Ring-Shaped Distributions of Monoenergetic Electron Beams Generated via Density Discontinuities in a Two-Stage Gas Cell

Using two-stage gas cells for laser wakefield acceleration experiments, we measure clear ring-shaped angular distributions of monoenergetic electron beams.~ These ``halo''-like structures are observed both on an on-axis and a magnet spectrometer imaging system.~ Initial assessment of the beam-halos suggests that they are most consistently generated in a gas cell where opposing flows create a type of density discontinuity between the stages.~ Generating such well-defined angular distributions of mono-energetic electrons may be useful for plasma-based X-ray sources.