Mirror degradation and heating in storage ring FELs

Storage Ring Free Electrons Lasers (SRFELs) in the UV are now utilized as laser facilities for time-resolved fluorescence experiments or two-colour experiments with synchronized FEL and synchrotron radiations. Such experiments set new requirements for the laser and, consequently, for the optics such as long duration lasing, high laser power and short wavelengths. As these features imply operation of the storage ring at higher energies, cavity mirrors are subjected to radiation from undulator harmonics under more drastic conditions: increased power leading to heating of the front mirror, and degradation induced by undulator radiation extending towards X-rays. Also, for short λ operation, new difficulties arise: some materials become more absorbant and scattering losses increase.