This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues.Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. t r a c tA new facility named SPARC_LAB has been recently launched at the INFN National Laboratories in Frascati, merging the potentialities of the former projects SPARC and PLASMONX. We describe in this paper the status and the future perspectives at the SPARC_LAB facility.

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This paper investigates the focusing properties of linear magnetic undulators, i.e., devices characterized by weak defocusing properties in the horizontal (wiggling) plane and strongly focusing in the vertical plane. The problem of identifying the conditions that ensure the existence of the electron beam eigenstates in the undulator lattice for a given working point of electron beam energy E b and resonant wavelength r is studied. For any given undulator lattice, a bandlike structure is identified defining regions in the ðE b ; r Þ plane where no periodic matching condition can be found, i.e., it is not possible to transport the electron beam so that optical functions are periodic at lattice boundaries. Some specific cases are discussed for the SPARC FEL undulator.

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Higher order harmonic generation in a free-electron laser amplifier operating in the superradiant regime [R. H. Dicke, Phys. Rev. 93, 99 (1954).] has been observed. Superradiance has been induced by seeding a single-pass amplifier with the second harmonic of a Ti:sapphire laser, generated in a -Barium borate crystal, at seed intensities comparable to the free-electron laser saturation intensity. Pulse energy and spectral distributions of the harmonics up to the 11th order have been measured and compared with simulations. DOI: 10.1103/PhysRevLett.108.164801 PACS numbers: 41.60.Cr, 41.50.+h, 42.55.Vc Nonlinear harmonic generation is widely used to extend the operation of optical lasers to the UV-vacuum ultra violet (UV-VUV) spectral region, where research methods for the investigation of matter [1-4] require ultrashort coherent pulses. Frequency up-conversion to the 10-100 nm range may be accomplished with high harmonics generated in gas (HHG) [5,6], where the active medium is a low density noble gas. The emission of high-energy photons, however, is inherently coupled with ionization, and the use of a nonlinear optical medium poses limitations to the conversion efficiency at the shortest wavelengths. In the spectral region where ionization processes are dominant, harmonic generation may still be obtained in freeelectron lasers (FELs). The mechanism of frequency up-conversion is based on the nonlinear density modulation of an electron beam at a given seed wavelength, seed . The frequency components of the modulated beam enforce the collective emission process at the resonant wavelength r ¼ u ð1 þ K 2 =2Þ=2 2 % seed and its harmonics [ is the Lorentz factor of the electrons, u is the period, and K ¼ eB u u =ð2 m e cÞ is the deflection parameter of the undulator] [7]. The seed transfers its longitudinal coherence properties to the FEL pulse [8], and schemes based on this principle were proposed to improve the longitudinal coherence of FELs operating in a self amplified spontaneous emission mode [9][10][11], where the temporal pulse structure is dominated by stochastic fluctuations associated with the initial shot noise [12]. The seed amplification, combined with the generation of coherent harmonics, has been demonstrated first in the midinfrared [8,13] and then in the UV-VUV range [14][15][16][17]. User facilities based on the frequency up-conversion of a seed laser in the VUV soft-x-ray region of the spectrum are now in operation and provide radiation with unprecedented properties of longitudinal coherence [18]. Coherent harmonics have been observed when seeding a single-pass FEL amplifier, and include the 3rd, 5th [19], and 7th harmonics [20]. The process of harmonic generation is expected to extend to higher orders when the FEL operates in the regime of super-radiance [21], in which a short optical pulse slips over the electron beam and increases its energy while maintaining a self-similar shape [22,23]. In this regime, the radiation pulse has a peak power that increases with the square of the distance z along...