The conceptual design of the laser-plasma-based soft X-ray Free Electron Laser at ELI-Beamlines involves the integration of a novel high-power, high-repetition-rate laser, plasma source, compact LPA-based electron beam accelerator, dedicated electron beam line with relevant diagnostics, undulator beam line, photon beam line with required diagnostics, as well as a photon beam distribution system. The proposed concept of the whole setup is optimized to produce high-quality, coherent X-ray pulses with femtosecond duration in the ‘water-window’ wavelength of the photon radiation, which will be used by the photon user community for expiring research in the field of biology to study biological structures and processes at the cellular and molecular level at high resolution. In addition, the laser-plasma-based soft X-ray FEL will extend the abilities of users in material science to study nanostructures and thin films. In the frame of this report, we present the conceptual design for the full setup, which will be incorporated into the existing infrastructure of ELI-Beamlines. Furthermore, we discuss the key obstacles and the role of this project in the EuPRAXIA joint activity.

The plasma accelerator-based Free Electron Laser research program at ELI-ERIC (ELI-Beamlines, Czech Republic) intends to utilize the unique qualities of plasma accelerators to build FELs with remarkable brightness, coherence, and pulse length. The program is based on the novel high-power, high-repetition-rate laser system, which is under preparation at ELI-Beamlines. The program entails expanding the LUIS experimental setup to test and validate the performance of the laser-plasma accelerator-based extreme ultra-violet (EUV) FEL, integrating a high-power laser, plasma source, and electron beam transport line with relevant diagnostics to create a comprehensive test bed for the development of the EuPRAXIA LPA-based FEL. The plasma accelerator-based FEL development program at ELI-Beamlines represents an innovative effort to expand the capabilities of FEL technology and open new possibilities for scientific research and industrial applications. In the frame of this report, we provide an overview of the relevant developments at ELI-ERIC (ELI-Beamlines) as well as the main challenges of this program.