Dynamical bunching and density peaks in expanding Coulomb clouds

TL;DR

This paper investigates the expansion behavior of non-neutral Coulomb clouds, revealing how high internal accelerations cause electron bunching and shock formation, with models validated by simulations.

Contribution

It introduces analytic fluid models that describe density bunching and shock formation in expanding Coulomb clouds, validated through PIC and N-particle simulations.

Findings

Bunching and shock formation occur in 2D and 3D systems with Gaussian initial densities.

One-dimensional systems do not exhibit these effects.

Initial density profiles and velocity chirp can tune the dynamics.

Abstract

Expansion dynamics of single-species, non-neutral clouds, such as electron bunches used in ultrafast electron microscopy, show novel behavior due to high acceleration of particles in the cloud interior. This often leads to electron bunching and dynamical formation of a density shock in the outer regions of the bunch. We develop analytic fluid models to capture these effects, and the analytic predictions are validated by PIC and N-particle simulations. In the space-charge dominated regime, two and three dimensional systems with Gaussian initial densities show bunching and a strong shock response, while one dimensional systems do not; moreover these effects can be tuned using the initial particle density profile and velocity chirp.