Electron beams seem too good to be true - wouldn't a beam of negatively charged particles travelling through a vacuum repel itself apart? In practice this is rarely an issue, as a beam current and energy are chosen such that any beam divergence is small over the distance of interest. However, when the beam propagates through air instead of a vacuum, collisions between beam electrons and air particles introduce a new source of scattering which increases the rate of particle divergence.
The aim of this paper is to demonstrate a method to reduce beam divergence in air, which involves sending the electron beam within a laser pulse. The laser ionises air particles into a plasma, and within the plasma, beam divergence can be reduced by self-generated magnetic fields.
To numerically support the assumptions in this anaylsis, a series of 2D Particle-In-Cell simulations were run in EPOCH (FORTRAN) to show that beam propagation through plasma can reduce divergence when compared to vacuum propagation. In Figure 2, the beam profiles in both plasma and vacuum backgrounds were compared to show the reduced beam divergence in plasma. The beam-plasma interaction is characterised further in Figures 3 and 4, to help describe this behaviour.
Reproduction of this work would involve running EPOCH simulations in 2D
At present, visualisation scripts to reproduce the figures are not present in EPOCH. However, the code does come with tools to read the SDF files which are output, and data can be processed to reproduce the figures.
Most electron beam physics is considered in the context of a vacuum, but there are applications to long-range electron beam transmission in air. As particle acceleration sources become more compact, we may have the chance to take particle beams out to the real world. The example provided in the paper describes that of x-ray backscatter detectors, where significantly stronger signals could be achieved by scanning objects with electron beams.
This paper forms the basis for a potential new mode of particle-beam research, and it is important to ensure the reproducibility of this work for groups who wish to explore the applications of this new technology.
The Reprohackers should attempt to reproduce Figures 2-4.