Beam quality preservation in the external injection scheme is one of the key missing milestones towards staging of plasma-wakefield accelerators, a prerequisite for their utilisation in particle physics or other applications requiring high energy beams. This topic will be studied at FLASHForward, a unique beam-driven plasma wakefield acceleration facility currently under construction at DESY (Hamburg, Germany), in the frame of the FLASHForward X-2 experiment. High-quality 1 GeV-class electron beams from the free-electron laser FLASH with μm-emittances, kA-scale currents, and less than 100 fs durations will be utilised to generate driver–witness pairs by using a mask in a dispersive section. In this contribution the physics case and the current status of the FLASHForward X-2 experiment are reviewed. The experimental installation is described, with a focus on the electron beamline. Electron beam dynamics and particle-in-cell simulations are presented.

Active plasma lenses have the potential to enable broad-ranging applications of plasma-based accelerators owing to their compact design and radially symmetric kT/m-level focusing fields, facilitating beam-quality preservation and compact beam transport. We report on the direct measurement of magnetic field gradients in active plasma lenses and demonstrate their impact on the emittance of a charged particle beam. This is made possible by the use of a well-characterized electron beam with 1.4 mm mrad normalized emittance from a conventional accelerator. Field gradients of up to 823 T/m are investigated. The observed emittance evolution is supported by numerical simulations, which suggests the potential for conservation of the core beam emittance in such a plasma lens setup.

A tunable plasma-based energy dechirper has been developed at FLASHForward to remove the correlated energy spread of a 681 MeV electron bunch. Through the interaction of the bunch with wakefields excited in plasma the projected energy spread was reduced from a FWHM of 1.31% to 0.33% without reducing the stability of the incoming beam. The experimental results for variable plasma density are in good agreement with analytic predictions and three-dimensional simulations. The proof-of-principle dechirping strength of 1.8  GeV/mm/m significantly exceeds those demonstrated for competing state-of-the-art techniques and may be key to future plasma wakefield-based free-electron lasers and high energy physics facilities, where large intrinsic chirps need to be removed.

The FLASHForward project at DESY is a pioneering plasma-wakefield acceleration experiment that aims to produce, in a few centimetres of ionised hydrogen, beams with energy of order GeV that are of quality sufficient to be used in a free-electron laser. The plasma is created by ionising a gas in a gas cell with a multi-TW laser system. The plasma wave will be driven by high-current-density electron beams from the FLASH linear accelerator. The laser system can also be used to provide optical diagnostics of the plasma and electron beams due to the <30 fs synchronisation between the laser and the driving electron beam. The project will explore both external and internal witness-beam injection techniques. The operation parameters of the experiment are discussed, as well as the scientific programme.