Evaluation of alumina and silica coating materials for the NIF final turning mirrors

One third of the exchanged final turning mirrors on the National Ignition Facility have been laser damaged by 3ω (351nm) target backscattered light. The mirrors are spectrally complex hafnia and silica multilayer dichroic coatings with high 1ω reflection for the forward propagating beam and low 3ω reflection to suppress propagation of target backscatter. Alumina, a wider bandgap high index material than hafnia, was selected to improve the 3ω laser damage resistance despite a lower refractive index. A 2-4x improvement in 3ω LIDT was demonstrated with adequate 1ω laser damage resistance. Nodule ejection was the dominate laser damage morphology. An 18% decrease in the refractive index between alumina and hafnia results in coating designs that are 3x thicker. Similarly, a decrease in refractive index also decreases the angular bandwidth resulting in increasing the number of mirror types from two to six over an angular range of 16 degrees. The significant improvement in 3ω LIDT has justified beginning a scale up demonstration to full aperture (0.3m2).