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

X-ray free electron lasers (FELs), which amplify light emitted by a relativistic electron beam, are extending nonlinear optical techniques to shorter wavelengths, adding element specificity by exciting and probing electronic transitions from core levels. These techniques would benefit tremendously from having a stable FEL source, generating spectrally pure and wavelength-tunable pulses. We show that such requirements can be met by operating the FEL in the so-called echo-enabled harmonic generation (EEHG) configuration. Here, two external conventional lasers are used to precisely tailor the longitudinal phase space of the electron beam before emission of X-rays. We demonstrate high-gain EEHG lasing producing stable, intense, nearly fully coherent pulses at wavelengths as short as 5.9 nm (~211 eV) at the FERMI FEL user facility. Low sensitivity to electron-beam imperfections and observation of stable, narrow-band, coherent emission down to 2.6 nm (~474 eV) make the technique a prime candidate for generating laser-like pulses in the X-ray spectral region, opening the door to multidimensional coherent spectroscopies at short wavelengths.

The status of development of ins&on devices for the 1.5-2 GeV synchrotron radiation source ELETTRA is described, including details of the prototype undulator, multipole wiggler, mechanical support structure and vacuum chamber. Plans for a novel source of circularly polarized radiation are also presented.

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