What limits the wavelength of an FEL? Challenge at the shortest wavelength lasing limit of linac-based FELs using the FELI linac with a thermionic gun

We have demonstrated that a medium-emittance (26π mm mrad), medium-current (60 A), 24 μs pulse 155 MeV electron beam from the FELI linac with a thermionic gun allows free electron laser (FEL) oscillations at 0.28–0.36 μm. The peak and average FEL powers are 1.8 MW and 4 mW at 0.35 μm. Gain estimation and effects of electron beam quality and filling factor on gain are also discussed. Experimental verification of gain estimates and the criteria on the electron beam quality and wavelength limit due to photon diffraction are tested with our data from visible- and ultraviolet-FEL oscillations.