes larger. Such a transition from quasilinear to nonlinear diffusion is observed without particle loss from the resonance region. The time-dependent diffusion coefficient can be explained by the effect of retarded friction caused by turbulent fluctuations.<>