Linac-based UV-FEL macropulse shape and gain estimate at the FELI

Linac-based UV-FEL macropulses were measured with an Si-photodetector using a 144-MeV electron beam from the FELI linac at the UV-FEL facility (FEL-3) with a 6.72-m long optical cavity and a 2.68-m (lambda(u) = 4 cm) long und ulator. The minimum rise-time of a 350-nm FEL macropulse shape is about 8 m u s, while the electron beam macropulse continues for 17 mu s. A net gain i s estimated from the rise-time of the 350-nm FEL macropulse and a gain redu ction due to the filling factor is also estimated. The estimated gain is 19 .0% for a 144-MeV, 132-A electron beam of epsilon(n) = 26 mm mrad and Delta E/E (FWHM) = 0.5%. The UV-FEL facility has broken the world record for the shortest wavelength oscillation of linac-based FELs with a thermionic gun by reaching 278 nm. These parameters can be used to find a saturation lengt h of a SASE device in the VUV range. (C) 1999 Elsevier Science B.V. All rig hts reserved.