With Fermi@Elettra full coherence is now available in the extreme ultraviolet

Free-electron lasers (FELs) are considered the most promising devices for the generation of light with laser-like properties in the extreme ultraviolet and X-ray spectral regions. FLASH at DESY (Germany) has been the first facility to provide users with FEL pulses in the VUV - soft X-ray spectral region for specific experiments. More recently LCLS at SLAC (USA) and SACLA at Spring8 (Japan) have extended this capability to the hard X-rays. All these FELs are based on the self-amplified spontaneous emission (SASE) mechanism and have allowed major breakthroughs in diffraction and spectroscopy applications, despite the relatively large intensity and photon-energy fluctuations and the limited longitudinal coherence inherent in the SASE mechanism.
Fermi@Elettra is the first FEL user facility that has been designed with a different approach in order to improve the coherence and the stability of its photon pulses.
After the demonstration of coherent photon pulses from the free electron laser done in December 2010, activities have been focused in 2011 toward improving the FEL and reaching its specification performance. The final goal of producing FEL pulses in the spectral range from 80 nm down to 20 nm with more than 100 μJ per pulse with a well-controlled longitudinal and transverse coherence has been finally achieved in June 2012 when pulses with more than 400 μJ at 52 nm have been measured corresponding to about 10¹⁴ photons per pulse.
A detailed characterization of the FEL pulses produced has been recently carried out and demonstrated how the FEL scheme adopted for Fermi@Elettra is able to produce FEL pulses with a high degree of longitudinal and transverse coherence.

Figure 1:    Measured FEL intensity at 32.5 nm. a) Measured growth of FEL energy as a function of the number of undulator sections used in the radiator, data are shown for both circular (red symbols) and horizontal (blue symbols) polarization. b) Typical shot-to-shot distribution of the pulse energy for 1000 consecutive FEL pulses.
 

Fermi@Elettra is based on the so-called high gain harmonic generation (HGHG) scheme that uses an external seed laser to initiate the FEL process. At Fermi@Elettra the FEL process starts when electron bunches which are accelerated in the linear accelerator up to 1.2 GeV interact the UV seed laser pulses produced by an optical parametrical amplifier. When the seeding process is optimized, the FEL radiation is amplified along the radiator undulators resulting in output pulses with an energy ranging from several tens of microjoules up to few hundreds of microjoules depending on the electron beam peak current. Figure 1(a) reports measurements of the exponential growth in the FEL intensity along the radiator undulator; the data show that for both circular (red) and horizontal (blue) polarization the FEL performance is in agreement with predictions based on numerical simulations.