Left panel : view of the Spring8 Compact SASE Source (SCSS) prototype (courtesy T. Hara). The High Harmonic seed at 160 nm, generated in a side chamber, is combined with the electron beam after a magnetic chicane.
Right panel : Two-dimensional image of the XUV beam at 160 nm (horizontal : wavelength; vertical : spatial profile), measured after one 4m-undulator section .Bottom to top : HH seed, unseeded SASE FEL, seeded FEL. The energy output of seeded FEL (250 nJ per pulse) corresponds to an amplification factor of 2780 from SASE and 460 from HH seed. The seeded FEL wavelength is slightly red-shifted whereas the spectral width is maintained, making possible the amplification of broadband ultra-short seed pulses.
Collaborations : M.-E. Couprie et al., SOLEIL Synchrotron (St-Aubin, France)
T. Hara et al., SPring8 Compact SASE Source (SCSS), XFEL Project/RIKEN (Hyogo, Japan)
L. Giannessi et al., ENEA & INFN/LNF (Frascati, Italy)
For producing intense ultra-short pulses in the XUV, the combination of laser- and accelerator-based sources will obviously play a major role in the design of the fourth-generation FEL. In the collaboration with Soleil-Synchrotron and SPring-8 Compact SASE Source (SCSS, SPring-8, Japan, SASE is for Self-Amplified Spontaneous Emission), we have recently coupled a seed harmonic source at 160 nm (5th harmonic of IR laser) to the LINAC FEL amplifier [G. Lambert, T. Hara et al., Nature Physics 4, 296 (2008)].
Amplification has been demonstrated at low seed level (few nJ per pulse) with large amplification factors (~500 for the seed and ~2500 for the SASE-FEL). The saturation of the FEL emission is reached faster than in the SASE mode. Moreover, the spectral properties of the seeded FEL are improved. We are now working on the extrapolation to shorter seed wavelength. A parallel collaborative program has been developed at SPARC LINAC FEL (ENEA & INFN/LNF, on support of EUROFEL design studies); its objective is to demonstrate the High Gain Harmonic Generation (HGHG: the FEL cavity is tuned on the seed harmonic).