Giant laser needs a revamp to achieve nuclear fusion

There's more than one way to spark a star. Although the world's biggest laser missed a key target date on the road to producing clean energy via nuclear fusion, an independent review panel says the technology holds enough promise to continue the quest ? with a few modifications.

The US National Ignition Facility (NIF) at Lawrence Livermore National Laboratory in California is the latest in a series of lasers built to study nuclear fusion reactions. The goal is to one day replace uranium-based nuclear fission with cleaner, safer fusion reactions to drive power plants.

Inside a star, nuclear fusion squeezes hydrogen nuclei together to make helium nuclei, releasing huge amounts of energy. Stars are so massive they can sustain these fusion reactions due to high pressure. On Earth, a quick laser pulse aimed at a hydrogen target should be able to create similar heat and pressure. In the case of NIF, the goal was to reach what is called ignition ? the point at which the nuclear reaction puts out more energy than was needed to start it off.

Deadline missed

The US Congress had set a deadline of 30 September 2012 to achieve ignition, one that NIF failed to meet. Now a review panel convened by the US National Research Council has come up with several recommendations for fusion developers, including looking at new types of targets, a different laser design and even replacing the laser with beams of heavy ions.

NIF's approach was to fire a 192-beam laser at a metal shell the size of a pencil eraser, holding a ball of frozen hydrogen. This produces a burst of X-rays that heats and compresses the hydrogen, fusing the nuclei in a brief implosion.

When NIF was being built in the 1990s, computer models predicted that short laser pulses delivering 1.8 megajoules of energy would create the pressures needed for ignition. The giant laser surpassed this energy level last year but still wasn't achieving enough pressure.

Until we know why NIF fell short, the panel recommends trying out other options, such as shifting to a different type of laser. For instance, firing an electron beam through a mixture of krypton and fluorine produces bright laser pulses at a shorter wavelength. This technology is less mature, but if it works it could implode the targets more uniformly than NIF's lasers.

Change of target

Developers might also try changing the target. NIF was designed to fire its lasers at a metal cylinder because this was thought to be the best way to spread compression energy evenly over the hydrogen ball. But new optical techniques have fired laser pulses directly at the hydrogen and still seen uniform compression. The panel wants to test this technique at NIF's energy levels.

Another suggestion is to get rid of the lasers entirely and fire heavy ions from a particle accelerator, akin to the ones used to recreate the conditions of the big bang at facilities like the Large Hadron Collider near Geneva, Switzerland. Ion beams can transfer energy to targets just as efficiently as lasers, although for now the beam energies we can achieve are far short of what's needed for fusion.

Mike Dunne, programme director for laser fusion energy at Livermore, praised the new report for continuing to support inertial confinement techniques ? the idea of heating and compressing a target. But that isn't the only avenue being explored. Teams at the Joint European Torus in Culham, UK and the ITER test reactor, under construction in Cadarache, France, are continuing to chase magnetic confinement, which uses magnetic fields to hold hot hydrogen for a few seconds. Only time will tell which, if any, method will be the first to build a star on Earth.

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