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.
Highlights
Non-neutral plasma systems arise in a variety of physical contexts ranging from astrophysics [1,2,3]; accelerator technologies [4,5,6,7]; ion and neutron production [8,9,10,11,12,13]; sources for electron and ion microscopy [14,15]; to high power vacuum electronics [16,17,18]
The researchers in the ultrafast electron microscopy (UEM) and the ultrafast electron diffraction (UED) communities have conducted substantial theoretical treatment of initially extremely short bunches of thousands to hundreds of millions of electrons that operate in a regime dominated by a virtual cathode (VC) limit [36,40,43,44,45] which is akin to the Child-Langmuir current limit for beams
We have shown that a shock occurs in the transverse, but not longitudinal, direction during expansion of pancake-like charged particle distributions typical of those used in ultrafast electron microscope (UEM) systems
Summary
Non-neutral plasma systems arise in a variety of physical contexts ranging from astrophysics [1,2,3]; accelerator technologies [4,5,6,7]; ion and neutron production [8,9,10,11,12,13]; sources for electron and ion microscopy [14,15]; to high power vacuum electronics [16,17,18]. Free expansion of clouds of charged single-specie particles starting from rest have been well studied both analytically and computationally [15,21,23,26,27,31,32,33,34,35,36,37,38,39], and a number of studies have found evidence of the formation of a region of highdensity, often termed a “shock,” on the periphery of the clouds under certain conditions [14,23,24,25,26,31,34] One application of these theories that is of particular current interest is to high-density electron clouds used in next-generation ultrafast electron microscopy (UEM) development [40,41,42]. Recent work has indicated that a substantial high density region may form in the transverse direction [55], and Nparticle simulation results, as demonstrated, demonstrate a rapidly developed substantial ringlike shock circumscribing the median of the bunch when a bunch with initial Gaussian transverse-profile starts from sufficient density. Particle-incell (PIC) methods, utilizing Warp [58], and N-particle simulations are used to validate the analytical predictions for peak emergence
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