Surf's Up

By Alex Stone|Tuesday, February 22, 2005

Particle accelerators—essential tools in fundamental physics, materials science, and medical imaging—are fantastically large and expensive: The Relativistic Heavy Ion Collider on Long Island, for example, is 2.4 miles around and cost $150 million. But a new kind of accelerator may soon pack the same kind of power into a device that fits on a lab bench.

The idea is ingeniously simple. A high-energy laser pulses through a tube filled with plasma—a soup of charged atoms and electrons—creating a series of waves, analogous to the wake of a speedboat. “The electrons surf on the wave,” says UCLA physicist and engineer Warren Mori. “And because the speed of the wake is roughly equal to the speed of the boat, the velocity of the wave is very near the speed of light.”

Physicists have now found ways to control this surfing process. “The key result is that the beam particles come out all with the same energy. That’s a huge surprise,” says engineer Thomas Katsouleas of the University of Southern California. Uniform beam energy is essential for a useful accelerator. Experiments illustrate the promise of laser-driven accelerators. In one test physicists pumped particles to energies of 100 million electron volts over a distance of only a millimeter, 1/25 of an inch. That energy is sufficient to generate X-rays and radioisotopes needed to help diagnose and treat cancer.

Fundamental physics research will require scaling up the experiments significantly. “We’d ultimately like to keep the same rate of acceleration and extend from a millimeter to a meter [three feet],” Katsouleas says. “That would be 100 billion electron volts, which is higher than the largest existing linear accelerator.” That would also be a dramatic downsizing: The biggest linear accelerator, the Stanford Linear Accelerator, is two miles long.

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