Abstract
Storage ring Free Electron Lasers (FELs) are very promising tunable and powerful laser sources for the UV range. However, the relatively small gain obtained in the UV, requires the use of high reflectivity multidielectric mirrors. In addition, all the synchrotron radiation (not only the first harmonic where the laser operates) emitted by the undulator, consisting of a wide spectrum of the harmonics of the fundamental wavelength extending towards the X rays is received by the front mirror of the laser cavity. These higher harmonics are responsible for the mirror degradation. The reflectivity of the multidielectric mirrors has to remain at a high level even in the presence of synchrotron radiation covering a wide spectral range, leading to specific requirements for FEL optics. The FEL at 350 nm was obtained with Ion Beam Sputtering of Ta<SUB>2</SUB>O5/SiO<SUB>2</SUB>, with super polished substrates. Extension towards shorter wavelength requires to use another type of material such as HfO<SUB>2</SUB>/SiO<SUB>2</SUB>. Substrates and mirrors are characterized in roughness, transmission and total losses. The evolution of the mirror degradation is studied versus the deposition technology. The realization of high reflectivity mirrors submitted to the radiation from the undulator remains a challenging issue for the development of tunable short wavelength FELs.
Published Version
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