Stephen M. Maricle

Methods for mitigating surface damage growth in NIF final optics

We report a summary of the surface damage, growth mitigation effort at 3(omega) for fused silica optics at LLNL. The objective was to experimentally validate selected methods that could be applied to pre-initiated or retrieved-from- service optics, to stop further damage growth. A specific goal was to obtain sufficient data and information of successful methods for fused silica optics to select a single approach for processing NIF optics. This paper includes the test results and the evaluation thereof, for several mitigation methods for fused silica. The mitigation methods tested in this study are wet chemical etching, cold plasma etching, CO2 laser processing, and micro-flame torch processing. We found that CO2 laser processing produces the most significant and consistent results to halt laser-induced surface damage growth on fused silica. We recorded successful mitigation of the growth of laser-induced surface damage sites as large as 0.5-mm diameter, for 1000 shots at fluences in the range of 8 to 13 J/cm2. We obtained sufficient data for elimination of damage growth using CO2 laser processing on sub-aperture representative optics, to proceed with application to full- scale NIF optics.

CO2-laser polishing for reductoin of 351-nm surface damage initiation in fused silica

We have applied a carbon dioxide (CO2) raster scanning laser polishing technique on two types of fused silica flat optics to determine the efficacy of CO2-laser polishing as a method to increase the 351-nm laser damage resistance of optic surfaces. R-on-1 damage test results show that the fluence for any given 355-nm damage probability is 10-15 J/cm2 higher (at 3 ns pulse length, scaled) for the CO2-laser polished samples. Poor quality and good quality surfaces respond to the treatment such that their surface damage resistance is brought to approximately the same level. Surface stress and the resultant effect on wavefront quality remain key technology issues that would need to be addressed for a robust deployment.

UV-laser conditioning for reduction of 351-nm damage initiation in fused silica

This paper describes the effect of 355-nm laser conditioning on the concentration of UV-laser-induced surface damage sites on large-aperture fused silica optics. We will show the effect of various 355-nm laser conditioning methodologies on the reduction of surface-damage initiation in fused silica samples that have varying qualities of polishing. With the best, generally available fused silica optic, we have demonstrated that 355-nm laser conditioning can achieve up to 10x reduction in surface damage initiation concentration in the fluence range of 10-14 J/cm2 (355- nm at 3 ns).

Laser damage performance of fused silica optical components measured on the beamlet laser at 351 nm

A statistics-based model is being developed to predict the laser-damage-limited lifetime of UV optical components on the NIF laser. in order to provide data for the model, laser damage experiments were performed on the Beamlet laser system at LLNL. Three prototype NIF focus lenses were exposed to 351 nm pulses during four experimental campaigns, each consisting of 23 to 38 pulses at NIF relevant fluences. Each lens was sol-gel AR coated and all laser exposures were performed in a vacuum environment. Through inspections of the lens before, during and after the campaigns, pulse-to- pulse damage growth rates were measured for damage initiating both on the surfaces and at bulk inclusions. Radial growth rates measured for rear surface damage was typically 10x higher than that measured in the bulk or at the front surface. No significant correlation of growth rate to precursor type was indicated. For 5 J/cm2, 3 ns pulses the typical radial growth rate was nominally 20 micrometers /pulse. Average growth rates measured on three lenses made by two manufacturers were in good agreement. While the growth rate clearly increased with fluence, the data obtained was insufficient to quantify the dependence. The growth rates reported here were 20x-50x higher than values predicted from off-line studies of bare surfaces in air.

Effect of thermal annealing and second harmonic generation on bulk damage performance of rapid-growth KDP type-I doublers at 1064 nm

This paper discusses the results of thermal annealing and in-situ second harmonic generation (SHG) damage tests performed on six rapid growth KDP type 1 doubler crystals at 1064 nm (1(omega) ) on the Zeus automated damage test facility. Unconditioned (S/1) and conditioned (R/1) damage probability tests were performed before and after thermal annealing, then with and without SHG on six doubler crystals from the NIF-size, rapid growth KDP boule F6. The tests revealed that unannealed, last-grown material from the boule in either prismatic or pyramidal sectors exhibited the highest damage curves. After thermal annealing at 160 degree(s)C for seven days, the prismatic sector samples increased in performance ranging from 1.6 to 2.4X, while material from the pyramidal sector increased only modestly, ranging from 1.0 to 1.3X. Second harmonic generation decreased the damage fluence by an average of 20 percent for the S/1 tests and 40 percent for R/1 tests. Conversion efficiencies under test conditions were measured to be 20 to 30 percent and compared quite well to predicted behavior, as modeled by LLNL frequency conversion computer codes.

Development of practical damage-mapping and inspection systems

We have developed and are continuing to refine semi- automated technology for the detection and inspection of surface and bulk defects and damage in large laser optics. Different manifestations of the DAMOCLES system provide an effective and economical means of being able to detect, map and characterize surface and bulk defects which may become precursors of massive damage in optics when subjected to high-fluence laser irradiation. Subsequent morphology and evolution of damage due to laser irradiation can be tracked efficiently. The strength of the Damocles system is that it allows for immediate visual observation of defects in an entire optic, which can range up to 1-meter dimensions, while also being able to provide digital map and magnified images of the defects with resolutions better than 5 micrometers .

Automated damage test facilities for materials development and production optic quality assurance at Lawrence Livermore National Laboratory

The Laser Program at LLNL has developed automated facilities for damage testing optics up to 1 meter in diameter. The system were developed to characterize the statistical distribution of localized damage performance across large- aperture National Ignition Facility optics. Full aperture testing is a key component of the quality assurance program for several of the optical components. The primary damage testing methods used are R:1 mapping and raster scanning. Automation of these test methods was required to meet the optics manufacturing schedule. The automated activities include control and diagnosis of the damage-test laser beam as well as detection and characterization of damage events.

Laser conditioning methods of hafnia-silica multilayer mirrors

Large aperture multilayer hafnia silica high reflector coatings at 1064 nm, deposited by reactive electron-beam deposition, were prepared to examine different laser conditioning methods for manufacturing high fluence optics in the National Ignition Facility. Laser conditioning is a process where the damage threshold of the coating is increased or the damage that is created is minimized so that it does not grow upon further irradiation. Two laser conditioning methods were examined for coatings deposited from only oxide starting materials. Off-line laser conditioning consists of raster scanning a mirror past a 1 mm diameter Gaussian beam over the entire clear aperture; a process that takes approximately 24 hours per scan. On-line laser conditioning consisted of a large aperture 300 mm X 300 mm beam from the Beamlet laser that irradiated the entire full clear aperture of a series of mirrors; a process that was limited by a 2 - 4 hour shot rate. In both cases a six-step process was used with the mirror first irradiated at a low fluence, then successively higher fluences increased in equal increments up to the peak laser operating fluence. Mirrors that were only partially laser conditioned damaged catastrophically while fully conditioned mirrors survived fluences exceeding the safe operating Beamlet fluence. An alternative off-line laser conditioning method was examined for coatings deposited from hafnia or metallic hafnium sources. Single-step laser conditioning consists of off-line raster scanning an optic at the peak operating fluence, thus decreasing the laser conditioning cost by reducing the number of scans and required laser conditioning stations to process all the mirrors for the National Ignition Facility. Between pulses the optic is stepped approximately one fourth of the 1/e2 Gaussian beam diameter so each area of the coating is irradiated by different segments of the beam starting at a low fluence at the outer edge of the beam diameter and increasing to the peak fluence in the center of the beam. The one-step conditioning results appear positive, but the influence of the coating improvements due to the metallic hafnium process on laser conditioning is undefined.

3-ω damage threshold evaluation of final optics components using Beamlet Mule and off-line testing

A statistics-based model is being develop to predict the laser-damage-limited lifetime of UV optical components on the NIF laser. In order to provide data for the mode, laser damage experiments were performed on the Beamlet laser system at LLNL. An early protoype NIF focus lens was exposed to twenty 351 nm pulses at an average fluence of 5 J/cm2, 3ns. Using a high resolution optic inspection inspection system a total of 353 damage sites was detected within the 1160 cm2 beam aperture. Through inspections of the lens before, after and, in some cases, during the campaign, pulse to pulse damage growth rates were measured for damage initiating both on the surface and at bulk inclusions. Growth rates as high as 79 micrometers /pulse were observed for damage initiating at pre-existing scratches in the surface. For most damage sites on the optic, both on the surface and at bulk inclusions. Growth rates as high as 79 micrometers /pulse were observed for damage initiating at per- existing scratches in the surface. For most damage sites on the optic, both surface and bulk, the damage growth rate was approximately 10(Mu) m/pulse.