Turbulence and coherent structures in non-neutral plasmas

Abstract

Electron plasmas confined in Penning-Malmberg traps evolve as near-ideal two-dimensional (2D) fluids, allowing a quantitative study of shear flow instabilities, vortex formation, turbulence, and self-organization. At first, an overview of the main physical properties of non-neutral plasmas is briefly presented, and the characteristics and mode of operation of their confinement devices are described. The 2D model for the transverse plasma dynamics is explicitly derived, and its main features and limits of validity are critically discussed. The attention is then focused on the free relaxation of 2D turbulence, with a short review of recent experiments and theoretical analyses. Experimental results on the dynamics of the 2D turbulence in pure electron plasmas obtained in the Penning-Malmberg trap ELTRAP are reported. Different initial electron distributions, leading to different relaxed states, are considered. The dynamics is investigated by means of the Proper Orthogonal Decomposition technique. The analysis enables to identify the coherent structures which give the dominant contribution to the plasma turbulent evolution. Finally, the scaling properties of 2D turbulence are analyzed, showing that intermittency increases as the turbulence evolution proceeds, due to the development of strong fluctuations which give rise to non-Gaussian tails in the probability distribution functions of vorticity increments.