Figure 9. Transmittance curves for (a) a glass substrate, (b) 15 A film of thermally evaporated silver on a glass substrate. % TRANSMISSION Figure 11. SEM micrograph showing the type of coverage that can be obtained by spin coating 0.3 um diameter polystyrene spheres onto a glass substrate. The monolayer sphere coverage is 94%. MAX=100.00 T Figure 12. Transmittance curves for a continuous VO film annealed in air for 2.0 min at 480°C. Figure 13. SEM micrograph of an annealed VO, film after the polystyrene spheres were lifted off and the VO film was annealed in air. Figure 14. Transmittance spectra for island-type structures of VO2 on a glass substrate. These structures were produced by depositing VO on a polystyrene/glass substrate, lifting off the polystyrene spheres, and annealing the VO in air to form VO,. Manuscript Received Structural Damage and Analysis of the Nova Final Focusing Lenses John H. Pitts Lawrence Livermore National Laboratory Livermore, California 94550-0618 Two types of damage (cracking and pitting) have been observed on the Nova Pitting occurs because laser light is diffracted and scattered around an apodized groove, concentrating in an annular region on the vacuum face. It also occurs at random locations when contaminants, adhering to the lens vacuum face, absorb laser light. In both cases, thermal stresses produce pitting; however, this pitting is not of structural concern because any flaws exceeding the fracture toughness of the glass propagate toward the vacuum face where the highest tensile stresses in the lenses exist. Key Words: cracking; fracture toughness; laser damage; lenses; Nova; pitting; The final focusing lenses of the Nova laser concentrate up to 100 kJ of energy on fusion targets placed at the center of a target chamber (see fig. 1). Because the target chamber is evacuated, these lenses also function as a vacuum barrier. Under normal laser operation, a pressure difference of 100 kPa exists across these lenses; this pressure difference produces tensile stresses in the lenses. We routinely inspect these lenses because the glass is brittle and these tensile stresses could cause lens failure. If failure occurred, it could result in extensive damage to expensive equipment. During these routine inspections, we noted two forms of damage on the vacuum (tension) face of the lenses. First, we found a 30-mm-long crack at the center of the vacuum face of one lens. This damage occurred during a high-yield shot. We removed this lens immediately and pressure tested it to destruction [1]. Failure occurred when a 150-kPa pressure difference was placed across the lens for 100 minutes, which is only 1.5 times the magnitude of the pressure difference under normal operating conditions. We then made and tested four chevron-notched fracture toughness specimens from one of the remaining pieces of glass, and we found that the minimum fracture toughness [2] was 770 kPa ✔m with an average of 820 kPa m. We also performed one flexure test and found the tensile strength to be 39 MPa. These results compare favorably with published values [3,4] of 750 kPa m and 49 MPa, respectively. We originally thought that the lens cracked because the back reflection of laser light from a calorimeter concentrated laser energy at the damaged location. However, Smith et.al. [5] later showed that the damage was caused by acoustic waves that were generated when stimulated Brillouin scattering was produced in the lens. The wave began as a compression wave and then propagated to the periphery of the lens where it reflected back as a tension wave. The magnitude of the tension wave increased as the wave focused on the lens axis, where it produced stress levels that were high enough to crack the glass. |