Saturday, March 12, 2011

Laser heats up fusion quest

Physicists at the $3.5bn National Ignition Facility (NIF) say they have taken an important step in the bid to generate fusion energy using ultra-powerful lasers. By focusing NIF's 192 laser beams onto a tiny gold container, researchers have achieved the temperature and compression conditions that are needed for a self-sustaining fusion reaction – a milestone that they hope to pass next year.

Located at the Lawrence Livermore National Laboratory in California and officially opened last year, NIF will provide data for nuclear weapons testing as well as carry out fundamental research in astrophysics and plasma physics. The facility will also aim to fuse the hydrogen isotopes deuterium and tritium in order to demonstrate the feasibility of laser-based fusion for energy production.

These hydrogen isotopes will be contained within peppercorn-sized spheres of beryllium, which will be placed in the centre of an inch-long hollow gold cylinder – known as a hohlraum. By heating the inside of the hohlraum, NIF's laser beams will generate X-rays that cause the beryllium spheres to explode and, due to momentum conservation, the deuterium and tritium to rapidly compress. A shockwave from the explosion will then increase the temperature of the compressed matter to the point where the nuclei overcome their mutual repulsion and fuse.

One of the main aims of NIF is to achieve "ignition", which means that the fusion reactions generate enough heat to become self-sustaining. Researchers hope that by burning some 20–30% of the fuel inside each sphere the reactions will yield between 10 and 20 times as much energy as supplied by the lasers.

NIF first began testing the laser beams last year and now two groups at Lawrence Livermore have shown that they can obtain the desired conditions inside the hohlraum. They did this by using plastic spheres containing helium, rather than actual fuel pellets, since these were easier to analyse, and by combining their experimental measurements with computer simulations, the researchers found that the hohlraum converted nearly 90% of the laser energy into X-rays and that it heated up to some 3.6 million degrees Celsius. They also found that the sphere was compressed very uniformly, its diameter shrinking from around two millimetres to about a tenth of a millimetre.

"These results are better than we were hoping," says NIF boss Edward Moses. "People were concerned that we wouldn't be able to achieve the desired temperature and implosion shape, but those fears have proved unfounded." Moses says that the next step will be to replace the plastic spheres with beryllium ones containing unequal quantities of deuterium and tritium, in order to study how hydrodynamic stabilities might lead to asymmetrical implosions. The final step will then be to switch over to actual fuel pellets, which will contain equal quantities of the two hydrogen isotopes, and which, it is hoped, will ignite.

Moses says he hopes that ignition will take place in 2012. But he is keen not to raise expectations, having had to deal with many technical problems since construction started on NIF back in 1997. Indeed, he and his colleagues had predicted last January that ignition would be achieved by the end of 2010. "We might be able to reach ignition around spring or summertime next year," he says. "But there's a lot of physics that can run us off course in the meantime."

David Hammer, a plasma physicist at Cornell University in New York, says that the latest results are encouraging. However, he warns that the study was done without fully understanding the interactions taking place between the laser beams and plasma inside the hohlraum and that such interactions could wreck the very precise symmetry of the implosion needed for ignition.

The work is described in Phys. Rev. Lett. 106 085003 and 106 085004.

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