FERMI | Machine FERMI Status Report In 2024, FERMI’s activities were primarily focused on fine-tuning and executing experiments with external users. FERMI operated for 3,752 hours in 2024, of which 84% were dedicated to user operations and in-house experiments, while the remaining 16% were allocated to machine physics studies. The total number of available experimental hours, consistent with 2023 and amounting to a 40% reduction compared to previous years, was affected by the increased cost of electricity, which necessitated longer shutdown periods. Despite this constraint, the number of user experiments carried out was 19, compared to 21 in 2023, while in-house experiments increased from 2 in 2023 to 5. The actual availability of the FEL source for experiments relative to the scheduled time, excluding downtime caused by thunderstorms, was 94.7% (compared to 95.2% in 2023). These figures confirm the effectiveness of the strategy implemented in recent years to enhance machine stability. FERMI continues to be a leading international facility among research infrastructures utilizing free-electron lasers. This is demonstrated by the number of highimpact scientific publications—13 in 2024—accounting for approximately 40% of the total 34 publications. These data highlight the outstanding scientific quality of the results achieved by FERMI users. On the LDM beamline, the capability to perform quantum control experiments using extreme ultraviolet radiation was successfully demonstrated. The significance of this achievement was validated by the publication of the corresponding article in the prestigious journal Nature. At the TIMEX experimental station, by using a pulsed electron beam, the effect of FEL irradiation on a gold crystal was measured. Notably, an ultrafast recrystallization of the system was observed for the first time, induced by vibrational modes generated by FERMI pulses. At the DiProI station, experiments were conducted to study the evolution of surface acoustic waves generated through the interaction with infrared pulses, as a function of the transferred momentum. The ability to control these acoustic waves at the nanoscale is particularly relevant for applications in electronic circuits, such as filters, oscillators, and transducers based on acoustic wave manipulation. On the TIMER beamline, in collaboration with the machine physics team, experiments were conducted using a transient grating setup with two beams having orthogonal polarization. This configuration enabled the generation of a helical response in the magnetic system under investigation, highlighting that nonlinear optics experiments offer greater sensitivity compared to conventional spectroscopies when studying chiral systems. Regarding machine development, FEL-1 in the new Echo Enabled Harmonic Generation (EEHG) mode was successfully implemented in user experiments. These users benefited from the high spectral purity and stability provided by EEHG, in a spectral range previously only accessible with FEL-2 (see Fig. 1). The EEHG configuration on FEL-1 allowed the team to gain valuable experience with this more complex scheme compared to the original design. FEL-1 also played a role in early studies supporting the FERMI 2.0 upgrade. Specifically, investigations focused on the effect of the first dispersive section in the EEHG setup on the amplification of microbunching instability, which could pose a challenge for FERMI 2.0's proposed dual-stage configuration—an EEHG stage followed by a High-Gain Harmonic Generation (HGHG) stage, separated via the existing fresh-bunch technique. For this reason, a significant portion of beam time was devoted to studying mitigation strategies for microbunching instability, yielding promising results. Near-complete suppression was achieved by operating the LINAC with a single magnetic compressor. However, since FERMI 2.0 will require a dual-compression scheme to achieve peak currents in the 1–1.2 kA range, research continues into mitigation techniques suitable for this operating regime. An increase in maximum energy from the LINAC was achieved following the installation of two new accelerating sections, developed in collaboration with PSI, and through continued RF conditioning. The maximum energy now exceeds 1.5 GeV at 50 Hz repetition rate. In terms of further LINAC upgrades, it has been decided to proceed with the development of four additional highgradient accelerating cavities, again in collaboration with PSI. A fourth harmonic seeding scheme was implemented, extending the FERMI spectral range up to the nitrogen K-edge, although currently only on discrete harmonics of the seed. Using the new dual-modulator configuration of FEL-1, a seeding scheme based on an optical klystron (instead of a single modulator) was experimentally tested and validated. This approach allows the use of seed pulse energies several orders of magnitude lower than those required in conventional schemes. Further machine studies were focused on the evolution of beam modulation under space charge regime, and on identifying the origin of signals corresponding to off-resonance harmonics in the radiator, which may influence outcomes in specific experimental configurations. In 2025, FERMI continued to deliver highimpact user science while expanding its experimental capabilities. Several experiments at the LDM, DiProI, and TIMER beamlines explored nonlinear and time-resolved phenomena in atoms, molecules, and solids, confirming FERMI’s role as a premier facility for ultrafast spectroscopy and quantum control in the XUV domain. On the machine side, a dedicated campaign was carried out to test the LINAC at its highest energy, in combination with a short-wavelength seed, achieving outstanding FEL performance at the nitrogen and oxygen K-edges—well beyond the standard operational range of the facility. These results mark a significant step toward the goals of FERMI 2.0, demonstrating both the stability of the upgraded LINAC and the potential for extending FERMI’s spectral reach into the soft X-ray regime. In 2024, FERMI focused on user experiments, operating for 3,752 hours with 84% dedicated to research. Despite reduced beamtime due to energy costs, 19 user experiments and 13 high-impact publications were achieved. Key results included quantum control with extreme UV, ultrafast recrystallization in gold, and nanoscale acoustic wave studies. FEL-1 operated in EEHG mode, supporting both experiments and FERMI 2.0 upgrade studies. Advances included increased linac energy, spectral range extension, and microbunching instability mitigation, reinforcing FERMI’s role as a premier free-electron laser facility. Elettra Highlights 2024–2025 154
RkJQdWJsaXNoZXIy MjQ4NzI=