Lawrence Livermore Laboratory scientists achieved a critical milestone in the lab's five-decade pursuit of fusion ignition, according to a study published online today in the journal Science.
In a series of experiments which began last summer and concluded in early December, scientists at the lab's National Ignition Facility consistently achieved a critical first step for generating fusion ignition. The multibillion-dollar facility officially opened in May 2009.
The researchers' goal was to direct the facility's 192 powerful lasers at a 1.8 millimeter capsule, heating it to a high temperature and causing it to implode in a highly symmetrical fashion.
That symmetry is critical for success of fusion ignition, as the capsule is designed to hold the hydrogen-based fusion material. And it must shrink in an even manner to achieve ignition, since uniform compression would create the high temperatures necessary for fusion to occur. Achieving this step has eluded physicists in the past.
"The symmetry was within a few percent" of design specifications, said Siegfried Glenzer, plasma physics group leader at NIF and lead author of the Science study. And that level left some wiggle room, as success is defined as being within 10 percent of specifications, he said.
"We used a new technique that worked right off the bat to achieve the symmetry," Glenzer said. "So nature was kind to us."
As the name implies, fusion ignition is NIF's mission.
The challenges sound straightforward, but they are exceedingly complex: Heat the hydrogen fuel to millions of degrees, propagate the reaction and confine the reacting material in a stable state. The lasers, which Glenzer said were operating at 20 times the energy level of any previous laser system, also succeeded in heating the capsule to 3.3 million degrees Kelvin. The sun's surface is almost 6,000 degrees Kelvin, or nearly 11,000 degrees Fahrenheit.
Rob Goldston, a leading plasma physicist with Princeton University, called the results exciting and noted the scientists had overcome a key hurdle.
"They've gotten over it with this clever trick at this power level," Goldston said. "But at the next power level what will happen?"
The capsules in these experiments contained a hydrogen-based gas with only trace amounts of fusion material. But when NIF researchers attempt the next round of experiments, they will use solid fuel containing the fusion material deuterium and tritium.
And they will need to increase that degree of implosion by a factor of 30 — creating an even tinier, and hotter — core, said Jeff Atherton, director of NIF experiments.
At the same time, they'll have to maintain that all-critical symmetry, while working this time with solid fusion material, not a gas. Atherton said NIF researchers plan to start experiments this summer to attempt to reach fusion ignition.
"These studies are not at the level of compression that is fully relevant to an ignition experiment," said Goldston. "And so when you really make it tinier yet, and have a very, very high mass density in the core, then how perfectly symmetrical is it when it really counts?"
This key step toward achieving fusion with the lab's newest facility generated high excitement among NIF scientists, Atherton said.
"It proved that conditions are consistent with what we need for the next phase of the experiment," he said.
Suzanne Bohan covers science. Contact her at 510-262-2789. Follow her at Twitter.com/suzbohan.