A kilometers-long particle accelerator may epitomize big science, but a team of physicists has taken a key step toward doing the same job with a much smaller machine. The team has amped up the current in an experimental type of accelerator—known as a plasma wakefield accelerator—and shown that it can efficiently produce an intense beam of electrons accelerated to a precisely defined energy. Many challenges remain, but some physicists hope that someday such a scheme might be used to make much smaller particle colliders.
"It's certainly an important step," says Gerald Dugan, an accelerator physicist and professor emeritus at Cornell University who was not involved in the work. "At the same time there's a long way to go" to developing a practical technology.
Particle accelerators are essential tools for many types of science. Physicists use them in atom smashers—such as the 27-kilometer-long Large Hadron Collider in Switzerland. Materials scientists and structural biologists study samples using x-rays radiated by the beams in electron accelerators. Accelerators typically measure hundreds or thousands of meters in length and cost hundreds of millions of dollars.
That's in part because a conventional accelerator can boost a particle's energy only so fast. To accelerate charged particles such as electrons, physicists shoot bunches of them through a vacuum chamber called a radio-frequency (RF) cavity, which rings with radio waves much as an organ pipe rings with sound waves. The electrons gain energy by surfing the radio waves, and the rate of acceleration depends on the strength of the oscillating electric field within the waves. But there’s a limit to how strong that field can be. If it is too strong, it will rip electrons out of the metal walls of the cavity and produce sparks that can damage the machine.
Because of that limit, high-energy accelerators must be long. For example, many particle physicists hope to build a collider using two opposing straight-shot linear accelerators to fire a beam of electrons into a beam of positrons at energies of 500 gigaelectron volts (GeV). To reach the desired energy, the proposed International Linear Collider (ILC), which might be built in Japan, would have to be 40 kilometers long.
But there’s another possibility. Physicists can abandon RF cavities and create accelerating fields hundreds of times stronger in a plasma—a gas energized so that the atoms separate into electrons and ions. Researchers fire into the plasma a pulse of electrons or laser light. That "drive bunch" plows aside the negatively charged electrons in the plasma, but barely budges the heavier positively charged ions. So in its wake, a bubble of a positive charge opens, followed by a knot of negative charge as the plasma’s electrons flow back together. As a result, the drive bunch's wake produces an enormous electric field that can accelerate other electrons like a speedboat pulling a water-skier.
Seven years ago, researchers at SLAC National Accelerator Laboratory in Menlo Park, California, showed that they could achieve very high energies by shooting a drive bunch of electrons from the lab's famed 3-kilometer-long linear accelerator into a chamber of lithium plasma. In less than a meter, it accelerated stray electrons from the plasma to energies as high as 85 GeV—twice the input energy. But only a few electrons were accelerated, and they came out with a very wide range of energies, unsuitable for just about any application.
Now, the SLAC team has gone further by placing a specially tailored trailing bunch of electrons 200 micrometers behind the drive bunch—an impressive feat given that both move at near light speed. To manage that trick, the team actually divides a single bunch of electrons from SLAC's linear accelerator into a larger drive bunch and a smaller trailing bunch, as the team reports this week in Nature. In addition to increasing the number of electrons that are accelerated, the trailing bunch itself evens out the electric field in the wake so that all the electrons experience similar acceleration, reducing the energy spread to 1%. Crucially, the hefty trailing bunch soaks up energy lost by the drive beam with higher efficiency as it zooms from 20 GeV to 22 GeV over 36 centimeters. Such efficiency is a key parameter, and the level achieved—18%—is close to what's needed to make a practical accelerator, says SLAC's Mark Hogan.
"It's a huge step forward," says Thomas Katsouleas, an engineer and physicist at Duke University in Durham, North Carolina, who was not involved in the research. The energy spread and efficiency still need to be improved, he says, but "we're talking a factor of 2 or so, whereas before we were talking two orders of magnitude."
Still, many challenges remain, especially when it comes to putting an electron-driven system to practical use. Whereas a laser-driven system could potentially be used to power a compact x-ray source, an electron-driven system like the one at SLAC is likely to be good only for building a collider for particle physics. That’s because it still requires a conventional accelerator to produce the electrons, says Michael Downer, a physicist at the University of Texas, Austin. To match the energy of the proposed ILC, physicists would have to pass an electron bunch through hundreds of plasma cells in a row, Downer says, and nobody has demonstrated such "staging" so far.
Even more important, to make a collider like the ILC, the scheme the SLAC team used would have to be modified to accelerate positrons, which would interact with the plasma differently from electrons. Nobody knows just how to do that, Downer says. Revving up positrons is "about a factor of 10 times more difficult" than accelerating electrons, Downer estimates.