NIF’s ability to recycle optical components in the ultraviolet (UV) section of the NIF laser system is an integral aspect of the NIF design that allows it to operate above the laser damage threshold of the UV final optics, a unique capability unlike any other laser in the world.
Sustaining high-energy, high-power operation at an increasing shot rate is enabled by a number of advances developed specifically for the NIF final optics. Two technologies developed for fused silica final optics have greatly improved their laser damage resistance: improved grinding and polishing processes to reduce the number of microfractures, or damage precursors, on the optic’s surface; and a novel buffered oxide etch process called the Advanced Mitigation Process (see “Keeping NIF Optics on the Job”).
In addition, NIF incorporates a final optics recycle infrastructure—the Loop—that allows the operations team to monitor, exchange, repair, and rapidly return these optics to service as part of routine operations (see Spaeth, et al., “Optics Recycle Loop Strategy For NIF Operations Above UV Laser-Induced Damage Threshold,” accepted for publication in Fusion Science and Technology, 2015).
NIF recycles two types of fused silica final optics—the wedged focus lenses (WFLs) and the grating debris shields (GDSs). At current operating levels the installed population of these optics is exchanged at the rate of about 2.5 times per year. Individual optics can pass through the Loop as many as ten times before they are sent back to the manufacturer for refinishing.
The NIF Optics Team reached a significant milestone on June 9 when the Optics Mitigation Facility (OMF) in Bldg. 391 repaired the 5,000th optic since the Loop process was launched in August of 2009. During this time it is estimated that NIF delivered approximately 250 megajoules of additional ultraviolet energy on target over the amount that would have been achieved if the shot energy were limited to levels that could be sustained without the Loop process.
Commenting on the 5,000th optic milestone, Optics Loop Manager Jim Folta said, “Congratulations not only to OMF, but to all of Optics Processing! Thank you for working so hard to make the optics recycle loop a success, which allows NIF to achieve its missions. I’m proud to work with you all.”