Double stage seeded fel with fresh bunch injection technique at fermi

Seeding a FEL with an external coherent source has been extensively studied in the last decades as it can provide a way to enhance the radiation brightness and stability with respect to that available from SASE. An efficient scheme for seeding a VUV-soft x ray FEL uses a powerful, long wavelength external laser to induce on the electron beam coherent bunching at the harmonics of the laser wavelength [1]. When the bunching is further amplified by FEL interaction in the radiator, the scheme is called high gain harmonic generation (HGHG) [2]. The need for high power seed sources and small electron beam energy spread are at the main limits for direct extension of the HGHG scheme to short wavelengths. The fresh bunch scheme was proposed as a way to overcome these limitations [3]; the scheme foresees the FEL radiation produced by one HGHG stage as an external seed for a second HGHG stage. We report the latest results obtained at FERMI that uses the two-stage HGHG scheme for generation of FEL pulses in the soft xray regime. A characterization of the FEL performance in terms of power, bandwidth and stability is reported. Starting from the FERMI results we will discuss extension of the scheme toward shorter wavelengths. Copyright © 2013 CC-BY-3.0 and by the respective authors.