Fig. 1. Schematic of the FACET-II experimental area with the planned location of an additional small chicane and radiation diagnostic to be used for X-FEL experiments. The simulated longitudinal phase space evolution shows the compression of the electron beam to attosecond duration with percent-level bunching at XUV/soft X-ray wavelengths.

Fig. 2. COXINEL electron and photon beam measurements compared to simulations. Left: Electron beam spectrometer measurements and transverse distributions along the screens (top: measurements; bottom: simulations using the measured electron beam distribution as an input). Right: Undulator radiation transverse pattern (measured with a CCD camera and modeled using the transported electron beam without electron energy selection).

Fig. 3. Average power as a function of the number of drive bunches per second at a range of existing or planned plasma-wakefield research facilities (bottom left corner) and photon-science and high-energy-physics user facilities (top right). The blue arrow represents the leap towards a beam-driven plasma-based FEL by using high-average-power upgrades to FLASHForward as a gateway.

Fig. 4. Schematic view of the LUX beamline after upgrade. For simplicity, diagnostics, such as electron beam profile monitors, are not shown.

Fig. 5. The SIOM-FEL setup with planar undulators and transverse gradient undulators.

Fig. 6. Plasma-based X-FEL and other ultrabright light sources options as summarized in the UK X-FEL science case^[81].

Fig. 7. Schematic layout of the BELLA Center’s Laser-Plasma Accelerator FEL beamline. The inset shows the electron beam beta function (beam size squared) inside the undulator in (left) the optimally matched strong-focusing undulator, (middle) a mismatched strong-focusing undulator, and (right) an optimized natural-focusing undulator. The strong-focusing undulator allows for higher beam density over the full undulator length.

Fig. 8. Layout of the EuPRAXIA@SPARCLAB infrastructure.

Table 1. Summary of parameters for the facilities discussed in the text utilizing a laser-driven approach to plasma-FEL operation.

Table 2. Summary of parameters for the facilities discussed in the text utilizing a beam-driven approach to plasma-FEL operation. We note that both Strathclyde and EuPRAXIA are also aiming to study multiple plasma-based FEL approaches including hybrid LWFA–PWFA configurations. Facilities/groups labeled with an asterisk have not yet begun experimental operation and for those the target parameters have been listed.