Lasers Burst Across a Frontier

Hank Hogan

At Lawrence Livermore National Laboratory, researchers have crossed a threshold. They've constructed a laser that creates conditions until now found only in massive particle accelerators or in the centers of stars.

At Lawrence Livermore National Laboratory, researchers have crossed a threshold. They've constructed a laser that creates conditions until now found only in massive particle accelerators or in the centers of stars.

Thomas Cowan, a staff physicist, said the device pumped out 300 J in a burst 0.5 ps long, translating to 0.6 PW. That's about 1000 times the electrical generating capacity of the entire US. With such power come new capabilities.

"It represents the crossing of an energy frontier, in which lasers now have enough energy and can be focused to sufficiently high intensity that they can cause nuclear physics effects to become important," Cowan said.

He and his co-workers are investigating these laser-induced nuclear physics effects. In one experiment, they fired the laser at a gold disc 2 mm in diameter and 1 mm thick. The gold was mounted on a slab of copper-clad uranium. The researchers chose gold as the target for the petawatt laser because they wanted a relatively heavy element to maximize x-ray production.
When the laser fired, the gold evaporated, with the vaporized cloud forming a plasma of ions and electrons. When the laser pumped energy into that plasma, some electrons reached an energy of 100 million V -- a level comparable to that found in particle accelerators.

When these high-energy electrons struck the gold, they created high-energy x-rays. As the x-rays fanned out, they barreled into the uranium, causing fission to take place and creating short-lived by-products. Then the researchers pieced events together from the longer-lived descendants of the initial by-products. The energy was intense enough to create some antimatter.

Although this experiment has been achieved in accelerators before, lasers offer some unique, not to mention potentially useful, capabilities. Lasers can be focused onto small spots in space and fired for a short time. This means that research can be conducted on localized areas or on phenomena that are fleeting in nature. Cowan and his colleagues are continuing their nuclear physics experiments with the petawatt laser.