Issue Date: March 30, 2009
Optics Needs Of NIF Required New Production And Coating Methods
The laser jocks and engineers who designed and built the National Ignition Facility, located at Lawrence Livermore National Laboratory (LLNL), didn't go straight from a laser table to the stadium-esque proportions of NIF. Several smaller facilities have been the training grounds, including the Omega laser at the University of Rochester; the Central Laser Facility at the Rutherford Appleton Laboratory, in England; and the Gekko laser system at Osaka University's Institute of Laser Engineering, in Japan. But NIF represents about a 100-fold increase in power over existing laser facilities, and with that came several materials challenges.
Among the problems confronting LLNL chemical engineers was the need to grow crystals of potassium dihydrogen phosphate (KDP)—but not just any crystals. To meet NIF needs, the KDP must be large enough to slice into 40- by 40-cm sheets. KDP optics are used at NIF to change the polarization of the light, to orient the polarization of the laser beams, and to convert the beams from infrared to ultraviolet light before they hit the target. KDP normally grows at a rate of 1 mm per day, which means it would take two years to grow the 350-kg crystals needed for NIF, says Marcus Monticelli, a chemical engineer at LLNL.
Monticelli and colleagues adapted techniques pioneered in Russian laboratories to speed up crystal growth. A synthetic seed crystal is placed in a 6-foot-tall tank of supersaturated KDP solution. The tank is then cooled and constantly rotated to speed up mass transport and prevent crystal inclusions. A small amount of impurities, principally group 2 or 3 cations, is added to force the crystal to grow up instead of out, with an eye toward getting the size and orientation needed to cut the optics. The process is all finely tuned to generate an 800-lb crystal in a mere two months with as few imperfections as possible. Rotating too quickly or too slowly, adding too many or too few cations, or allowing the crystal to grow too quickly could yield unusable KDP. "The issue is getting that sweet spot," Monticelli says. LLNL expects that about 30 KDP optics will need to be replaced every year.
Other optics components in NIF include neodymium-doped phosphate laser glass used to amplify the beam. A special manufacturing process was developed by Hoya Corp., in Fremont, Calif., and Schott Glass Technologies, in Duryea, Pa., to pour a continuous slab of the glass, which is then cut down and polished. There are roughly two miles of the laser glass in NIF, cut to 2,000 slabs about 3–4 feet long, all with impurities controlled to about 1 ppm.
Although every effort was made to construct NIF as cleanly as possible, there are limits to how clean one can keep a facility that covers the area of three football fields. That presented additional challenges for the optics components, says James Fair, a chemical engineer at LLNL. Fair helped develop coatings that would repel dust and dirt, as well as be antireflective and withstand the lasers. The coatings are made through carefully controlled sol-gel processes to yield microporous silica with evenly sized silica nanoparticles. Functionalized coatings may also be applied to the nanoparticles to add aliphatic groups to the silica. "The inorganic nanospheres form the backbone, which is covered with an organic coating that is resistant to nonpolar contaminants," Fair says.
Fair adds that as hard as optical manufacturers may try, no material is perfect. Consequently, all defects in optical materials or coatings at NIF are identified and tracked over time. A better understanding of how imperfections evolve with exposure to the lasers may play into designing better manufacturing processes, materials, and coatings, for NIF and other stadium-sized laser facilities under construction or being planned in Europe and Asia, Fair says.
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