September 24, 2006

From zero to a billion electron volts in 3.3 centimeters

Super powerful and compact accelerators are coming.

In a precedent-shattering demonstration of the potential of laser-wakefield acceleration, scientists at the Department of Energy's Lawrence Berkeley National Laboratory, working with colleagues at the University of Oxford, have accelerated electron beams to energies exceeding a billion electron volts (1 GeV) in a distance of just 3.3 centimeters.

SLAC, the Stanford Linear Accelerator Center, boosts electrons to 50 GeV over a distance of two miles (3.2 kilometers) with radiofrequency cavities whose accelerating electric fields are limited to about 20 million volts per meter.

The electric field of a plasma wave driven by a laser pulse can reach 100 billion volts per meter, however, which has made it possible for the Berkeley Lab group and their Oxford collaborators to achieve a 50th of SLAC's beam energy in just one-100,000th of SLAC's length.

Impressive as this is, "It's the tip of the iceberg," says Leemans. "We are already working on injection" -- inserting an already energetic beam into an accelerating cavity -- "and staging," the handoff of an energetic beam from one capillary to the next and subsequently to others, until very high energy beams are achieved. "Brookhaven physicist Bill Weng has remarked that achieving staging in a laser wakefield accelerator would validate 25 years of DOE investment in this field."

Leemans's group and their collaborators look forward to the challenge with confidence. "In DOE's Office of Science, the High Energy Physics office has asked us to look into what it would take to go to 10GeV. We believe we can do that with an accelerator less than a meter long -- although we'll probably need 30 meters' worth of laser path."

While it's been said that laser wakefield acceleration promises high-energy accelerators on a tabletop, the real thing may not be quite that small. But laser wakefield acceleration does indeed promise electron accelerators potentially far more powerful than any existing machine -- neatly tucked inside a small building.