Researchers at Lawrence Livermore National Laboratory have demonstrated that big things can come from small packages. The scientists used a tabletop laser to ignite nuclear fusion in a gas jet of deuterium, an isotope of hydrogen. This successful experiment could lead to a tabletop neutron source. A laser beam ignites a puff of deuterium clusters pulsed from the bottom of a cryogenically cooled gas jet (which measures about 1 in. in diameter). The white spark is the fusion plasma.Such a source could be quite small, according to Todd Ditmire, a project leader in the Laser Science and Technology Program at Lawrence Livermore and one of the researchers. A 30-mJ laser firing 30-fs pulses would be enough, he said. To date, researchers have used a 35-fs laser pulse of 120 mJ at a wavelength of 820 nm focused down to a 200-mm spot and aimed at a deuterium gas jet cooled to 2170 °C, allowing 0.05-nm atomic clusters to form. As the radiation struck the gas, it ionized and compressed. As a result, the nuclei of deuterium combined to form helium, producing energy and neutrons. Each joule of laser energy created about 100,000 neutrons -- an efficiency approximating that of much larger lasers. The larger lasers have been used to investigate methods of achieving fusion ignition and gain, the point at which the energy from the reaction is equal to or greater than the energy being pumped into it. But even though the neutron efficiencies are similar, there is a fundamental difference between the larger lasers and the tabletop variety. "This technique will not scale up to levels needed for ignition and fusion gain," Ditmire said. "However, we might be able to exploit the scaling to, say, a 1-kJ laser, to make an intense source of fusion neutrons." A possible application for such a neutron source would be to investigate the materials that would go into future fusion reactors, which are expected to produce a flood of neutrons. Any material in the reactor would have to stand up to the bombardment. According to Ditmire, another possibility might be the use of the tabletop neutron source for neutron radiography. However, such applications are several years off.