The Electro-Optical Sampling (EOS) system uses the electro-optic effect to map the transient field of the beam onto a probe laser pulse, enabling direct reconstruction of the temporal current distribution. Temporal profiles acquired with the EOS were compared with independent measurements from the RF deflector, used as the reference diagnostic. The two techniques show good agreement in pulse duration, peak position, and main temporal features, with minor differences attributed to their intrinsic resolution and coupling conditions. These results confirm that the EOS is fully operational and provides reliable time-domain diagnostics complementary to the deflector, non invasive and compatible with the operation

The systematic comparison of two semi-analytical models of microbunching instability affecting electron beams in single pass or recirculating linear accelerators is reported. The comparison is comprehensive of numerous features of the instability, such as low and high gain contributions, either linearized or at second order, coherent synchrotron and edge radiation, Landau damping by transverse emittance and laser heater, investigated for single and double magnetic bunch length compression. The comparison is enriched by three different expressions – one of which is new – to calculate intrabeam scattering. The inclusion of this effect allows the recovery of agreement with published experimental observations, to date either in disagreement with theory, or in partial agreement by virtue of blind fitting of parameters.

FERMI has recently completed an early upgrade phase that has significantly extended its operational capabilities and positioned the facility on a clear path toward the future FERMI 2.0 program. The upgrade of the linac has enabled routine operation at increased beam energy, while the FEL-1 beamline can now be operated in EEHG mode with high reliability, complementing the established HGHG performance on both FELs. The introduction of a seed signal at 200 nm, together with a dedicated program of microbunching-instability suppression in the accelerator and transport lines, has markedly improved longitudinal beam quality. As a result, FERMI is already able to explore photon-energy ranges foreseen for the next-generation facility. User-relevant operation has been demonstrated at the nitrogen and oxygen absorption edges, and measurements have confirmed the generation of higher harmonics extending beyond 1 keV. These achievements show that the present machine configuration can access a substantially expanded spectral range, providing an important stepping-stone toward the full FERMI 2.0 upgrade.