Conductive Heat Transport and Microinstabilities in High-Beta, Magnetized, Weakly Collisional Plasma

Abstract

The regulation of conductive heat transport in high-beta, weakly collisional, magnetized plasma by microscale electromagnetic instabilities is investigated. The transport mediated by these instabilities can be a small fraction of the value expected by particle collisions. A temperature gradient oriented along a mean magnetic field can induce a heat flux that drives a number of kinetic microscale instabilities, including the electron-heat-flux-driven whistler instability (HWI), ion-heat-flux-driven slow mode instability (HSI), and ion-heat-flux-driven gyrothermal instability (GTI). I provide analytic arguments showing that many of these instabilities have marginally stable values of heat flux that scale with beta. I also ran a number of electromagnetic particle-in-cell (PIC) simulations of the HWI for two distinct initial conditions across a range of electron beta and temperature-gradient length scales. I found that, even out to the largest heretofore performed simulations, the steady-state heat flux scales as the inverse of the electron beta. I also present preliminary results using hybrid-PIC simulations of ion-heat-flux-driven micro-scale instabilities, which show the presence of the GTI for the first time. In order to investigate how the HWI saturates as a function of electron beta and scale separation, I used a number of methods to infer from the simulations an effective collision operator describing interactions between electrons and the HWI fluctuations. All of the methods presented in this thesis are either novel or are significant refinements of existing methods, and are results in their own right. I use the calculated effective scattering frequency from these methods to motivate a model for effective collisions by the HWI. The model, which is that of a resonance-broadened quasi-linear operator, shows that the HWI can regulate heat flux to the observed inverse-electron-beta scaling in the limit of astrophysical scale separation. Resonance broadening is a crucial ingredient for this extrapolation. I argue that without this feature, electrons with sub-thermal parallel velocities are not scattered by waves, resulting in a large, unphysical heat flux that would deviate from the scaling.