Two-stage seeded soft-X-ray free-electron laser


We report the first generation of coherent, tunable, variable-polarization, soft X-ray femtosecond pulses, generated by a seeded free-electron laser (FEL) operating in the fresh bunch, two-stage harmonic upshift configuration. Characterization of the radiation proves this FEL configuration can produce single-transverse-mode, narrow-spectral-bandwidth output pulses of several tens of microjoules energy and low pulse-to-pulse wavelength jitter at final wavelengths of 10.8 nm and below. The fresh bunch configuration enhances the FEL emission at high harmonic orders by avoiding a gain depression due to the energy spread induced by the first-stage FEL interaction. Coherent signals measured down to 4.3 nm suggest this configuration is directly scalable to photon energies that will enable scientific investigations below the carbon K-edge, including access to the L-edges of many magnetic materials, with an energy per pulse unlocking the gate for experiments in the soft X-ray region with close to Fourier-transform-limited pulses.

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Figure 1: Layout of FERMI's FEL-2 two-stage undulator line.
Figure 2: Second-stage spectral results.
Figure 3: Shot-to-shot FEL pulse energy fluctuations.


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The authors thank the FERMI engineering, technician, control and operator teams for their continuous support during the installation and commissioning of the FEL. The resources that made possible the construction of FERMI were obtained and managed by G. Comelli, A. Franciosi and C. Rizzuto. The authors also acknowledge the contributions of S. Milton, project director (2007–2010), who oversaw the final definition of the machine parameters and its physical realization; the advice and support of the FERMI Machine and Scientific Advisory Committees and consultants M. Cornacchia, W. Barletta, S. Tazzari and S. Biedron; and, finally, assistance from numerous individuals from sister FEL labs. This work was funded by the FERMI project of Elettra-Sincrotrone Trieste, partially supported by the Ministry of University and Research (grant nos FIRB-RBAP045JF2 and FIRB-RBAP06AWK3).

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All authors contributed to the commissioning and realization of the experiments. W.F. and L.G. oversaw the manuscript production, with the most significant text contributions provided by E.A. and E.F. (FEL and spectral data reduction), M.D. (laser systems), B.D. (undulators), G.P. and S.D.M. (linac and electron-beam phase spaces) and M.Z. (FEL optical diagnostics). E.A., L.F., G.D.N, F.P. and M.Z. carefully read and improved the final drafts. The FERMI project was coordinated by L.G. (physics), F.P. (user experiments), D.Z. (engineering and installation) and M.S. (project director, 2011–present).

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Correspondence to W. M. Fawley or L. Giannessi.

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Allaria, E., Castronovo, D., Cinquegrana, P. et al. Two-stage seeded soft-X-ray free-electron laser. Nature Photon 7, 913–918 (2013).

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