title: Superradiant cascade in a seeded free-electron laser.
abstract: We report measurements demonstrating the concept of the free-electron laser (FEL) superradiant cascade. Radiation (λ(rad) = 200 nm) at the second harmonic of a short, intense seed laser pulse (λ(seed) = 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.
authors.affiliation: ENEA C.R. Frascati, Via E. Fermi, 45 00044 Frascati, Roma, Italy.
authors.forename: L
authors.initials: L
authors.lastname: Giannessi
authors.affiliation: INFN-LNF, Via E. Fermi, 40 00044 Frascati, Roma, Italy.
authors.forename: M
authors.initials: M
authors.lastname: Bellaveglia
authors.affiliation: INFN-LNF, Via E. Fermi, 40 00044 Frascati, Roma, Italy.
authors.forename: E
authors.initials: E
authors.lastname: Chiadroni
authors.affiliation: INFN-Roma Tor Vergata and Università di Roma Tor Vergata, Via della Ricerca Scientifica, 1-00133 Rome, Italy.
authors.forename: A
authors.initials: A
authors.lastname: Cianchi
authors.affiliation: SOLEIL, L'Orme des Merisiers Saint-Aubin, BP 48 91192 GIF-sur-Yvette, Cedex, France.
authors.forename: M E
authors.initials: ME
authors.lastname: Couprie
authors.affiliation: ENEA C.R. Frascati, Via E. Fermi, 45 00044 Frascati, Roma, Italy.
authors.forename: M
authors.initials: M
authors.lastname: Del Franco
authors.affiliation: INFN-LNF, Via E. Fermi, 40 00044 Frascati, Roma, Italy.
authors.forename: G
authors.initials: G
authors.lastname: Di Pirro
authors.affiliation: INFN-LNF, Via E. Fermi, 40 00044 Frascati, Roma, Italy.
authors.forename: M
authors.initials: M
authors.lastname: Ferrario
authors.affiliation: INFN-LNF, Via E. Fermi, 40 00044 Frascati, Roma, Italy.
authors.forename: G
authors.initials: G
authors.lastname: Gatti
authors.affiliation: ENEA C.R. Frascati, Via E. Fermi, 45 00044 Frascati, Roma, Italy and SOLEIL, L'Orme des Merisiers Saint-Aubin, BP 48 91192 GIF-sur-Yvette, Cedex, France.
authors.forename: M
authors.initials: M
authors.lastname: Labat
authors.affiliation: Department of Physics and Astronomy, Particle Beam Physics Laboratory, UCLA, Los Angeles, California 90095, USA.
authors.forename: G
authors.initials: G
authors.lastname: Marcus
authors.affiliation: Università La Sapienza, Piazzale Aldo Moro, 1 00185 Roma, Italy.
authors.forename: A
authors.initials: A
authors.lastname: Mostacci
authors.affiliation: ENEA C.R. Frascati, Via E. Fermi, 45 00044 Frascati, Roma, Italy.
authors.forename: A
authors.initials: A
authors.lastname: Petralia
authors.affiliation: Università degli Studi di Milano and INFN-Mi, Via Celoria, 16 20133 Milano, Italy.
authors.forename: V
authors.initials: V
authors.lastname: Petrillo
authors.affiliation: ENEA C.R. Frascati, Via E. Fermi, 45 00044 Frascati, Roma, Italy.
authors.forename: M
authors.initials: M
authors.lastname: Quattromini
authors.affiliation: ISM-CNR Via del Fosso del Cavaliere, 100 00133 Roma, Italy.
authors.forename: J V
authors.initials: JV
authors.lastname: Rau
authors.affiliation: Sincrotrone Trieste S.C.p.A., Area Science Park, S.S. 14 Km 163.5, I-34149 Trieste, Italy.
authors.forename: S
authors.initials: S
authors.lastname: Spampinati
authors.affiliation: ENEA C.R. Frascati, Via E. Fermi, 45 00044 Frascati, Roma, Italy.
authors.forename: V
authors.initials: V
authors.lastname: Surrenti