Laser induced damage and fracture in fused silica vacuum windows

Laser induced damage, that initiates catastrophic fracture, has been observed in large (up to 61-cm diameter), 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 (tensile) side of the lens. The damage can lead to catastrophic crack growth if the flaw (damage) size exceeds the critical flaw size for SiO2. If the elastic stored energy in the lens is 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 (i.e. tensile stress) 2) the ratio of the window thickness to flaw depth and 3) secondary crack propagation. Fracture experiments have been carried out 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 used to develop design criteria for a ''fail-safe'' lens (that is, a lens that may catastrophically fracture but will not implode). Specifically the window must be made thick enough such that the peak tensile stress is less than 500 psi (3.4 MPa) and the corresponding ratio of the thickness to critical flaw size is less that 6. Under these conditions a properly mounted window, upon failure, will break into only two pieces and will. not implode. One caveat to these design criteria is that the air leak through the window fracture and into the vacuum must be rapid enough too reduce the load on 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 (i.e. secondary) crack growth.