Berkeley, CA, October 1, 2004--Researchers at the Department of Energy's Lawrence Berkeley National Laboratory have taken a giant step toward realizing the promise of laser wakefield acceleration, by guiding and controlling extremely intense laser beams over greater distances than ever before to produce high-quality, energetic electron beams.
For a quarter of a century physicists have been trying to push charged particles to high energies with devices called laser wakefield accelerators. In theory, particles accelerated by the electric fields of laser-driven waves of plasma could reach, in just a few score meters, the high energies attained by miles-long machines using conventional radio-frequency acceleration. Stanford's linear accelerator, for example, is two miles long and can accelerate electrons to 50 GeV (50 billion electron volts). Laser wakefield technology offers the possibility of a compact, high-energy accelerator for probing the subatomic world, for studying new materials and new technologies, and for medical applications.
In plasmas, researchers have generated electric fields a thousand to ten thousand times greater than in conventional accelerators � but these large fields exist only over the short distance that a laser pulse remains intense; for tightly focused beams, that distance is typically only a few hundred micrometers (millionths of a meter). The resulting beams are of relatively poor quality, with particle energies so widespread that less than one percent have enough punch for scientific applications.
The Berkeley Lab researchers achieve high-quality beams by first shaping a channel through hydrogen gas with powerful, precisely timed laser pulses, then accelerating bunches of electrons through the plasma inside the channel. Because of the controlled accelerator length and the characteristics of the channel, there are several billion electrons in each bunch within a few percent of the same high energy, more than 80 MeV (80 million electron volts).
The work was done by the L'OASIS group (L'OASIS stands for Laser Optics and Accelerator Systems Integrated Studies), led by Wim Leemans of the Center for Beam Physics in Berkeley Lab's Accelerator and Fusion Research Division. To analyze their successful experiment, the group collaborated with the Tech-X Corporation of Boulder, Colorado, using the VORPAL plasma simulation code to model their results on supercomputers at DOE's National Energy Research Scientific Computing Center (NERSC). The researchers report their results in the 30 September 2004 issue of Nature.
"The plasma-channel technique is an essential step toward developing compact laser wakefield accelerators with multiple stages, which can produce focused, ultrafast, high-energy bunches of electrons to compete with state-of-the-art machines using conventional radio-frequency acceleration," says Leemans. "But the new technique has already suggested novel sources of radiation, such as the efficient generation of femtosecond x-rays" � x-ray pulses measured in quadrillionths of a second � "and coherent terahertz and infrared radiation."