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 (λu=4 cm) long undulator. The minimum rise-time of a 350-nm FEL macropulse shape is about 8 μs, while the electron beam macropulse continues for 17 μs. A net gain is estimated from the rise-time of the 350-nm FEL macropulse and a gain reduction due to the filling factor is also estimated. The estimated gain is 19.0% for a 144-MeV, 132-A electron beam of εn=26 mm mrad and ΔE/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 length of a SASE device in the VUV range.