Tunability experiments at the FERMI@Elettra free-electron laser

Abstract

FERMI@Elettra is a free electron-laser (FEL)-based user facility that, after two years of commissioning, started preliminary users' dedicated runs in 2011. At variance with other FEL user facilities, FERMI@Elettra has been designed to deliver improved spectral stability and longitudinal coherence. The adopted scheme, which uses an external laser to initiate the FEL process, has been demonstrated to be capable of generating FEL pulses close to the Fourier transform limit. We report on the first instance of FEL wavelength tuning, both in a narrow and in a large spectral range (fine-and coarse-tuning). We also report on two different experiments that have been performed exploiting such FEL tuning. We used fine-tuning to scan across the 1s-4p resonance in He atoms, at ≈23.74 eV (52.2 nm), detecting both UV-visible fluorescence (4p-2s, 400 nm) and EUV fluorescence (4p-1s, 52.2 nm). We used coarse-tuning to scan the M 4,5 absorption edge of Ge (∼29.5 eV) in the wavelength region 30-60 nm, measured in transmission geometry with a thermopile positioned on the rear side of a Ge thin foil.

Radiation characteristics of extreme UV and soft X-ray sources

“…Much more quickly, λ p can be influenced through a change of K u by variation of B 0 with variable gap undulators [88]. Typical values of the free-electron laser FLASH are K u = 1.23, λ u = 27.3 mm and E kin = 1.2 GeV which results in a fundamental wavelength of λ p = 4 nm [89].…”

Optical Klystron Enhancement to Self Amplified Spontaneous Emission at FERMI

“…FEL-1 [14] was designed to operate in HGHG mode, but it has been found to be suitable to realize an optical klystron in a high-gain FEL, by simply tuning both the modulator and the radiators at the same wavelength and exploiting the dispersive section to enhance the bunching induced by the spontaneous emission produced in the modulator. The wavelength tuning for both modulator and radiators is realized by changing the undulator gap [32,33]. The FERMI electron bunch has a strong requirement on the intrinsic energy spread σ δ that derives from the HGHG FEL mode: in order to efficiently lase at the harmonic n of the seed laser, σ δ should be smaller than ρ/ √ n 2 + 1.…”

Time resolved X-ray absorption spectroscopy in condensed matter: A road map to the future

“…As mentioned before for seeded FELs, like FERMI, the photon energy can be scanned across a much broader range with respect to SASE FELs, for variable gap undulators are used allowing an easy tuning of few percent across the absorption edge photon energy [53]. In this respect FERMI is so far the only FEL M a n u s c r i p t 10 where the tuning of the photon energy is permitted almost overall the entire operational photon energy range, i.e.…”