Since the discovery of lasers, the quest for new powerful sources of radiation has become an exciting fields of both fundamental and applied research. Indeed, light sources are nowadays one of the most powerful tools for matter investigation. Free electron lasers (FELs) represent the next step in the evolution of the light sources, with the potential to overcome the lack of coherent sources in the x-ray spectral range. After a general introduction to the characteristics and mechanisms of seeded FEL I will introduce the FERMI FEL facility, which is the first user facility in the XUV spectral range based seeded high-gain harmonic generation. Then I will report different approaches that I followed to improve the performance of the FERMI FEL. In particular, I will focus on experiments aimed at measuring and controlling the polarization of the FEL pulse, and at generating multi-color FEL pulses for pump-probe experiments. Furthermore I will present the experiments I carried out in order to assess the effect on the FEL performance of a laser heater, which is a device that is used to control the energy spread of the electron beam
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