Extension in far-infrared of the CLIO free-electron laser

The infrared free-electron laser offers the advantage of a potential large tunability since the FEL gain itself remains substantially high throughout the infrared spectral range, and the reflectivity of metal mirrors used in the optical cavity remains also close to unity. The main limitation comes from the diffraction of the optical beam due to the finite size of the vacuum chamber of the undulator and other optical cavity elements. At CLIO, we have optimized the accelerator from 50 to 8 MeV, with a satisfactorily high electron current. By using toroidal mirrors, we take advantage of optical guiding at long wavelengths in the undulator vacuum chamber. We report here the results of the first measurements with this new configuration, leading to an FEL tunable from 3 to 120 μm.