Electron bunches internally injected into plasma accelerators reach relativistic energies in giga-volt-per-metre-level fields, reducing emittance growth due to space charge and ultimately yielding high-brightness beams.Density downramps with steep gradients have provided an effective method of controllable injection into beam-driven plasma-wakefield accelerators, but previously have relied on the fields of an intense particle beam or laser to preionise the acceleration stage. Here, injection using downramps formed via optical ionisation is experimentally shown to be compatible with discharge-based-acceleration stages, producing bunches with up to six times higher energy than with laser-only preionisation. Results demonstrating injection of charge using a purely discharge-preionised acceleration stage and a low-energy injection laser suggest the feasibility of a scalable, high-repetition-rate injection stage—potentially attractive for high-average-power applications.

Radiofrequency linacs accelerate thousands of bunches per second, which should be matched by beam-driven plasma wakefield accelerators (PWFAs) if their benefits as high-acceleration-gradient energy boosters are to be fully exploited. However, demonstrations to date have accelerated only ~10 bunches per second. At FLASHForward, key issues are being solved to bridge this gap. Analytic models have been developed to show how to generate bunch pairs from the photocathode with the longitudinal shape optimised for plasma acceleration, thus reducing stray radiation compared to a collimator system. To deal with large energy depositions from rapid plasma creation and acceleration events benchmarked models have been built to determine the heating of the plasma source at kHz repetition rates, so that remedial measures can be developed. Furthermore, we have seen that ionisation induced by the wakefield-perturbed plasma can limit the maximum repetition rate. Finally, PWFAs must produce large energy gains for photon science or particle physics applications. We recently demonstrated acceleration of bunches from 1.2 to > 1.7 GeV over 0.5 m of plasma, with < 2% energy spread.