Storage Ring Free Electron Laser (FEL) are attractive, full of promise, tuneable and powerful laser sources for the UV range. High reflectivity dielectric mirrors should be produced in order to allow lasing at very short wavelength, with a long stability in a strongly harsh environment and to optimize the extracted FEL power required for most of the newest applications. The front mirror of the laser cavity receives all the synchrotron radiation (SR) emitted by the wiggler, which is the responsible for the mirror degradation, combined with the contamination by the vacuum residuals. We are tackling the problem of tests and manufactures of reliable robust mirrors and explore themes such as resistance analysis of UV mirrors to FEL multiscale power, broadband (X-UV) mirror robustness. Under drastic SR conditions, multiscale wavelength damages could be observed. Specific measurement techniques, able to investigate localised spatial modification induced by the non uniform synchrotron radiation are presented. A local crystalline structure modification of the high index material appears together with a severe increase of the roughness. © 2001 SPIE–The International Society for Optical Engineering.

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