Accurate measurements of slice electron beam parameters at the undulator in seeded Free Electron Lasers

The operation of modern Free Electron Lasers (FELs) necessitates precise knowledge of electron beam properties at the undulator to ensure the level of control required by increasingly demanding experiments. In seeded FELs, where only electrons inter- acting with the seed laser contribute to the process, it is crucial to determine the local values of these properties. We present a novel method, based on accurate modeling of the FEL process in High-Gain Harmonic Generation (HGHG), to accurately retrieve the electron beam energy spread, current, and laser-induced energy modulation. Understanding these values is essential for enabling advanced FEL schemes and optimally setting advanced seeding schemes such as Echo Enabled Harmonic Generation (EEHG). We describe the method and provide an experimental application to the FERMI FEL-1, where an energy spread in the range of 40-100 keV with a fewkeV accuracy is measured.