Laser fusion comes into the open ... and takes another step.

For 30 years, researchers at the Lawrence Livermore (Calif.) National Laboratory have been investigating the possibility of using short, powerful pulses of laser light to initiate the fusion of atomic nuclei. But because the research was classified, scientists could release very little information about these experiments and their results.

In a major policy change, the Department of Energy announced last December that it will gradually declassify information related to its inertial-confinement fusion program. The decision allowed researchers at Livermore and elsewhere to begin describing their work at open meetings and publishing detailed results in journals.

Four papers in the Oct. 24 PHYSICAL REVIEW LETTERS now reveal details of experiments conducted over the last few years involving Livermore's immensely powerful Nova laser. The laser's light was used to create X rays for irradiating a tiny spherical capsule containing gaseous hydrogen fuel.

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In the Livermore scheme, the Nova laser generates 10 beams of ultraviolet light at a wavelength of 351 nanometers. These beams enter a special cylindrical vessel known as a Hohlraum and strike its interior walls, which are made of uranium. The interaction between the ultraviolet light and the uranium produces X rays, which irradiate the pinhead-size, fuel-containing capsule (see illustration).

The X rays vaporize the capsule's outer plastic coating, creating a plasma that pushes against the inner sphere of fuel. This action compresses the fuel, which typically consists of a mixture of deuterium and tritium gas, to extremely high densities at a temperature greater than 100 million [degree]C. In principle, such densities can get high enough to prompt the fusion of nuclei. The resulting nuclear reactions create helium nuclei and release energy.

In their four papers, the Livermore researchers describe experiments designed to test their understanding and control of the fusion process. They particularly wanted to check whether the X rays produced in the Hohlraum irradiated the capsule evenly enough to ensure that the fuel was compressed uniformly. They were also concerned about the possibility that instabilities during the compression phase could disrupt the process.

"These experiments ... allowed detailed comparisons to simulations and permitted a deeper understanding of the sensitivity of the implosion process to factors such as laser power balance," the researchers conclude. The results indicate that the approach developed at Livermore could be scaled up to achieve nuclear fusion.

... and takes another step

As the next step toward achieving nuclear fusion using lasers, the Department of Energy last month announced its decision to proceed with the National Ignition Facility (NIF). Construction is to begin at a Livermore site in 1996, and the facility, which will cost $1.07 billion, should be in operation by the year 2002.

The project design includes a laser system capable of generating 500 trillion watts of power. That's 10 times the power produced by Livermore's Nova laser, which currently ranks as the most powerful in the world. This power level should produce enough heat in an imploding capsule to achieve self-sustaining nuclear fusion.