Most FELs employ the mechanism of self-amplified spontaneous emission (SASE) from a relativistic electron beam to generate intense femtosecond pulses in the x-ray spectral region. Such SASE FELs are characterized by a broad bandwidth and relatively poor longitudinal coherence, and offer a rather limited control over the spectro-temporal properties. The limitations of a SASE FEL can be overcome by using an external laser to trigger the amplification process. Echo-enabled harmonic generation (EEHG), alone or in combination with the high-gain harmonic generation scheme (HGHG) is currently the most promising candidate to extend the operation of externally-seeded FELs into the soft x-ray region. Here, we discuss the plan at FERMI for the upgrade of the second FEL line in order to reach ~2 nm at the fundamental emission wavelength. In the first step, coherent radiation at ~10 nm will be generated with an EEHG layout and used as a seed in an HGHG stage on a fresh part of the electron beam. The experience with EEHG at the FEL-1 line will be an important step towards the final realization of the FERMI FEL as a reliable source of highly coherent radiation at ~2 nm and below.

The FERMI free-electron laser (FEL) facility has recently achieved a significant milestone with the successful implementation of the echo-enabled harmonic generation (EEHG) scheme in the FEL-1 amplifier line. This advancement is part of a broader upgrade strategy aiming at expanding the covered spectral range of the facility to the entire water window and beyond. Through this upgrade, the maximum photon energy of FEL-1 has been doubled and spectral quality has been enhanced. The updated FERMI FEL-1 is the first user facility operating in the spectral range 20-10 nm utilizing the EEHG scheme. It will serve also as the ideal test bench for conducting new machine studies in the perspective of future developments. In this contribution, we present the results obtained during the commissioning phase and the first user experiments.

The mechanisms that drive short-range modulations in the longitudinal phase space of accelerated electron bunches, otherwise known as the microbunching instability, have undergone intensive study. The various collective interactions between charged particles within the bunch, and their environment, can degrade the quality of these bunches, eventually making them unsuitable to drive light sources such as free-electron lasers (FELs). Although the most common method for removing this instability at X-ray FELs – namely, the laser heater – has proven to be very useful in improving the performance of these facilities, alternative methods to achieve this goal are active areas of research. In this contribution, we present experimental evidence of the influence of transverse Landau damping on mitigating the microbunching instability.