Schemi per la produzione di impulsi di luce ultracorti (sub-10fs) nel Laser ad Elettroni Liberi FERMI

Free-electron lasers (FELs) are capable of generating intense pulses of a few fs from VUV to hard X-rays. This possibility is opening a wide range of new scientific opportunities. The time scale of a few fs allows probing ultrarapid, out-of-equilibrium dynamics, driving transitions in regimes where excitation is not exhausted by fast decaying side channels, such as the Auger effect. Ultrashort pulses can probe THz-stimulated dynamics, such as coherent phonons or collective excitations in condensed matter, paving the way for the observation of coherent, field-dependent phenomena. When sort pulses are combined with a correspondingly high peak power, nonlinear optics is possible at EUV/soft X-ray wavelengths. These are examples showing the importance of generating short intense pulses at a FEL facility such as FERMI. In this thesis work, we discuss some of the schemes available to produce ultrashort photon FEL pulses. The work starts from a briefly review from analytical and theoretical point of view, highlighting the limitation in the short pulse generation due to the finite gain bandwidth of a FEL. FERMI, as a seeded FEL source, aims at generating close to the Fourier limit pulses. The interplay between pulse duration, spectral purity, and quality of the electron beam longitudinal phase space are analyzed. Several options available for the production of short pulses are then addressed from a computational point of view. In some promising cases it was finally possible to carry out experiments which were compared to the theoretical predictions. In the last three years, we have deepened the analysis of mainly two schemes: the superradiance cascade scheme, consisting in multiple harmonic jumps, with a small harmonic number, to quickly saturates the bunch, reaching the overbunched condition and to allows just the first peak to fully evolve in the superradiant evolution. This condition allows to reach a few fs pulses, due to the shortening of the bunching factor and the overbunched condition. The second peak is cleared by the out-of-resonance condition. The second one is the scarpered beam condition, in which the longitudinal length is reduced by the implementation of the scraper, which is composed by two blades that scatter away the tails of the electron charge distribution. By removing the outer charge, only the core distribution is allowed to propagate, resulting in a shorter electron bunch. We have proved that this will also reduce the FEL pulse duration. The first method, superradiance, was tested experimentally at FERMI in several conditions. This method has some limitations in terms of applicability, requiring specific undulator configurations to properly work, but it is now considered sufficiently mature to be applied in user experiments. The second method seems promising and applicable in the long wavelength range of FERMI but is still in the stage of simulation work. Before discussing the FEL proprieties, we want to briefly review some of the major schemes and concepts that concerns the production of ultrashort FEL pulses.