Pseudochaos and low-frequency percolation scaling for turbulent diffusion in magnetized plasma

TL;DR

This paper analyzes how low-frequency turbulence in magnetized plasma leads to pseudochaotic transport, revealing a specific scaling law for turbulent diffusion and describing the transport process with fractional diffusion equations.

Contribution

It introduces the concept of pseudochaos in plasma turbulence and derives a new scaling law for turbulent diffusion at low frequencies, extending understanding of transport regimes.

Findings

Diffusion coefficient scales as Q^{2/3} in the pseudochaotic regime

Pseudochaotic transport exhibits slow mixing and vanishing Kolmogorov-Sinai entropy

Transition to Bohm scaling occurs when pseudochaos is relaxed

Abstract

The basic physics properties and simplified model descriptions of the paradigmatic "percolation" transport in low-frequency, electrostatic (anisotropic magnetic) turbulence are theoretically analyzed. The key problem being addressed is the scaling of the turbulent diffusion coefficient with the fluctuation strength in the limit of slow fluctuation frequencies (large Kubo numbers). In this limit, the transport is found to exhibit pseudochaotic, rather than simply chaotic, properties associated with the vanishing Kolmogorov-Sinai entropy and anomalously slow mixing of phase space trajectories. Based on a simple random walk model, we find the low-frequency, percolation scaling of the turbulent diffusion coefficient to be given by $D/\omega\propto Q^{2/3}$ (here $Q\gg 1$ is the Kubo number and $\omega$ is the characteristic fluctuation frequency). When the pseudochaotic property is relaxed the percolation scaling is shown to cross over to Bohm scaling. The features of turbulent transport in the pseudochaotic regime are described statistically in terms of a time fractional diffusion equation with the fractional derivative in the Caputo sense. Additional physics effects associated with finite particle inertia are considered.