Characterization of micron scale periodic fluence variation and its possible impact on laser damage

Cones machined into the surface of the final fused silica optics on the NIF have been used to remove laser induced damage from exposure to high fluence 351 nm laser light. When applied to the input surface of an optic, a shadow is created on the exit surface due to the divergence of the laser light by the cone walls. In recent years input surface cones have been utilized to shadow exit surface damage and thus arrest its continued growth. The expanding waves from the cone walls interfere with the incident beam to create a high fluence intensification at the exit surface. This intensification has the characteristic periodic spatial variation on a scale of the order of the 351 nm wavelength. The question arises as to how the damage density probability, ρ(Φ), is affected by this variation as compared to a uniform fluence. Does it follow the local periodic variation, or is it averaged over that variation. We consider both cases, how it can be predicted by direct measurement of the intensification as opposed to costly damage tests, and how we might measure the effect directly.