Operation of the “CLIO” FEL at long wavelengths and study of partial guiding in the optical cavity

We study the influence of a partially guided propagation in the CLIO infrared FEL, both theoretically and experimentally. The waveguide is of planar type, guiding only in the vertical plane, and it is limited to the undulator length. The light propagation in the rest of the cavity is in free space. Therefore, the laser mode is in an intermediate regime of propagation, which involves a combination of free-space and waveguide propagation. Numerical simulations, based on FFT like propagation, have been performed in order to estimate the total losses in the FEL optical cavity. These losses and the laser transverse profile are calculated for different wavelengths and radius of curvature, either of spherical or toroidal mirrors. A new RF coupler installed on the accelerator has allowed us to operate the FEL at energies down to 16 MeV. The available spectral range is now 3–90 μm with spherical mirrors. Tests are underway with toroidal mirrors.