Phase Mixing in Turbulent Magnetized Plasmas

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

Fluidization of collisionless plasma turbulence

In a collisionless, magnetized plasma, particles may stream freely along magnetic-field lines, leading to phase "mixing" of their distribution function and consequently to smoothing out of any "compressive" fluctuations (of density, pressure, etc.,). This rapid mixing underlies Landau damping of these fluctuations in a quiescent plasma-one of the most fundamental physical phenomena that make plasma different from a conventional fluid. Nevertheless, broad power-law spectra of compressive fluctuations are observed in turbulent astrophysical plasmas (most vividly, in the solar wind) under conditions conducive to strong Landau damping. Elsewhere in nature, such spectra are normally associated with fluid turbulence, where energy cannot be dissipated in the inertial scale range and is therefore cascaded from large scales to small. By direct numerical simulations and theoretical arguments, it is shown here that turbulence of compressive fluctuations in collisionless plasmas strongly resembles one in a collisional fluid and does have broad power-law spectra. This "fluidization" of collisionless plasmas occurs because phase mixing is strongly suppressed on average by "stochastic echoes", arising due to nonlinear advection of the particle distribution by turbulent motions. Besides resolving the long-standing puzzle of observed compressive fluctuations in the solar wind, our results suggest a conceptual shift for understanding kinetic plasma turbulence generally: rather than being a system where Landau damping plays the role of dissipation, a collisionless plasma is effectively dissipationless except at very small scales. The universality of "fluid" turbulence physics is thus reaffirmed even for a kinetic, collisionless system