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Philosophical transactions of the Royal Society of London / A Mathematical, physical and engineering sciences Series A 377(2151), 20180392 - (2019). doi:10.1098/rsta.2018.0392, The FLASHForward experimental facility is a high-performance test-bed for precision plasma wakefield research, aiming to accelerate high-quality electron beams to GeV-levels in a few centimetres of ionized gas. The plasma is created by ionizing gas in a gas cell either by a high-voltage discharge or a high-intensity laser pulse. The electrons to be accelerated will either be injected internally from the plasma background or externally from the FLASH superconducting RF front end. In both cases, the wakefield will be driven by electron beams provided by the FLASH gun and linac modules operating with a 10 Hz macro-pulse structure, generating 1.25 GeV, 1 nC electron bunches at up to 3 MHz micro-pulse repetition rates. At full capacity, this FLASH bunch-train structure corresponds to 30 kW of average power, orders of magnitude higher than drivers available to other state-of-the-art LWFA and PWFA experiments. This high-power functionality means FLASHForward is the only plasma wakefield facility in the world with the immediate capability to develop, explore and benchmark high-average-power plasma wakefield research essential for next-generation facilities. The operational parameters and technical highlights of the experiment are discussed, as well as the scientific goals and high-average-power outlook., Published by Soc., London

Physical review letters 122(3), 034801 (2019). doi:10.1103/PhysRevLett.122.034801, A tunable plasma-based energy dechirper has been developed at FLASHForward to remove thecorrelated energy spread of a 681 MeV electron bunch. Through the interaction of the bunch withwakefields 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 densityare in good agreement with analytic predictions and three-dimensional simulations. The proof-of-principledechirping 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 energyphysics facilities, where large intrinsic chirps need to be removed, Published by APS, College Park, Md.

Precise tailoring of plasma-density profiles has been identified as one of the critical points in achieving stable and reproducible conditions in plasma wakefield accelerators. Here, the strict requirements of next generation plasma-wakefield concepts, such as hybrid-accelerators, with densities around 10 17 cm −3 pose challenges to target fabrication as well as to their reliable diagnosis. To mitigate these issues we combine target simulation with fabrication and characterization. The resulting density profiles in capillaries with gas jet and multiple in- and outlets are simulated with the fluid code OpenFOAM. Satisfactory simulation results then are followed by fabrication of the desired target shapes with structures down to the 10 µm level. The detection of Raman scattered photons using lenses with large collection solid angle allows to measure the corresponding longitudinal density profiles at different number densities and allows a detection sensitivity down to the low 10 17 cm −3 density range at high spatial resolution. This offers the possibility to gain insight into steep density gradients as for example in gas jets and at the plasma-to-vacuum transition.