Total reflection amorphous carbon mirrors for vacuum ultraviolet free electron lasers

Sputter-deposited amorphous carbon coatings of high optical quality and very high radiation stability will be used as reflecting optical elements for free electron laser (FEL) applications in the vacuum ultraviolet (VUV) and soft x-ray spectral regions. Their reflectivity at grazing angles of 2° is typically 94–96% for photon energies between 50 and 250 eV. Using the focused radiation (λ=98 nm) of the VUV FEL at Deutsches Elektronen-Synchrotron DESY the damage threshold of such coatings was determined to 0.07 J/cm2. Ripples with a spatial period of 76 nm were found on the surfaces damaged by the FEL radiation.