Double stage seeded fel with fresh bunch injection technique at fermi@elettra

During the month of October 2012 the commissioning of the light source FEL-2 at FERMI reached an important milestone. Fermi FEL-2 is the a seeded FEL operating with a double stage cascade in the "fresh bunch injection" mode [1]. The two stages are two high gain harmonic generation FELs where the first stage is seeded by the 3rd harmonic of a Ti:Sa laser system, which is up converted to the 4th-12th harmonic. The output of the first stage is then used to seed the second stage. A final wavelength of 10.8 nm was obtained as the 24th harmonic (and eventually 48th) of the seed wavelength at the end of the two frequency conversion processes, demonstrating that the FEL is capable of producing single mode narrow bandwidth pulses with energy of several tens of micro Joules. We report on the experimental characterisation of the FEL performances in this configuration.