The National Ignition Facility (NIF) is intentionally operated with the final fused silica glass (SiO2) optics exposed to fluences and intensities with the potential to induce damage that will grow with additional laser exposure. Therefore, the NIF operates a recycle loop to refurbish final optics by mitigating any initiated surface damage to arrest its growth. However, the morphology of filamentary damage, caused by local self-focusing in the bulk of a silica optic, adds additional complexity to optics mitigation and provides a limitation to optic reusability. This study evaluates techniques for mitigating isolated and clustered filamentary damage. Optical microscopy before and after NIF installation was used to determine the efficacy of filamentary mitigation after a series of high fluence and intensity laser exposures. The challenges and success rate of the methods are compared for various filamentary damage mitigations strategies.
Exit surface damage on high value fused silica final optics on the NIF is sometimes too large to be mitigated with the currently used technique of removing the damage by CO2 laser machining a cone into the surface. To extend the service life of the optic, a 2 cm diameter shadow is created at the damage using a programmable spatial light modulator at the front end of the laser system. The use of this shadow technique is limited by the obscuration due to the large size of the shadow. An alternative approach is to create the shadow by machining a cone on the input surface opposite the damage. This reduces the shadow a rea, and thus the obscuration by several orders of magnitude. Additional benefits in service life of optics would be realized if the shadow cone size could be increased from current 600 m diameter. There are fabrication challenges encountered when the cone size is increased. To overcome this problem, the shadow performance of a hexagonal array of four 600 m diameter cones has been tested. We report on shadow leakage, bulk damage, and exit surface intensification issues presented by this array and techniques to address those issues.
In this work, an update will be provided on the deployment of shadow cone blockers on NIF grating debris shields. These cones have been demonstrated to reduce the fluence in excess of 11 J/cm2 of an incident 351 nm nanosecond-scale pulse to below the growth threshold of most damage sites. Furthermore, shadow cones will be characterized in terms of their transmission reduction and intensification at downstream surfaces, giving insight into the technique’s broader potential applications and limitations.
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