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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. 

Figure 2: Measured single shot of FEL at 32.5 nm and seed laser spectra (dashed red and continuous blue lines respectively).

Typical shot-to-shot distribution of the FEL pulse energy shows a quasi-Gaussian distribution characterized by a standard deviation of about 10%, (Figure 1(b)) these fluctuations reflect the shot-to-shot variations in the electron-beam parameters and may be improved by stabilizing the electron beam.
The most important feature of Fermi@Elettra is that the seed laser transfers its coherence properties to the electron beam, allowing an improved longitudinal coherence of the FEL with respect to what achievable with SASE. Typical FEL pulses at Fermi are characterized by a single Gaussian mode spectrum with a bandwidth of about 20 meV. Figure 2 reports a typical single shot spectrum of Fermi operated at 32 nm plotted together with the seed laser spectrum; both spectra are plotted in photon energy with respect to their central energy. Results clearly show the single mode spectrum emission. Since the measured bandwidth is close to the transform limit for the expected FEL pulse length it is possible to state that FEL pulses are close to be fully coherent.
Another important feature of Fermi is related to the use of variable gap APPLE–II undulators in the radiator. Indeed, APPLE-II undulators, used for the first time in a high gain FEL, gives control on the polarization of the light pulses.
Our work demonstrates that Fermi@Elettra can produce X-ray pulses with unprecedented shot-to-shot wavelength stability, low intensity fluctuations, close to transform-limited bandwidth, transverse and longitudinal coherence and variable polarization. The achieved performance allowed to open the first call for experiments that will be held in December this year.
 

This research was conducted by the FERMI team at Elettra - Sincrotrone Trieste.


Reference

E. Allaria, R. Appio, L. Badano, W.A. Barletta, S. Bassanese, S.G. Biedron, A. Borga, E. Busetto, D. Castronovo, P. Cinquegrana, S. Cleva, D. Cocco, M. Cornacchia, P. Craievich, I. Cudin, G. D'Auria, M. Dal Forno, M.B. Danailov, R. De Monte, G. De Ninno, P. Delgiusto, A. Demidovich, S. Di Mitri, B. Diviacco, A. Fabris, R. Fabris, W. Fawley, M. Ferianis, E. Ferrari, S. Ferry, L. Froehlich, P. Furlan, G. Gaio, F. Gelmetti, L. Giannessi, M. Giannini, R. Gobessi, R. Ivanov, E. Karantzoulis, M. Lonza, A. Lutman, B. Mahieu, M. Milloch, S.V. Milton, M. Musardo, I. Nikolov, S. Noe, F. Parmigiani, G. Penco, M. Petronio, L. Pivetta, M. Predonzani, F. Rossi, L. Rumiz, A. Salom, C. Scafuri, C. Serpico, P. Sigalotti, S. Spampinati, C. Spezzani, M. Svandrlik, C. Svetina, S. Tazzari, M. Trovo, R. Umer, A. Vascotto, M. Veronese, R. Visintini, M. Zaccaria, D. Zangrando and M. Zangrando,

Highly coherent and stable pulses from the FERMI seeded free-electron laser in the extreme ultraviolet,
Nature Photonics 6, 699 (2012). DOI:10.1038/nphoton.2012.233

Last Updated on Tuesday, 23 October 2012 10:08