Keeping NIF Optics on the Job

NIF is the largest optical instrument ever built, with a total optical surface area of eight-tenths of an acre—about 40 times the surface area of the giant Keck telescope in Hawaii. For the past two decades, LLNL researchers have been diligently pursuing better ways of preventing or mitigating laser damage to the optics in the ultraviolet section of the laser—about five percent of the total number of large optics in the system—in order to extend their lifetimes.

The infrared light, known as one-omega or 1ω, generated in NIF’s main laser system is tripled in frequency to ultraviolet (3ω) light in the final optics assemblies (see Final Optics). The wedged focus lenses that direct NIF’s 192 beams to the center of the Target Chamber are made of fused silica, due to its high ultraviolet light transparency, high damage threshold, and excellent surface quality. Fused silica can be damaged, however, when microscopic defects introduced during manufacturing and processing, called damage precursors, are exposed to NIF’s ultra-high ultraviolet laser energies. When the defects absorb and transfer energy to the optic, they can initiate damage sites that grow with subsequent laser shots and eventually could render the optic inoperative.

Years of study have produced a number of techniques for preventing damage precursors and for repairing damaged optics so they can be returned to service. The advanced mitigation process (AMP), for example, uses optimized hydrofluoric acid to remove the defect layers found on fractured surfaces associated with fine scratches or impurities. AMP significantly reduces damage sites caused by precursors, primarily at lower fluences (energies per unit area)—less than about 10 joules per square centimeter.

The causes of damage at higher energies, however, have been harder to diagnose, especially in situations where multiple laser shots are fired in rapid succession (see "Probing Energy Loss in High-Rep-Rate Lasers"). Recent research has revealed that precipitates of trace impurities found in processing chemicals are the most significant damage precursors in the high-fluence range. The work has demonstrated that by minimizing the presence of precipitates during chemical processing and drying, the damage produced by high-fluence shots can be reduced by more than two orders of magnitude. The new process, called AMP3, could significantly extend the functional lifetime of NIF optics, helping to further reduce operational costs and improve performance.

AMP3 also enables a 60 percent increase in the amount of energy that can pass through an optic without damage. Recent results show a continued progression toward zero damage through a reduction of 2,000 precursors in damage density, which doubles the damage threshold. Scaling of the AMP3 process to full-size NIF optics is now under way.

For more information, see "Sleuthing an Optical Mystery," Science & Technology Review, January-February 2015; and "Enhanced Damage Resistance for NIF Optics," Science & Technology Review, September 2011.

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