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Is fusion success in sight?

LLNL
At the National Ignition Facility, the power of 192 lasers will be focused on a
gold-plated cylinder like this one, containing a pea-sized pellet of fusion fuel.


Experiments at the National Ignition Facility have given researchers confidence that they'll achieve a milestone in nuclear fusion sometime this year.

The tests involved blasting a cylinder the size of a pencil eraser, known as a "hohlraum," with 192 laser beams and seeing whether researchers could tweak the energy to create the right kind of implosion. The results suggested that they could - and that the $3.5 billion blaster in California just might produce the world's first controlled fusion reaction, with more energy coming out than going in.

For more than a half-century, scientists have been trying to harness the nuclear fusion reaction to generate what could be prodigious amounts of energy. The reaction involves crushing together light atoms (like hydrogen) so forcefully that they fuse into heavier atoms (like helium). Each reaction converts a tiny amount of mass from the atoms directly into energy.

When you multiply that demonstration of E=mc2 by trillions, you start producing power on the scale of an H-bomb or the sun.

The research reported by the National Ignition Facility, or NIF, represents a step toward actual energy production in a controlled reaction. But there are still many steps to go before scientists reach that break-even point. Even if NIF is successful, it will take years to adapt the technology for commercial applications. And that's the most optimistic view.

Jeffrey Atherton, the program director for target experimental systems at NIF, is an optimist.

"The potential for NIF and fusion energy as the game-changer [for energy resources] is enormous," he told me. He acknowledged that commercial fusion was far from a sure bet, but said the technology had to be included in the nation's portfolio of energy research. "You have to invest in things that could have risk associated with them, but also have enormous benefits should they play out," Atherton said.

The initial tests at NIF, built at the Lawrence Livermore National Laboratory in Northern California, have made Atherton and his colleagues feel more comfortable about the investment. Those tests are detailed today in a research paper published online by the journal Science.

"When we extrapolate the results of the initial experiments to higher-energy shots on full-sized hohlraums, we feel we will be able to create the necessary hohlraum conditions to drive an implosion to ignition later this year," Siegfried Glenzer, plasma physics group leader at Lawrence Livermore National Laboratory, told me in an e-mail.

What's a hohlraum?
The term "hohlraum" comes from the German words for "hollow area." Hohlraums are hollow, gold-plated cylinders that are structured to spread the energy from the laser beams into a inward-pointing blast of X-rays, all focused on a target the size of a small pea. The target is a precisely machined, spherical pellet of beryllium, containing the stuff to be imploded.

For the real ignition shots, the targets will be filled with a cryogenically cooled dollop of deuterium-tritium fusion fuel. Deuterium and tritium are two isotopes of hydrogen that are particularly well-suited for fusion. If the researchers do it right, that tiny bit of fuel would be compressed by a factor of 1,000 or more, and reach temperatures approaching 100 million degrees Celsius (180 million degrees Fahrenheit). That's hotter than the sun.

For the tests described in the Science paper, the hohlraums were smaller, the targets were filled with plain old hydrogen and helium, and the temperatures reached a mere 3.3 million degrees C (6 million degrees F). The resulting reaction fell far short of break-even fusion, but Atherton said it confirmed that NIF was on the right track.

"The point is that we were doing it at a scale that's about 20 times larger than has been done, with a laser power that accordingly is about 20 times higher than has been done, with a precision and efficiency that hasn't been done before," he said.

Dealing with uncertainty
One big challenge was to aim the laser beams so precisely that the target was heated evenly. If the heating is the slightest bit uneven, the fusion fuel will splurt away before it implodes enough to create the pressure and temperature required for ignition. That's essentially what happened at NIF's predecessor, the $200 million Nova laser facility. But researchers said they were satisfied with the uniformity of heating at NIF.

"We also demonstrated a very elegant way of tuning the symmetry of the laser beams, by making very subtle changes in the color of the wavelength in the cone of these beams," Atherton said.

Glenzer told me the wavelength-tuning trick "was predicted to work, but could only be tested on full NIF experiments described in this paper." More than 90 percent of the laser energy was absorbed by the hohlraums - which is more than was predicted by the pre-test simulations.

The experiments demonstrated that researchers could "overcome the biggest physics uncertainty in laser fusion - namely, we showed that we can heat hohlraums to temperature and radiation symmetry close to what is needed for ignition," Glenzer said.

Atherton echoed those comments in more down-to-earth terms: "Given the very positive results out of last summer and fall, we do feel much more confident about the feasibility of fusion as an energy source," he told me.

The tests described in Science were conducted last year at an energy level of 0.7 megajoules. Since then, NIF has ramped up to the 1-megajoule level, and Atherton said "our ignition experiments will be operating at a laser energy of 1.2 or 1.3 megajoules this summer."

Energy source of the future?
The results impressed other experts. "They're ahead of the curve predicted," Mike Dunne, director of the Central Laser Facility of Britain's Rutherford Appleton Laboratory, told Science.

"It's definitely a very capable and interesting machine," said Charles Seife, a longtime science writer and journalism professor at New York University who wrote a book about the fusion quest titled "Sun in a Bottle."

However, it remains to be seen whether reality will follow the predicted path. In his book, Seife shows that the course of true fusion never did run smooth, despite repeated predictions that success was just a few years and a few (million? billion?) dollars away. The classic joke is that fusion is the "energy source of the future - and always will be."

Even Atherton acknowledges that NIF's nanosecond-long shots can't be harnessed for commercial purposes in the near term. The shots would have to occur "10 times per second, as opposed to once every few hours, or days, or pick your unit of time," he said.

Researchers say NIF could blaze a trail for more commercially viable concepts. For example, the Laser Inertial Fusion Engine, or LIFE, would use laser shots to generate neutrons for a hybrid fusion-fission reaction.

However, in the long run, it may turn out that one of the other approaches to fusion will be more fruitful. Maybe it'll be the $13 billion magnet-based ITER project taking shape in France. There are also a number of dark-horse candidates - such as the low-cost, high-voltage system currently being funded by the Navy, or the levitating-magnet system that came into the spotlight just this week.

Atherton said NIF would almost certainly be the first technology to reach the break-even point - but he said it made sense to investigate other paths to fusion as well. "We don't look at this as a competition, as much as that we're all in a race to develop clean energy resources," he said. He recognized that other energy technologies - including biofuels, solar, wind and safer fission reactors - also had to be funded.

"Many people who study this and try to take a considered, balanced perspective actually believe that it's important to invest in all of these technologies," Atherton said.

Beyond energy production
Atherton pointed out that fusion research isn't aimed exclusively at commercial energy production. The knowledge gained at NIF could also be applied to astrophysics and nuclear physics - that is, the science behind what happens in stars. "There's a whole wealth of basic science that could be done with this type of burning-plasma creation," he said. "That could give physicists the ability of doing experiments looking inward instead of outward."

There's yet another big reason why the U.S. Department of Energy has spent billions of dollars on NIF: "The physical conditions created with an ignition-type target can be used to study important physics questions related to the safety and reliability of the nuclear stockpile," Atherton said.

Seife suspects that the weapons issue is the key to NIF's existence, but he hasn't been able to put his finger on how exactly the research being conducted there benefits the U.S. nuclear weapons program. He wonders whether NIF is actually less about nuclear physics - and more about keeping nuclear physicists employed.

"NIF isn't truly about energy," Seife writes in his book. "It is not about keeping our stockpile safe, at least not directly. It is about keeping the United States' weapons community going in the absence of nuclear tests."

Is NIF on the right track for nuclear fusion? Is the promise of nearly limitless energy worth the billions of dollars being spent on fusion research? Or is fusion research really a matter of national security rather than energy production? Feel free to weigh in with your comments below.

More on the fusion quest:


Another paper published today on the Science Express Web site - "Charged-Particle Probing of X-ray-driven Inertial-Fusion Implosions" - sheds additional light on the reactions expected to take place at NIF.

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