Non-equilibrium plasma physics near criticality: phase-space cascades, wave-flow interactions, edge-of-chaos structures and turbulent transport

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The question of how large a temperature difference can be maintained between the core and the edge of the plasma in a fusion machine opens a window onto some of the most fundamental theoretical questions about the structure of nonlinear, non-equilibrium states in magnetised plasma. These questions revolve around the ways in which free energy extracted from macroscopic inhomogeneities of the system (in the case of a fusion plasma, from the background temperature gradient) in the form of electromagnetic fluctuations is distributed in phase space (particles' positions and momenta) so as to give rise to a chaotic, multiscale (=turbulent) state that accommodates a steady flow of energy from macro to micro scales and its return to its inevitable resting place in the form of the thermal motion of the particles. This distribution of energy is then what determines how the turbulent plasma transports heat and momentum. This project's aim is to follow up on a number of recent developments in our understanding of these phenomena and investigate: the implications of the nonlinear suppression of energy flows into phase space [1,2] for transport models; the delicate balance between a chaotic field of plasma excitations ("drift waves") and self-generated regular "zonal" flows [3] that maintains the nonlinear state of a plasma near the critical threshold separating the turbulent and quiescent phases; the emergence of persistent coherent structures at this so-called "edge of chaos" [4] --- and how all this fits together and conspires to set limits on the background gradients. The project will have analytical, numerical and possibly experimental [5] components, depending on rate of progress and student's inclinations.1. A. A. Schekochihin et al., J. Plasma Phys. 82, 905820212 (2016)2. T. Adkins & A. A. Schekochihin, J. Plasma Phys. 84, 905840107 (2018)3. G. J. Colyer et al., Plasma Phys. Control. Fusion 59, 055002 (2017)4. F. van Wyk et al., J. Plasma Phys. 82, 905820609 (2016)5. M. F. J. Fox et al., Plasma Phys. Control. Fusion 59, 034002 (2017)