The FERMI FEL user facility: recent results and future plans.

Allaria E.

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VI - Fisica applicata, acceleratori e beni culturali
Aula 29C-2 - Giovedì 20 h 09:00 - 13:00
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FERMI is the Free Electron laser Facility located at the Elettra laboratory in Trieste. The FEL covers the VUV to soft X-ray photon energy range with two FELs, FEL-1 and FEL-2, both based on the High Gain Harmonic Generation seeded mode (HGHG). The HGHG scheme consists in preparing the electron beam phase space in a first undulator, called modulator, where the interaction with an external laser, the seed, induces a controlled, periodic modulation, in the beam energy distribution. The beam then traverses a dispersive section, which converts the energy modulation into a density modulation. The higher order harmonic components of this modulation retain the phase and amplitude properties of the seed. The density modulated beam is then injected into a long FEL amplifier where the startup of the amplification process is enhanced by the presence of the modulation. The modulation depth may be tuned by varying the seed intensity or the dispersion, in order to reach saturation and efficient energy extraction at the end of the amplifier. The HGHG cascade scheme is implemented in FERMI FEL-1 to generate fully coherent radiation pulses in the VUV spectral range. The seed signal, continuously tunable typically in the range 230--260 nm, is obtained from a sequence of nonlinear harmonic generation and mixing conversion processes from an optical parametric amplifier. The radiation resulting from conversion in the FEL up to the 13th harmonic with sub-100fs pulse duration, is routinely delivered to user experiments. The amplitude of the energy modulation necessary to initiate the HGHG process grows with the order of the harmonic conversion. The induced energy dispersion has a detrimental effect on the high gain amplification in the final radiator at higher harmonic orders. These high orders can still be reached with a double stage HGHG cascade, where the harmonic conversion is repeated twice. The double conversion is done with the fresh bunch injection technique on the second FEL line at FERMI: FEL-2. FEL-2 is composed of a first stage, analogous to FEL-1, followed by a delay line, a magnetic chicane slowing down the electron beam with respect to the light pulse generated in the first FEL. The light pulse from the first FEL is therefore shifted to a longitudinal portion of the beam which is unperturbed by the seed in the first FEL. The light from the first FEL functions as a short wavelength seed for the second FEL. This scheme was implemented for the first time on FERMI FEL-2 and was used to demonstrate the seeded FEL coherent emission in the soft-X rays, up to harmonic orders of 65, and larger. We will report on the first 8 years of experience after the first lasing in 2010 and on the future plans.

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