UV dielectric multilayers mirrors for Free Electron Lasers

Storage Ring Free Electron Lasers (FELs) are brand new laser sources where a relativistic electron beam passing through an undulator emits synchrotron radiation at a wavelength lambda which is tunable according to the electron beam energy and the magnetic field intensity. An overview of the SRFELs presently operating in the UV range is presented. The easy tunable FEL operation in the whole UV range is mainly limited by the mirrors losses, which should be maintained low even in the very hostile environment generated by the broadband high power spectrum of the undulator, extending to X rays, and which action lead to the mirrors degradation. Dedicated mirror measurements have been then developed in LURE, in the visible and in the UV range, for checking the mirrors performances before insertion in the optical cavity and after degradation. In order to insure a wide tunability range and large output power of the FEL, Low total losses are crucial for FEL oscillation at shorter wavelengths. New FEL sources, for instance at the ELETTRA synchrotron source, could then provide a widely tunable and powerful laser light down to 200 nm.