Laser heater commissioning at an externally seeded free-electron laser

Theory Group–ENEA C.R. Frascati, 00044 Frascati (Rome), Italy(Received 27 September 2014; published 18 December 2014)FERMIisthefirstuserfacilitybaseduponanexternallyseededfree-electronlaser(FEL)andwasdesignedto deliver high quality, transversely and longitudinally coherent radiation pulses in the extreme ultravioletand soft x-ray spectral regimes. The FERMI linear accelerator includes a laser heater to control thelongitudinal microbunching instability, which otherwise is expected to degrade the quality of the highbrightnesselectronbeamsufficientlytoreducetheFELoutputintensityandspectralbrightness.Inthispaper,wepresenttheresultsoftheFERMIlaserheatercommissioning.Forthefirsttime,weshowthatoptimizingthe electron beam heating at an upstream location (beam energy, 100 MeV) leads to a reduction of theincoherent energy spread at the linac exit (beam energy, 1.2 GeV). We also discuss some of the positiveeffectsofsuchheatingupontheemissionofcoherentopticaltransitionradiationandtheFELoutputintensity.