The goal of the National Ignition Facility is to create a miniature star on earth to ensure the safety of the nation’s nuclear stockpile.

More than 15 years have passed since the United States last tested a nuclear device. By joining with other nations in abstaining from testing, the United States has helped slow the global arms race. But the decision has other consequences. Without testing, we need alternative ways of monitoring the nation’s nuclear arsenal.

To plug the knowledge gap, nuclear scientists run computer simulations of weapon systems and explosions. That helps them understand whether the warheads are safe and working as designed. These simulations employ some of the world’s most powerful computers, but like any computer model they need to be compared with real data. Which is where the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory in Livermore, California, comes in.

The NIF was built to simulate much of the physics of nuclear weapons. It enables scientists to trigger and study fusion reactions—essentially tiny thermonuclear explosions. “We can study the physics of weapon performance, including how nuclear fuel is actually burned,” says Livermore physicist Ed Moses. “We can look at how nuclear materials behave at extremely high pressures and temperatures.”

By tracking the course of these reactions, scientists can ensure that their models are accurate. “The only way to tell if the models are right is to create the conditions they simulate,” Moses added. “That is what the NIF can do and it is a unique facility to do it. The goal is to ensure the safety and efficacy of our strategic stockpile without nuclear testing.” It will be the cornerstone of the government’s Stockpile Stewardship Program.

The work will begin in the fall of 2010. During the experiments, a thimble-sized gold target in a warehouse-sized building at Livermore will be illuminated with an array of 192 lasers. When fired up, they will become the most powerful light show ever assembled in a laboratory. Briefly, they will ignite a fusion reaction similar to that which powers the sun.

In a fusion reaction, energy is released when two light nuclei are squeezed together to form a heavier one. At the sun’s heart, the fuel is hydrogen and helium nuclei. To recreate a similar process on earth, physicists know that two isotopes of hydrogen—deuterium and tritium—are the best candidates. The challenge is to produce and control the terrifying temperatures and pressures needed to force the isotopes together.

That’s what the NIF is designed to do. At its heart is a cylindrical gold target. When the lasers strike the target—96 from below, the others from above—the gold casing soaks up the light and emits highenergy X-rays, bathing the inside of the cylinder in intense radiation.

The fuel itself will be stored inside a small ball of beryllium held within the cylinder. When the X-rays hit the ball, the outer surface will blow off at high speeds, driving the target inward at about a million miles per hour. The resulting shock wave will then compress the hydrogen isotopes at pressures of up to 100 billion atmospheres. That’s enough to fuse the deuterium and tritium into helium nuclei. Very briefly, a tiny star will burn inside Livermore.

Weapons researchers will learn about the behavior of fusion detonations, and energy researchers will probe the reactions in a bid to build a future generation of power stations. Fusion reactions release no greenhouse gases and the hydrogen isotopes needed to power the process can be extracted from seawater. If the process can be safely controlled, it could form the basis of a limitless supply of carbonfree energy for fueling our future without pollution or global warming.

While this will not be the first time that fusion will have been triggered by scientists, the Livermore reaction will be unique in one important respect. In other fusion experiments, researchers have watched the reaction flare and then die out. The ignition facility will deliver a far bigger spark than previous experiments, enough for the nuclear fire to burn by itself. The fusion reaction will last less than a billionth of a second, but for that short time it will be self-sustaining. Previously, this has only been achieved in nuclear weapons explosions.

The Livermore device is not designed to maintain the kind of high repetition rate needed to produce electricity. Other efforts have developed technologies for this purpose. Livermore researchers plan to vary the laser and target conditions to work out which configuration produces the most energetic reaction.

“One of the holy grails of 21st-century science is harnessing fusion as an energy source,” said Barry Walker, a laser researcher at the University of Delaware, Newark. “Laser fusion at NIF, and the basic science behind it, is one of the most interesting results that could come from this facility.”