Study of the ``CLIO'' FEL properties at long wavelengths

Abstract The optical gain of the infrared FEL “CLIO” has been increased at low electron energy by modifying the gun pulser. Therefore, the power emitted at long wavelengths (larger than 25 μm) has been increased by more than an order of magnitude and the maximum wavelength, initially at 50 μm, has been extended to 60 μm. Measurements of laser and cavity extraction efficiencies have provided an experimental method to estimate the optical diffraction losses with respect to the wavelength.