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

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, $O$ (10 pC/MeV) spectral density and per-cent-level energy spread, all with excellent reproducibility.