Free-electron lasers (FELs) are promising devices for generating light with laser-like properties in the extreme ultraviolet and X-ray spectral regions. Recently, FELs based on the self-amplified spontaneous emission (SASE) mechanism have allowed major breakthroughs in diffraction and spectroscopy applications, despite the relatively large shot-to-shot intensity and photon-energy fluctuations and the limited longitudinal coherence inherent in the SASE mechanism. Here, we report results on the initial performance of the FERMI seeded FEL, based on the high-gain harmonic generation configuration, in which an external laser is used to initiate the emission process. Emission from the FERMI FEL-1 source occurs in the form of pulses carrying energy of several tens of microjoules per pulse and tunable throughout the 65 to 20 nm wavelength range, with unprecedented shot-to-shot wavelength stability, low-intensity fluctuations, close to transform-limited bandwidth, transverse and longitudinal coherence and full control of polarization

We report the first generation of coherent, tunable, variable-polarization, soft X-ray femtosecond pulses, generated by a\ud seeded free-electron laser (FEL) operating in the fresh bunch, two-stage harmonic upshift configuration. Characterization\ud of the radiation proves this FEL configuration can produce single-transverse-mode, narrow-spectral-bandwidth output\ud pulses of several tens of microjoules energy and low pulse-to-pulse wavelength jitter at final wavelengths of 10.8 nm and\ud below. The fresh bunch configuration enhances the FEL emission at high harmonic orders by avoiding a gain depression\ud due to the energy spread induced by the first-stage FEL interaction. Coherent signals measured down to 4.3 nm suggest\ud this configuration is directly scalable to photon energies that will enable scientific investigations below the carbon K-edge,\ud including access to the L-edges of many magnetic materials, with an energy per pulse unlocking the gate for experiments\ud in the soft X-ray region with close to Fourier-transform-limited pulses

Extreme ultraviolet and X-ray free-electron lasers (FELs) produce short-wavelength pulses with high intensity, ultrashort duration, well-defined polarization and transverse coherence, and have been utilized for many experiments previously possible only at long wavelengths: multiphoton ionization, pumping an atomic laser and four-wave mixing spectroscopy. However one important optical technique, coherent control, has not yet been demonstrated, because self-amplified spontaneous emission FELs have limited longitudinal coherence. Single-colour pulses from the FERMI seeded FEL are longitudinally coherent, and two-colour emission is predicted to be coherent. Here, we demonstrate the phase correlation of two colours, and manipulate it to control an experiment. Light of wavelengths 63.0 and 31.5nm ionized neon, and we controlled the asymmetry of the photoelectron angular distribution by adjusting the phase, with a temporal resolution of 3as. This opens the door to new short-wavelength coherent control experiments with ultrahigh time resolution and chemical sensitivity