Storage ring (SR) Free Electron Laser (FEL) are attractive and full of promise tunable and powerful laser sources for the UV range. Concerning the optical cavity, the relatively small gain obtained in the UV calls for the necessity to use high reflectivity multilayer mirrors with reliable longevity in synchrotron environment. It is also crucial to limit their absorption in order to optimize the extracted power required for most of the applications. Indeed, the front mirror of the laser cavity receives not only the first harmonic where the lasers operates but all the synchrotron radiation emitted by the undulator: a wide spectrum extending towards X rays. These short wavelengths are responsible for the mirror degradation which results from changes in the coating materials (high induced absorption, color centers, heating…) as well as from carbon contamination due to cracked hydrocarbons originating from the residual vacuum atmosphere. Deposition technologies which allow the manufacture of very dense oxide coatings with low absorption and high reflectivity in UV spectral region were optimized and characterized for this purpose. We report here degradation studies performed on UV mirrors for Storage Ring Free Electron Lasers down to wavelengths as short as 200 nm. Thus, reactive electron beam evaporation (REB), Ion plating (IP) but mainly Plasma and Ion Assisted Deposition techniques providing dense films of low absorption also in the UV region close to the electronic band gap of the only useful oxide materials (SiO2, Al2O3, HfO2) are employed. HfO2/SiO2 (for use down to wavelengths of about 250 nm) have been deposited by plasma ion assisted deposition (PIAD). The changes of their optical and structural properties caused by the irradiation, have been characterized by VUV - and UV - spectrophotometry, X ray diffraction, absorption calorimetry and total losses measurements. The degradation tests have been performed at the synchrotron facilities Super ACO (Orsay), Elettra (Trieste) and Delta (Dortmund) using irradiation set ups installed under ultra high vacuum. The comparative study of the degradation behavior of same batch mirrors irradiated at different conditions has been performed, which allows to discriminate between the different deposition parameters, to find optimal deposition conditions and to give information on the degradation phenomena which occur under extreme environmental conditions.

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