Exact Nonlinear Plasma Oscillations

TLDR

Trapped particles in collisionless plasmas lead to a disparity in the distribution function that challenges conventional linearized theory and raises questions about Landau damping and wave existence. The authors present an exact elementary solution for stationary nonlinear electrostatic waves in collisionless plasmas, show that adding trapped particles yields arbitrary traveling wave solutions, reveal that small‑amplitude limits lack a linear term, and demonstrate that while singular distribution functions can recover linearized theory, these waves do not exhibit Landau damping and their physical relevance depends on collisions and stability.

Abstract

The problem of a one-dimensional stationary nonlinear electrostatic wave in a plasma free from interparticle collisions is solved exactly by elementary means. It is demonstrated that, by adding appropriate numbers of particles trapped in the potential-energy troughs, essentially arbitrary traveling wave solutions can be constructed.When one passes to the limit of small-amplitude waves it turns out that the distribution function does not possess an expansion whose first term is linear in the amplitude, as is conventionally assumed. This disparity is associated with the trapped particles. It is possible, however, to salvage the usual linearized theory by admitting singular distribution functions. These, of course, do not exhibit Landau damping, which is associated with the restriction to well-behaved distribution functions.The possible existence of such waves in an actual plasma will depend on factors ignored in this paper, as in most previous works, namely interparticle collisions, and the stability of the solutions against various types of perturbations.