# Bright electron bunches from a plasma-wakefield accelerator with a steep density down-ramp

## TL;DR

This paper shows how plasma accelerators can produce high-quality electron bunches with low emittance and high brightness for use in particle physics and photon science.

## Contribution

The novel contribution is demonstrating reproducible generation of ultra-bright electron bunches in a plasma accelerator with low emittance and high charge per unit energy.

## Key findings

- Electron bunches with mm-mrad normalized emittance were injected and accelerated in a plasma.
- The bunches achieved O(10 pC/MeV) spectral density and per-cent-level energy spread.
- The results show excellent reproducibility and potential for future ultra-bright bunch production.

## Abstract

High-brightness electron bunches drive fundamental research in particle physics and photon science. Key to achieving a high brightness is to have a low transverse emittance, which ensures that the bunch can be tightly focussed. In radiofrequency accelerators a low initial emittance can be rapidly degraded due to space charge forces, which are greatly diminished once the electron bunch attains a relativistic velocity. A plasma accelerator can maintain orders-of-magnitude higher accelerating fields than radiofrequency accelerators, while multiple techniques exist to create a low emittance electron bunch directly inside the plasma accelerator structure. Plasma accelerators therefore offer a possibility to create high-brightness bunches in wakefields driven even by low-quality drive bunches. Here we demonstrate the injection and gigavolt-per-metre acceleration of electron bunches with mm-mrad normalised emittance, \documentclass[12pt]{minimal}
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Bright electron bunches power particle physics and photon science. Here, reproducible bunches were generated in a plasma with simultaneous high charge per unit energy and low emittance, advancing a scheme to produce ultra-bright bunches in future.