Radiation damage to amorphous-carbon optical coatings

The multi-mJ, 21-nm soft-x-ray laser at the PALS facility was focused onto the surfaces of amorphous carbon (a-C) coatings, developed for heavily loaded XUV/x-ray optical elements. AFM (Atomic Force Microscopy) images show a 3-micrometer expansion of the irradiated material. Raman spectra, measured with an Ar⁺ laser microbeam in both irradiated and unirradiated areas, confirm a high degree of graphitization in the irradiated layer. In addition to this highfluence (∼ 1 J/cm²), single-shot experiment, it was necessary to carry out an experiment to investigate the consequences of prolonged XUV irradiation at relatively low fluence. A high-order harmonic (HH) beam generated at the LUCA facility of the CEA/Saclay Research Center was used as a source of short-wavelength radiation delivering high-energy photons to the surfaces at a low single-shot fluence but with high-average power. a-C irradiated at low fluence, (< 0.1 mJ/cm²) by many HH shots exhibits an expansion of several nanometers. Although it is a less dramatic change of surface morphology than that due to single-shot x-ray-laser exposures, even the observed nanometer-sized changes caused on an a-C surface by an HH beam could influence the reflectivity of a grazing incidence optical element. These results seem to be important for estimating damage to surfaces of highly irradiated optical elements developed for guiding and focusing the ultra-intense XUV/x-ray beams provided by the new generation of sources (VUV FEL and XFEL in Hamburg; LCLS in Stanford) because, up to now, only melting and vaporization, and not graphitization, have been taken into account.