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.

The injection of a seed in a free-electron laser (FEL) amplifier reduces the saturation length and improves the longitudinal coherence. A cascaded FEL, operating in the high-gain harmonic-generation regime, allows us to extend the beneficial effects of the seed to shorter wavelengths. We report on the first operation of a high-gain harmonic-generation free-electron laser, seeded with harmonics generated in gas. The third harmonics of a Ti:sapphire laser, generated in a gas cell, has been amplified and up-converted to its second harmonic (λ(rad)=133 nm) in a FEL cascaded configuration based on a variable number of modulators and radiators. We studied the transition between coherent harmonic generation and superradiant regime, optimizing the laser performances with respect to the number of modulators and radiators.

We report the first experimental implementation of a method based on simultaneous use of an energy chirp in the electron beam and a tapered undulator, for the generation of ultrashort pulses in a selfamplified spontaneous emission mode free-electron laser (SASE FEL). The experiment, performed at the SPARC FEL test facility, demonstrates the possibility of compensating the nominally detrimental effect of the chirp by a proper taper of the undulator gaps. An increase of more than 1 order of magnitude in the pulse energy is observed in comparison to the untapered case, accompanied by FEL spectra where the typical SASE spiking is suppressed.

SPARC (acronym of ‘‘Sorgente Pulsata ed Amplificata di Radiazione Coerente’’, i.e. Pulsed and\ud Amplified Source of Coherent Radiation) is a single pass free-electron laser designed to obtain high gain\ud amplification at a radiation wavelength of 500 nm. Self-amplified spontaneous emission has been\ud observed driving the amplifier with the high-brightness beam of the SPARC linac. We report measurements\ud of energy, spectra, and exponential gain. Experimental results are compared with simulations from\ud several numerical codes

We report measurements demonstrating the concept of the free-electron laser (FEL) superradiant cascade. Radiation ( rad ¼ 200 nm) at the second harmonic of a short, intense seed laser pulse ( seed ¼ 400 nm) was generated by the cascaded FEL scheme at the transition between the modulator and radiator undulator sections. The superradiance of the ultrashort pulse is confirmed by detailed measurements of the resulting spectral structure, the intensity level of the produced harmonics, and the trend of the energy growth along the undulator. These results are compared to numerical particle simulations using the FEL code GENESIS 1.3 and show a satisfactory agreement. Fourth generation light sources open the way towards the exploration of molecular and atomic phenomena, yielding unprecedented benefits to a wide range of scientific disciplines [1][2][3][4][5][6]. Free-electron lasers (FELs), operating in self-amplified spontaneous emission (SASE) mode, have demonstrated the capability of reaching the subnanometer wavelength range at the femtosecond time scale [7,8]. The multielectron dynamics in atoms and molecules involved in chemical transformation processes evolves on a femtoto attosecond time domain. Thus, ultrashort pulses in the XUV spectral range are the potential tool to control and map the collective electronic and nuclei rearrangements. In a SASE FEL the generation of radiation is due to the passage of a relativistic electron beam through the periodic magnetic field of an undulator with maximum field amplitude B u and period u , inducing emission at a resonant2 (linear undulator) and its higher order harmonics, where is the Lorentz factor of the electrons and K ¼ eB u u =ð2m e cÞ the deflection parameter of the undulator. At the onset of saturation, the electrons emit coherently [9] over a characteristic frequency bandwidth Á!=! $ , where is the Pierce parameter [10,11]. Considering that typical values of the Pierce parameter range from 10 À3 to 10 À4 , the associated FWHM pulse length at the Fourier limit is ¼ 2 ffiffiffiffiffiffiffiffiffiffiffiffiffi ffi 2 lnð2Þ p L c =c, where L c ¼ rad =4 is known as cooperation length and, at rad ¼ 1 nm, assumes values in the range c $ 0:6-6 fs. When the electron bunch is longer than L c , several independent processes of SASE amplification can occur, leading to a pulse structure far from the Fourier limit and composed of a number of independent spikes [12]. A selective amplification of only one of these spikes has been demonstrated by shaping the electron beam phase space, in order to enable the field growth only in a limited portion of the bunch [13,14]. A single mode may also be generated by seeding the FEL amplifier with an external source. Seeding with a pulse shorter than c results in a spike of rms duration given by the cooperation length, at the onset of saturation when P sat % P beam (P beam is the electron beam power). Pulses shorter than the cooperation length may still be obtained at intensities higher than the saturation level, when the FEL emits in the superra...