Coherent Pulses from a Seeded Free-Electron Laser in the Extreme Ultraviolet

We report results of a free-electron laser operating with the high gain harmonic generation configuration in the wavelength rage from 60 to 20 nm. With the recent experiments done at the FERMI free-electron laser facility in Trieste we demonstrated the capability of the used method to produce highly coherent pulses allowing to improve the performances of FELs in the EUV spectral range overcoming some of the limitations that are typical of systems based on the self amplified spontaneous emission (SASE). Taking advantage of the seeding scheme adopted for FERMI high photon energy stability and a very high longitudinal and transverse coherence have been measured. The operation of the FERMI FEL facility will allow improving the use of the FEL light for those experiments that require a small spectral bandwidth or a precise control of the photon energy.