Nuclear mattersUnderstanding nuclear ignition better

The U.S. nuclear weapons are aging (think the beginning of the cold war), and the U.S. government is turning to researchers and scientists at universities such as University of California-San Diego to figure out safe and reliable ways to estimate their longevity and to understand the physics of thermonuclear reactions in the absence of underground testing currently prohibited under law. One of them is Hoanh Vu, a research scientist in the Electrical and Computer Engineering Department at the UCSD Jacobs School of Engineering.

Under a recent three-year, $510,000 grant from the National Nuclear Security Administration (NNSA), Vu and his colleagues at Los Alamos National Laboratory (NM), Lodestar Research Corporation and the Laboratory for Laser Energetics at the University of Rochester in New York, are using computer simulation tools to figure out how to successfully achieve controlled, miniaturized nuclear ignition of spherical fuel pellets in laboratory environments using lasers as energy drivers.

“The way nuclear weapons work is that there is a spherical core of deuterium-tritium that is driven by a radiation source to nuclear ignition; we know that these bombs work because they have worked underground,” Vu said. “But the nuclear materials inside these fuel cores, primarily deuterium-tritium, and the radiation sources that drive these cores to nuclear ignition, have a relatively short shelf life. We don’t know with any certainty if these weapons still work. Since we can’t test them on a full scale, what do we do? We actually look at the physics and scale down the problem so we can test the viability of these weapons in a safer and more controlled environment.

“These weapons are massive so we try to scale it down to pellets that are millimeter-size,” Vu continued. “The method of choice for compressing these fuel pellets is using a laser, which provides a radiation source.”

Vu said the primary lasers for these studies are the Omega laser at the University of Rochester (NY) and the newly built National Ignition Facility at Lawrence Livermore National Laboratory (CA).

“What we would like to do is take the laser and shine the laser onto what we call a hohlraum, a cylindrically shaped black-body radiator made of high-Z materials (typically gold), in the middle of which the miniaturized fuel pellet is placed,” he explained. “The material on the wall of the hohlraum absorbs the laser energy, heats up, and becomes a plasma. The plasma in turn irradiates off its newly acquired energy,