• V. Petrillo
  Universita' degli Studi di Milano, Milano
• M. Ferrario
  INFN/LNF, Frascati (Roma)
• A.R. Rossi
  Istituto Nazionale di Fisica Nucleare, Milano

We study both analytically and numerically the effects of a laser pulse on the longitudinal phase space of an electron beam in the stage of extraction from the cathode. We show how the interaction can produce modulations in the longitudinal momentum distribution.

• V. Petrillo, C. Maroli
  Universita' degli Studi di Milano, Milano
• M. Ferrario
  INFN/LNF, Frascati (Roma)
• L. Giannessi
  ENEA C.R. Frascati, Frascati (Roma)

We have written and numerically solved a set of 1-d FEL equations for electrons and radiation without the need of the slowly varying envelope approximation (SVEA). The equations, which take into account both forward and backward waves, have been applied to the case of a very short beam, as long as few wavelenghts, and to the case of long beams with short density modulations.

• L. Giannessi, F. Ciocci, G. Dattoli, M. Del Franco, A. Petralia, M. Quattromini, C. Ronsivalle, E. Sabia, I.P. Spassovsky, V. Surrenti
  ENEA C.R. Frascati, Frascati (Roma)
• D. Alesini, M. Bellaveglia, M. Castellano, E. Chiadroni, L. Cultrera, G. Di Pirro, M. Ferrario, L. Ficcadenti, D. Filippetto, A. Gallo, G. Gatti, E. Pace, B. Spataro, C. Vaccarezza, C. Vicario
  INFN/LNF, Frascati (Roma)
• A. Bacci, V. Petrillo, A.R. Rossi, L. Serafini
  Istituto Nazionale di Fisica Nucleare, Milano
• M. Bougeard, B. Carré
  CEA, Gif-sur-Yvette
• F. Briquez, M.-E. Couprie, M. Labat
  SOLEIL, Gif-sur-Yvette
• A. Cianchi
  Università di Roma II Tor Vergata, Roma
• F. Frassetto, L. P. Poletto
  LUXOR, Padova
• G. Lambert
  LOA, Palaiseau
• G. Marcus, J.B. Rosenzweig
  UCLA, Los Angeles, California
• M. Moreno, M. Serluca
  INFN-Roma, Roma
• A. Mostacci
  Rome University La Sapienza, Roma
• J.V. Rau, V. Rossi Albertini
  ISM-CNR, Rome

SPARC is a single pass free electron laser test facility realized in collaboration between the main Italian research institutions and devoted to experiments of light amplification in different beam conditions. While the laser was commissioned in self amplified spontaneous emission (SASE) mode during the last year, the operation in seeded mode has been recently demonstrated. The amplifier has been seeded with the second harmonic of the Ti:Sa driver laser generated in a crystal and with higher order VUV harmonics generated in a gas cell. The comparison between seeded and unseeded FEL emission will be discussed. The laser has been also operated in a new SASE configuration with a strongly chirped longitudinal e-beam phase space resulting from the RF compression. The chirp has been compensated by accordingly tapering the undulator gaps. Spectra with and without taper have been collected. An increase of about a factor 5 of the pulse energy in combination with spectra with a single longitudinal coherence region have been detected in presence of the taper. The combination of the chirp with the input seed is under study.