Performance of multiple cone arrays as shadow cone blockers on NIF

Abstract

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.