Seminarieserie inom subatomär, högenergi- och plasmafysik: Michael Nastac

Abstract: Collisional systems relax to Maxwell-Boltzmann equilibria. Collisionless plasmas and self-gravitating systems, despite the lack of frequent interparticle collisions, nevertheless exhibit quasi-equilibrium nonthermal states with a degree of universality. I will discuss a theoretical framework that seeks to explain equilibria arising from “violent relaxation” in both plasmas and gravitational systems. The central idea is that (closed) collisionless systems tend simultaneously toward states of minimum energy and maximum entropy. This process is driven by a turbulent cascade of a particular kind: in phase space, but reminiscent of the Batchelor scalar turbulence in fluid dynamics. This framework may be able to explain how nonthermal distributions arise in a variety of contexts, from flat-top and power-law distributions in the solar wind to the universal equilibria of galaxies and dark-matter halos.