Continuous-Melt Process Yields Rare Optical Glass for U.S. Lab

The National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory (Livermore, CA) will require thousands of kilograms of high-optical quality, neodymium-doped, phosphate laser glass in its program to use the world's largest laser to heat fusion fuel to thermonuclear ignition. But an economical way of making the glass has been hard to find. Now, two U.S. manufacturers have developed a continuous-melt process that promises to drastically cut the costs of the rare glass, which had previously been made only in batches.

SCHOTT North America, Inc. (Duryea, PA), one of two laser-glass vendors working on the NIF project, has successfully demonstrated the new process. The second vendor, Hoya Corp. (Fremont, CA), is scheduled to begin similar glass melting operations within the next month. To date, Schott has produced more than 250 of the roughly meter-sized glass slabs needed for NIF's demanding optical specifications. In addition, Livermore reports that the glass has been produced at a rate 20 times faster than could be achieved using the existing one-slab-at-a- time batch melting technology.

More than 3,500 laser glass slabs will be needed for the NIF unit. Each slab is about 80 cm long, 45 cm wide, 4 cm thick, and weighs approximately 100 pounds. The costs for the development of the continuous melting process has been shared equally between Livermore and the French Commissariat a L'Energie Atomique (CEA). The CEA plans to purchase a similar quantity of slabs for their Laser Megajoule that will begin construction later this decade.

A Schott Glass Technologies official examines a pallet of laser glass slabs produced by a continuous-melt process for Livermore's National Ignition Facility.

While initial attempts by Schott and Hoya in 1999 demonstrated that continuous melts could occur, certain glass specifications were not achieved at that time. In particular, the glass product contained trace quantities of hydroxyl group contamination that originated from small amounts of moisture in the surrounding air and in the initial glass raw materials. Attempts to remove the moisture-derived contamination in the glass degraded the other glass properties. But over the past six months, Livermore, Schott, and Hoya have carried out cooperative research specifically aimed at moisture contamination. Schott's current melting effort is said by Livermore to be the first successful demonstration of this research and improved technology. Both vendors will begin production runs using this technology in early 2001.

The NIF, currently under construction at Livermore, is one of the cornerstones of the U.S. Dept. Energy's Stockpile Stewardship Program. Under this program, the fusion experiments at the facility will be used to help sustain confidence in the U.S. military's nuclear weapons stockpile without actual testing the weapons. The facility will also produce additional benefits in basic science and fusion energy, says Livermore.

Edited by Gordon Graff