August 1, 2012 | 9
Blink and you’ll miss it. Don’t blink, and you’ll still miss it.
Imagine a device capable of delivering more power than all of the world’s electric plants. But this is not a prop for the next James Bond movie. A new laser at Lawrence Berkeley National Laboratory was put through its paces July 20, delivering pulses with a petawatt of power once per second. A petawatt is 1015 watts, or 1,000,000,000,000,000 watts—about 400 times as much as the combined instantaneous output of all the world’s electric plants.
How is that even possible? Well, the pulses at the Berkeley Lab Laser Accelerator (BELLA) are both exceedingly powerful and exceedingly short. Each petawatt burst lasts just 40 femtoseconds, or 0.00000000000004 second. Since it fires just one brief pulse per second, the laser’s average power is only about 40 watts—the same as an incandescent bulb in a reading lamp.
BELLA’s laser is not the first to pack so much power—a laser at Lawrence Livermore National Laboratory, just an hour’s drive inland from Berkeley, reached 1.25 petawatts in the 1990s. And the University of Texas at Austin has its own high-power laser, which hit the 1.1-petawatt mark in 2008. But the Berkeley laser is the first to deliver petawatt pulses with such frequency, the lab says. At full power, for comparison, the Texas Petawatt Laser can fire one shot per hour.
The Department of Energy plans to use the powerful laser to drive a very compact particle accelerator via a process called laser wakefield acceleration, boosting electrons to high energies for use in colliders or for imaging or medical applications. Electron beams are already in use to produce bright pulses of x-rays for high-speed imaging. An intense laser pulse can ionize the atoms in a gas, separating electrons from protons to produce a plasma. And laser-carved waves in the plasma [blue in image at right] sweep up electrons [green], accelerating them outward at nearly the speed of light.
BELLA director Wim Leemans says that the project’s first experiments will seek to accelerate beams of electrons to energies of 10 billion electron-volts (or 10 GeV) by firing the laser through a plasma-based apparatus about one meter long. The laser apparatus itself is quite a bit larger, filling a good-size room [see top photo]. For comparison, the recently repurposed Stanford Linear Accelerator Center produced electron beams of 50 GeV from an accelerator 3.2 kilometers in length.