- 11:06 02 October 2012 by Jeff Hecht
Bad news for star power. The world's largest laser has missed a deadline that was key to its goal of producing safe, clean energy via nuclear fusion, the same process that powers the sun.
The 192-beam laser forms the heart of the US National Ignition Facility at Lawrence Livermore National Laboratory in California, the world's leading laboratory for laser fusion research. But the lab has still not succeeded in creating a nuclear fusion reaction that makes more energy than it consumes, a milestone known as ignition. The US Congress, which funds NIF, had said the lab must do so by 30 September.
In nuclear fusion, hydrogen nuclei are squeezed together to form helium nuclei, releasing huge amounts of energy. The hope is that fusion might one day replace uranium-based fission reactors as a cleaner source of nuclear power, but no controlled fusion reaction on Earth has ever hit the ignition point.
Magnetic confinement fusion – using magnetic fields to confine a plasma at the pressures needed to achieve fusion – is generally considered the most advanced technique. That is what is used in the experimental Joint European Torus (JET) in Culham, UK, and the test reactor ITER, under construction in Cadarache, France.
By contrast, NIF creates fusion reactions by imploding tiny balls of frozen hydrogen using pulses from its giant laser. The resulting pressure squeezes the hydrogen atoms for a few billionths of second and they fuse to form helium, releasing energy. NIF's push to meet the congressional deadline represented an ambitious attempt to pip magnetic confinement fusion to the post, but that looks unlikely now.
Though computer models predicted that NIF was on track to create fusion reactions that produce more energy than they consume, this has turned out not to be case. The key problem seems to be achieving the required pressure of 300 gigabars (300 billion atmospheres).
"We've reached 150 to 200 gigabars in implosions, but that's still off by a factor of 2-ish," Ed Moses, Lawrence Livermore's principal associate director for NIF and photon sciences, told New Scientist.
Achieving the required pressure requires four things: an implosion velocity of 370 kilometres per second, creating a perfectly spherical hot spot at the centre of the imploded hydrogen pellets, mixing the plasma properly once it has formed, and very even compression. "We can do all of these things. The trouble is that we can't do them all at once," says Moses. "Like squeezing on a balloon, something might pop out."
These problems do not surprise David Hammer, a Cornell University engineer who served on an NIF review panel three years ago. "They started the ignition programme more optimistically than they should have," he told New Scientist. "They thought they could do it as an engineering programme", he says, tweaking the laser from shot to shot until it worked.
NIF still has money to fund ignition work for one more year. "They need to do systematic experiments to understand the level of laser plasma instability and come up with innovative ways to control it," says Hammer.
Others will be watching. The Laser Megajouleexperiment in France had planned to follow in NIF's footsteps in attempting ignition – but those plans could change, or at least be delayed, if NIF fails. Similarly, a joint fusion effortbetween NIF and two labs in the UK could now be shelved.
Even without ignition, though, the lab is unlikely to shut down because of its ability to double up as a nuclear weapon simulator. The implosion of hydrogen pellets is similar to the way a hydrogen bomb works. That means NIF can be used to update knowledge about nuclear weapons and simulate how the US nuclear weapons stockpile might be affected by its age.
I believe that nuclear fusion is one of the major answers to our need to replace fossil fuels as our energy source's, and might even provide enough power to make intergalatic travel a reality .
Edited by The Ratiocinator, 06 October 2012 - 06:05 PM.