Scientists are creeping closer to their goal of creating a controlled fusion-energy reaction by mimicking the interior of the sun inside the hardware of a laboratory. In the latest incremental advance, reported Wednesday online in the journal Nature, scientists in California used 192 lasers to compress a pellet of fuel and generate a reaction in which more energy came out of the fuel core than went into it.
There's still a long way to go before anyone has a functioning fusion reactor, something that physicists have dreamed of since Albert Einstein was alive. A fusion reactor would run on a common form of hydrogen found in sea water, would produce minimal nuclear waste and couldn't have the kind of meltdown that can occur in a traditional nuclear-fission reactor.
"You kind of picture yourself climbing halfway up a mountain, but the top of the mountain is hidden in clouds," said Omar Hurricane, the lead author of the Nature paper, in a teleconference with journalists. "And then someone calls you on your satellite phone and asks you, 'How long is it going to take you to climb to the top of the mountain?' You just don't know."
Hurricane and other scientists at the Lawrence Livermore National Laboratory, home of the multi-billion-dollar National Ignition Facility, took pains to calibrate their claims of success. This was not fusion "ignition," the ultimate ambition of the NIF. The experiment overall requires much more energy on the front end-all those laser shots-than comes out the back end.
Only about 1 percent of the energy from the laser actually winds up in the fuel, according to Debra Callahan, a co-author of the Nature paper. Most of the laser energy gets absorbed by surrounding material - a gold cylinder called a hohlraum, and a plastic capsule within that - before it reached the fuel, which coats the inside of the capsule and is made of two hydrogen isotopes, deuterium and tritium.
But the experiment worked as hoped. When briefly compressed by the laser pulses, the isotopes fused, generating new particles and heating up the fuel further and generating still more nuclear reactions, particles and heat. This feedback mechanism is known as "alpha heating" and is an important goal in fusion research.
"They've got a factor of about 100 to go," said Mark Herrman, director of the Pulse Power Sciences Center at the Sandia National Laboratory, a sister institution to the Livermore lab. "We want a lot of fusions. They made 5 million billion fusions, but we want more than that. We want 100 times than what they made."
To frame the challenge further: Even if ignition is achieved in coming years, the contraption required is so extremely elaborate and capital-intensive-total cost of the NIF operation is in the realm of $5 billion-that it may be of limited practical application for generating electricity to power someone's toaster.
Still, the new result represents progress in the fusion-energy field and came as a relief for Lawrence Livermore scientists after early efforts produced energy yields lower than what had been predicted from computer models. The process requires exquisite precision in operating the lasers to compress the fuel pellet by a factor of 35, like squeezing a basketball to the size of a pea, Callahan said.
The latest technique modified the laser pulses to create a better-shaped implosion of the pellet.
"The real significance of this is, we're now matching our models, we have our feet back on the ground where we know where to go forward," said Jeff Wisoff, principal associate director of NIF and photon science at the lab. "We have a number of knobs we can turn."
NIF is funded by the National Nuclear Security Administration, and does fusion research only part of the time. Usually it is engaged in tests that help scientists understand the processes involved in nuclear weapons explosions.
Another fusion energy strategy, developed at the Princeton Plasma Physics Laboratory in New Jersey and widely used by researchers around the world, uses giant magnets to confine the hot plasma in which the fusion reactions occur.
Stewart Prager, director of the Princeton laboratory, applauded the new results reported in Nature by the California team, saying, "It's the first sign that they're getting what we call self-heating."
He's optimistic about fusion energy in the long run.
"In 30 years, we'll have electricity on the grid produced by fusion energy-absolutely," Prager said. "I think the open questions now are how complicated a system will it be, how expensive it will be, how economically attractive it will be."
The short-term problem is funding. Congress appropriated about $500 million for fusion energy science in the 2014 budget, a boost of more than $100 million from the tight budgets of the previous two years, but fusion advocates want more.
Rep. Rush Holt, D-N.J., a physicist who spent 10 years working at the Princeton lab, said Wednesday that the United States is losing leadership in fusion energy research to Europe, Japan, South Korea and China.
"It's nowhere close to making your electric meter run backwards," Holt said of fusion energy. "But the reason other countries are now investing more than we are-this is a sad story in itself-is that the country that was the world's leader in fusion research is no longer."