Generation and measurement of ultrashort FEL pulses at SPARC_LAB

Vittoria Petrillo,  M. P. Anania, M. Artioli, A. Bacci, M. Bellaveglia, E. Chiadroni, A. Cianchi, F. Ciocci, G. Dattoli, D. Di Giovenale, G. Di Pirro, M. Ferrario, G. Gatti, L. Giannessi, A. Mostacci, P. Musumeci, A. Petralia, R. Pompili, C. Ronsivalle, A. R. Rossi, E. Sabia, V. Shpakov, C. Vaccarezza, and F. Villa, LNF-INFN, Frascati, Italy ; ENEA, Frascati, Italy; University La Sapienza, Rome; University Tor Vergata , Rome; INFN-Section and University of Milan

Laser pulse shaping and velocity bunching (VB) in the linac have been used at SPARC_LAB for producing few tens femtoseconds electron beams with consequent generation of few femtoseconds FEL radiation pulses. The undulator has been used in the double role of radiation source and diagnostics for the characterization of the electron beam. Furthermore, when two electron beamlets with energy difference larger than the FEL parameter are generated by illuminating the cathode with two ps-spaced laser pulses, followed by a rotation of the longitudinal phase space by VB, 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. The experiment has been repeated also in the seeded mode, showing increased stability and coherence.