We present the experimental demonstration of a new scheme for the generation of ultrashort pulse trains based on free-electron-laser (FEL) emission from a multipeaked electron energy distribution. Two electron beamlets with energy difference larger than the FEL parameter have been generated by illuminating the cathode with two ps-spaced laser pulses, followed by a rotation of the longitudinal phase space by velocity bunching in the linac. The resulting self-amplified spontaneous emission FEL radiation, measured through frequency-resolved optical gating diagnostics, reveals a double-peaked spectrum and a temporally modulated pulse structure. DOI: 10.1103/PhysRevLett.111.114802 PACS numbers: 41.60.Cr, 42.55.Àf, 42.65.Ky Radiation pulses with attosecond to femtosecond time scales represent a real possibility for a breakthrough in science and technology, permitting unprecedented insights into the ultrafast electron and nuclear dynamics [1][2][3]. The time-resolved study of electron rearrangements could lead to significant advances in the understanding of intermolecular processes, chemical bond breaking and formation, and the interaction of photoactivated molecules with their environment.Trains of ultrashort radiation pulses enable stroboscopic electron imaging [4] and the investigation of the response accompanying collective electron motion in nanomaterials [5]. They also find further applications in other technical fields, such as the enhancement of transmission or reflectivity in materials, resonant inelastic x-ray scattering, or the ab initio phasing of nanocrystals [6].Sequences of spikes have been synthesized by means of the high harmonic generation driven by lasers in gases [7] and regularly used in experiments [4,8], but are severely limited in efficiency approaching the keV range.Free-electron lasers (FELs) are capable of producing high brightness pulses in the x-ray spectral region [9][10][11][12]. The FEL, in the self-amplified spontaneous emission (SASE) mode of operation [13], generates radiation with limited temporal coherence [14], time duration of the order of the electron bunch length and structured in a chaotic succession of random peaks. The typical time scale of these radiation spikes is set by the FEL Pierce parameter [13]. Several techniques have been explored to increase longitudinal coherence, stability, and/or to shorten the FEL pulse time scale towards the attosecond domain. The amplification of one single SASE spike has been demonstrated by compressing the electron beam close or below the FEL coherence length [15,16], by using a chirped bunch energy combined with a matched undulator taper [17][18][19], or by spoiling the whole electron beam except a limited fraction [20,21], a technique that has also been implemented to produce double pulse two-color radiation for pump and probe experiments [22]. Short single or multiple pulses have also been produced in seeded or cascaded FELs [23][24][25][26][27], with increased coherence and shot to shot stability. More sophisticated seeding concept...

The theory of free-electron lasers is extended to include the new coupling between an electron beam and optical wave propagating at an angle 5 in an arbitrary harmonic. The coupling allows the laser to be tuned to a wider range of wavelengths and to include the effects of emittance in the electron beam. The formulation of the results in terms of coupling constants means that the existing knowledge of high gain, low gain, weak optical fields, strong optical fields, and short pulses in freeelectron lasers can be immediately generalized to off-axis propagation in an arbitrary harmonic.

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