Laser interplay stokes fusion uncertainty.

Fusion researchers are skewering laser beams with laser beams to see how the interactions might affect laboratory efforts to spark nuclear fusion. Two research groups this week report that criss-crossing lasers can squander or misplace energy that fusion labs will need to set off wee thermonuclear blasts.

Many beams will intersect in a pair of billion-dollar lasers to be built in the next decade. The 192-beam National Ignition Facility at Lawrence Livermore (Calif.) National Laboratory (LLNL) and the 240-beam Laser Megajoule at Bordeaux, France, will support nuclear weapons and energy research (SN: 10/19/96, p. 254). Fired into the ends of a gold cylinder containing a BB-size hydrogen fuel pellet, overlapping beams will generate heat. This heat creates a uniform bath of X-rays that will implode the pellet.

The beams also rip electrons from gas molecules inside the cylinder, creating a hot cloud of electrons and ions, or plasma, which sometimes reflects beams away or otherwise interferes with target illumination.

In experiments that overlap two or three beams in a plasma, researchers found an unexpected rise in stimulated Raman scattering, an energy-sapping laser-plasma interaction. "This is very bad for fusion," says team leader Christine Labaune of the Ecole Polytechnique's intense laser laboratory in Palaiseau, France, who worked with researchers from LLNL and the University of Alberta in Edmonton. However, another undesirable interaction diminished.

Labaune described her findings this week at the annual meeting of the Division of Plasma Physics of the American Physical Society (APS) in New Orleans.

At the same meeting, a team of U.S. government, university, and private scientists reported that, under some conditions, about one-tenth of a beam's energy can leap to a sister beam when identical-wavelength lasers intersect in a plasma. Slightly tweaking laser colors will probably make the problem moot, explains Kenneth B. Wharton of LLNL, who led the team. Otherwise, such energy shifts could cause uneven pellet irradiation and a fusion fizzle, he says.

Understanding multiple-laser effects is vital since interactions such as Raman scattering pose a challenge to laboratory fusion, says Barrett Ripin of APS in College Park, Md., who heads a committee that advises LLNL on laser fusion. However, the problem of energy transfer between beams described in Wharton's "beautiful piece of scientific work" seems surmountable, he adds.