Laser-induced damage and fracture in fused silica vacuum windows

Jack H. Campbell; Patricia A. Hurst; Dwight D. Heggins; William A. Steele; Stanley E. Bumpas

doi:10.1117/12.274227

13 May 1997 Laser-induced damage and fracture in fused silica vacuum windows

Jack H. Campbell, Patricia A. Hurst, Dwight D. Heggins, William A. Steele, Stanley E. Bumpas

Author Affiliations +

Proceedings Volume 2966, Laser-Induced Damage in Optical Materials: 1996; (1997) https://doi.org/10.1117/12.274227
Event: Laser-Induced Damage in Optical Materials: 1996, 1996, Boulder, CO, United States

Abstract

Laser induced damage, that initiates catastrophic fracture, has been observed in large, fused silica lenses that also serve as vacuum barriers in high-fluence positions on the Nova and Beamlet lasers. In nearly all cases damage occurs on the vacuum side of the lens. The damage can lead to catastrophic crack growth if the flaw size exceeds the critical flaw size for SiO₂. If the elastic stored energy in the lens in high enough, the lens will fracture into many pieces resulting in an implosion. The consequences of such an implosion can be severe, particularly for large vacuum systems. Three parameters control the degree of fracture in the vacuum barrier window: (1) the elastic stored energy, (2) the ratio of the window thickness to flaw depth and (3) secondary crack propagation. Fracture experiments have ben carried our on 15-cm diameter fused silica windows that contain surface flaws caused by laser damage. The results of these experiments, combined with data from window failures on Beamlet and Nova have been sued to develop design criteria for a 'fail-safe' lens. Specifically the window must be made thick enough such that the peak tensile stress is less than 500 psi and the corresponding ratio of the thickness to critical flaw size is less than 6. Under these conditions a properly mounted window, upon failure, will break into only tow pieces and will not implode. One caveat to these design criteria is that the air leak through the window before secondary crack growth occurs. Finite element stress calculations of a window before and immediately following fracture into two pieces show that the elastic stored energy is redistributed if the fragments 'lock' in place and thereby bridge the opening. In such cases, the peak stresses at the flaw site can increase leading to further crack growth.

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Jack H. Campbell, Patricia A. Hurst, Dwight D. Heggins, William A. Steele, and Stanley E. Bumpas "Laser-induced damage and fracture in fused silica vacuum windows", Proc. SPIE 2966, Laser-Induced Damage in Optical Materials: 1996, (13 May 1997); https://doi.org/10.1117/12.274227

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