Phase Mixing in Turbulent Magnetized Plasmas

Abstract

Landau damping (phase mixing) is perhaps the most salient feature of weakly collisional plasmas. Phase mixing plays a crucial role in kinetic plasma turbulence-- it transfers energy to small velocity space scales, and provides a route to dissipation to the turbulent cascade. Phase mixing has been well understood in the linear limit for nearly seventy years, however, we do not yet fully understand the behavior of phase mixing in presence of a fluid-like turbulent cascade--a common scenario in weakly collisional systems.

In this thesis, we consider simple models for kinetic passive scalar turbulence that simultaneously incorporate phase mixing and turbulent cascade, in order to study the effects of turbulence on phase mixing. We show that the nonlinear cascade scatters energy in the phase space so as to generate a turbulent version of the plasma echo. We find that this stochastic plasma echo suppresses phase mixing by reducing the net flux to small velocity space scales.

Further, we study the problem of compressive fluctuations in the solar wind at scales larger than the ion Larmor radius (the so-called inertial range). The compressive perturbations at these scales are passively mixed by the Alfvenic turbulence. Hence, the general results regarding kinetic passive scalar turbulence are directly applicable to this problem. We find that the suppression of phase mixing by the stochastic plasma echo is key to the persistence of the turbulent cascade of compressive fluctuations at scales where these fluctuations are expected to be strongly damped.

A new code, Gandalf was developed for the GPU architecture using the CUDA platform in order to study these systems, in particular to study solar wind turbulence in the inertial range.