Long the domain of megascience, nuclear fusion conjures up images of massive lasers housed in cavernous buildings. Now physicists have shown that deuterium nuclei can fuse when hit by short, powerful pulses of a tabletop laser. Although the feat--reported last month at a meeting of the American Physical Society and also in tomorrow's issue of Nature--won't provide a new source of energy, it could lead to a handy source of neutron beams for probing materials.
A team led by Todd Ditmire at Lawrence Livermore National Laboratory in California released deuterium gas through a small jet into a vacuum chamber in short pulses at -170 degrees Celsius. That's cold enough that the deuterium atoms try to form tiny droplets--clusters of a few thousand atoms. "These cluster targets have the ability to absorb a large amount of energy," says team member Jason Zweiback. Pulses from the laser beam, compressed to high energies by mirrors and other optical equipment, ionize the atoms, creating a small spherical plasma of deuterium ions and electrons. When the clusters blow apart, deuterium ions collide with enough energy to fuse into helium nuclei. The fusion reactions spawn thousands of neutrons, which the team could detect.
The total energy released is far too little to make the deuterium clusters useful for energy production. Instead, the team envisions the tabletop laser as a convenient neutron source for materials science. One application would be testing the reactor chambers for fusion power plants, because the energy of individual neutrons corresponds to those in future reactors. Thus the device could offer an alternative to today's expensive neutron sources, either nuclear reactors or spallation sources in which an accelerator shoots protons at a metal target.
The concept appeals to Charles Rhodes, a physicist at the University of Illinois, Chicago. "A fair amount of neutrons are coming out in a hurry, so it is a very bright source," he says. "It could have considerable practicality."