30 May 2003 Influence of BK7 substrate solarization on the performance on hafnia and silica multilayer mirrors
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Transport mirrors within the National Ignition Facility, a 192-beam 4-MJ fusion laser at 1053 nm, will be epxosed to backscattered light from plasmas created from fusion targets and backlighters. This backscattered light covers the UV and visible spectrum from 351 - 600 nm. The transport mirror BK7 substrates will be intentionally solarized to absorb >95% of the backscattered light to prevent damage to the metallic mechanical support hardware. Solarization has minimal impact on the 351- 1053-nm laser-induced damage threshold or the reflected wavefront of the multilayer hafnia silica coating. Radiation sources of various energies were examined for BK7 darkening efficiency within the UV and visible region with 1.1 MeV gamma rays from a Cobalt 60 source ultimately being selected. Finally, bleaching rates were measured at elevated temperatures to generate a model for predicting the lifetime at ambient conditions (20°C), before solarized BK7 substrates exceed 5% transmission in the UV and visible region. Over a 30-mm thickness, BK7 glass will bleach in 10 years to 5% transmission at 600 nm, the most transmissive wavelengths over the 351 - 600 nm regions.
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Christopher J. Stolz, Christopher J. Stolz, Joseph A. Menapace, Joseph A. Menapace, Francois Y. Genin, Francois Y. Genin, Paul R. Ehrmann, Paul R. Ehrmann, Philip E. Miller, Philip E. Miller, Gregory T. Rogowski, Gregory T. Rogowski, } "Influence of BK7 substrate solarization on the performance on hafnia and silica multilayer mirrors", Proc. SPIE 4932, Laser-Induced Damage in Optical Materials: 2002 and 7th International Workshop on Laser Beam and Optics Characterization, (30 May 2003); doi: 10.1117/12.472052; https://doi.org/10.1117/12.472052

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