Speaker
Description
The FERMI FEL, as an externally seeded facility, provides extraordinary temporal coherence, pulse-to-pulse stability, and spectral purity, overcoming the stochastic limitations inherent in self-amplified spontaneous emission (SASE). However, its maximum photon energy is primarily limited by the achievable harmonic multiplication before energy spread and microbunching degradation disrupt the seeding process. To extend the operational range of the FERMI FEL-2 line toward shorter wavelengths, three key enablers were implemented during a one-week campaign in May 2025: the minimization of the electron bunch slice energy spread, an increase in electron beam energy, and the use of a shorter seed wavelength. High spectral purity lasing on the FEL-2 line was achieved with energies of several microjoules down to the 80th harmonic (2.47 nm), near the oxygen K-edge; here, the final detected energy was further limited by losses within the optical transport system. Furthermore, by leveraging non-linear harmonic generation, the 1 keV photon energy limit was surpassed, demonstrating stable, fully coherent radiation at 1.1 nm. In this talk, I will present the technical details of this temporary upgrade, its limitations, and potential advancements to enable regular operation and user experiments in this extended short-wavelength regime.