Shattered Beamlet Lens Blamed on Timing Error

Kathleen G. Tatterson

LIVERMORE, Calif. -- A high-power experiment on Lawrence Livermore National Laboratory's Beamlet laser destroyed one of the setup's lenses and damaged another. No injuries were reported, but officials say that the September incident caused $70,000 in damage.

Scientists were operating the laser at a 15-kJ, 20-ns pulse -- the highest power yet tested on the prototype laser -- when the 61-cm-diameter, 3.5-cm-thick lens shattered. Investigators reported that the operators used incorrect parameters and mistimed the operation. The errors caused a phenomenon called stimulated Brillouin scattering; the mistimed bandwidth amplified the light to an intensity too great for the lenses to handle.

Researchers plan to replace the damaged lenses with square ones that will stand up to high-power use. At press time, laboratory officials expected to have the Beamlet back online by mid-November.

The incident occurred just 15 months after a similar mishap damaged the Beamlet in the same position in the laser chain. Investigators blamed that incident on pre-existing lens damage.

Inspections before the latest occurrence indicated no pre-existing damage. The laboratory had implemented protective measures based on the lessons learned from the April 1995 incident, and those measures were in place at the time of the most recent experiment, said E. Michael Campbell, associate director for lasers.

Precautions taken

The Beamlet is a working prototype of the laboratory's National Ignition Facility (NIF), a proposed $1 billion frequency-tripled, Nd:glass laser system comprising 192 identical beams similar to the Beamlet (Photonics Spectra, September 1996, p. 34). Despite the setbacks, researchers still expect the facility to go online in September 2002.

Campbell said that the laboratory's Nova laser has automatic checking functions that regulate the timing of bandwidth added to the laser beam, and that the proposed ignition facility will as well, preventing a repeat of the Beamlet accident.

"The NIF design has always had the automatic system, and we will install a similar system on the Beamlet," he said. "This incident is not grounds to question Beamlet or NIF's safety, environmental impact or performance."