30 August 2005 Radiation damages to amorphous-carbon optical coatings
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The multi-mJ, 21-nm soft-x-ray laser at the PALS facility was focused on the surface of amorphous carbon (a-C) coating, developed for heavily loaded XUV/x-ray optical elements. AFM (Atomic Force Microscopy) images show 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 consequences of prolonged XUV irradiation at relatively low fluence. High-order harmonic (HH) beam generated at the LUCA facility in CEA/Saclay Research Center was used as a source of short-wavelength radiation delivering high-energy photons on the surface at a low single-shot fluence but with high-average power. a-C irradiated at a low fluence, i.e., < 0.1 mJ/cm2 by many HH shots exhibits an expansion for several nanometers. Although it is less dramatic change of surface morphology than that due to single-hot x-ray-laser exposure even the observed nanometer-sized changes caused by the HH beam on a-C surface could influence reflectivity of a grazing incidence optical element. These results seem to be important for estimating damages to the surfaces of highly irradiated optical elements developed for guiding and focusing the ultraintense XUV/x-ray beams provided by new generation sources (i.e., VUV FEL and XFEL in Hamburg; LCLS in Stanford) because, up to now, only melting and vaporization, but not graphitization, have been taken into account.
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L. Juha, M. Bittner, M. De Grazia, J. Feldhaus, J. Gaudin, S. Guizard, S. Jacobi, M. Kozlova, J. Krasa, J. Krzywinski, H. Merdji, C. Michaelsen, T. Mocek, R. Nietubyc, M. Jurek, J. Polan, A. R. Prag, B. Rus, R. Sobierajski, B. Steeg-Keitel, M. Stoermer, M. Stupka, V. Vorlicek, J. Wiesmann, and J. Wild "Radiation damages to amorphous-carbon optical coatings", Proc. SPIE 5917, Fourth Generation X-Ray Sources and Optics III, 59170F (30 August 2005); doi: 10.1117/12.617125; https://doi.org/10.1117/12.617125

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