• 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.

• A. Marinelli, J.B. Rosenzweig
  UCLA, Los Angeles, California

Longitudinal space-charge forces can be a major source of micro-bunching instability. We will discuss a three-dimensional theoretical model for the high frequency limit of space-charge interactions leading to density modulation at the optical scale. Particular emphasis will be given to the effect of transverse thermal motion on the angular distribution of micro-bunching and to its connection to the physics of Landau damping in longitudinal plasma oscillations. A comparison with the results of high resolution molecular dynamics simulations will also be discussed.