Theory of electron and ion holes as vortices in the phase-space of collision-less plasmas

Abstract

This article studies the vortical nature and structure of phase-space holes -- nonlinear B.G.K. trapping modes found in the phase-space collision-free plasmas. A fluid-like outlook of the particles' phase-space is explored, which makes it convenient to analytically identify electron and ion holes as vortices -- similar to that of ordinary two-dimensional fluids. A fluid velocity is defined for the phase-space of the electrons and ions, continuity and momentum equations describing the flow of the phase-space fluid representing the particle system are then developed. Pressure formation and associated diffusion in phase-space of such systems is introduced and a vorticity field of the phase-space is then defined. Using these equations, electron holes and ion holes are analytically identified as vortices in the phase-space of the plasma. A relation between Schamel's trapping parameter ($\beta$), hole speed ($M$), hole phase-space depth ($-\Gamma$) and hole potential amplitude ($\chi_0$) is derived. The approach introduces a new technique to study the phase-space holes of collision-less plasmas, allowing fluid-vortex-like treatment to these kinetic structures. Phase-space distribution functions for electron hole regions can then be analytically derived from this model, reproducing the schamel-df equations and thus acting as a precursor to the pseudo-potential approach, avoiding the need to assume a solution to the phase-space density.