Longitudinal crossover and the dynamics of uniform electron ellipsoids focused by linear chirp

TL;DR

This paper analyzes the dynamics of electron bunches in ultrafast electron microscopy, comparing analytic models and simulations to understand longitudinal focusing and space charge effects.

Contribution

It introduces extended analytic approaches for characterizing longitudinal focus regimes and compares them with N-particle simulations, revealing mechanisms of emittance growth.

Findings

Identification of bounce-back and crossover regimes in longitudinal focus

Comparison of analytic models with simulation results

Insights into emittance growth mechanisms

Abstract

High resolution single-shot non-relativistic ultrafast electron microscopy(UEM) relies on adaptive optics to compress high intensity bunches using Radio Frequency (RF) cavities. We present a comprehensive discussion of the analytic approaches available to characterize bunch dynamics as an electron bunch goes through a longitudinal focal point after an RF cavity where space charge effects can be large. Methods drawn from the Coulomb explosion literature, the accelerator physics literature and the analytic Gaussian model developed for UEM are compared, utilized and extended in some cases. In particular the longitudinal focus may occur in two different regimes, a bounce-back regime and a crossover regime; and we characterize the critical point separating these regimes in the zero-emittance model. Results from N-particle simulations using efficient multipole methods are compared to the theoretical models revealing features requiring extensions of the analytic approaches; and in particular mechanisms for emittance growth and transfer are discussed.