Exponential Separation of Neighbouring Trajectories in Fluid and Plasma Turbulence

摘要

The time behaviour of spatial relative diffusion of neighbouring charged particles in turbulent plasmas is shown to have three different time-regimes. An exponential separation of the orbits appears in an intermediate time regime (DUPREE'S clumps). Comparison is performed with analoguous regimes found in Brownian motion, and with fluid turbulence where the exponential regime has recently been observed. From the theoretical point of view, the exponential regime corresponds to a lack of validity of the usual quasi-linear approximation for small initial velocity differences: in this case fluctuations are no longer small corrections, and renormalization appears to be necessary. This gives rise to a SUZUKI scaling regime. The similarity in the results obtained for relative diffusion in plasma and fluid turbulence suggests a further comparison of general processes in these two separate fields. (ERA citation 09:012991)