Early particle accelerators were the size of trash cans. Today they sprawl over an area as big as some golf courses. Tomorrow, will they girdle the globe? Physicists hope not. In this week's Physical Review Letters, researchers report an important step toward using lasers to speed particles on their way. The technique could one day be used to build miniature accelerators that would outdo today's titans at recreating the high energies of the early universe.
In a traditional accelerator, electrons or other subatomic particles gain energy by surfing giant electromagnetic waves made in large resonant vessels. The waves can only attain a certain size, however, before breaking down. As a result, particle accelerators are huge (kilometers in size) and pricey (billions of dollars). So over the last few decades, physicists have begun playing with lasers, which can generate electromagnetic tidal waves a hundred times steeper when they are focused on an ionized gas, or plasma.
One obstacle has been figuring out how to inject large numbers of electrons at just the right moment so they can ride these precipitous waves. Christopher Moore and colleagues of the Naval Research Laboratory in Washington, D.C., figured they might be able to do that with a laser, too. The team focused a tabletop terawatt laser on a tiny spot inside a chamber filled with krypton gas. The idea was that the powerful burst would rip off some 18 electrons from each atom; the naked electrons would quiver and be expelled by the large electromagnetic field created by the laser. The team had thought the electrons would tend to peel off in two directions, following the general direction of the electric field which liberated them. But even better, they found, the electrons shot off in two tightly focused beams. "It was not what we expected," Moore says. Theorists will have to sort out why the team was so lucky.
"People were skeptical [about plasma accelerators] in part because there was no good plan for an injector," says Chris Clayton, a plasma physicist at the University of California, Los Angeles. Now there is one, he says. But other obstacles remain. For instance, the injector and the rest of a laser-powered accelerator will have to be aligned and synchronized with incredible precision. Still, says Clayton, "engineers are clever, so you never know."