Laser fusion nears crucial milestone

This could be the year the National Ignition Facility (NIF) finally lives up to its name. The facility, which boasts the world’s largest laser, is designed to trigger fusion by imploding a target pellet of hydrogen isotopes, thereby releasing more energy than will go into the shot. NIF’s managers think that the end of their two-year campaign for break-even energy, or ‘ignition’, is in sight. “We have all the capability to make it happen in fiscal year 2012,” says Ed Moses, director of the US$3.5-billion facility, at the Lawrence Livermore National Laboratory in California.
But even if the champagne corks do get popped, the method — a form of ‘inertial confinement’ fusion — faces an uncertain future. Would success mean that the US Department of Energy (DOE) will be ready to develop it into an economically viable energy source? And if so, is NIF’s laser-based approach the best one? An interim report released on 7 March by a US National Academies panel concludes that it is still too early to tell, and recommends that fusion scientists explore alternative technologies for imploding the fuel.
Glen Wurden, a plasma physicist at Los Alamos National Laboratory in New Mexico, agrees, saying that scientists working on inertial confinement should be wary of putting all their eggs in the laser basket. “It’s premature right now,” he says. He points to the troubles that have plagued a competing approach to fusion — magnetic confinement — and its flagship project ITER, a $21-billion international fusion experiment under construction at St-Paul-lez-Durance, France. Wurden blames ITER’s delays and ballooning costs on a premature commitment to a technology known as a tokamak, a doughnut-shaped cage within which powerful electromagnets confine a fusion plasma.
Despite early confidence, bolstered by favourable computer models, NIF too has lagged behind schedule. “It thought it had ignition in the bag,” says Wurden. Instead, NIF’s approach to heating and compressing the hydrogen isotopes has proved troublesome. In what is known as indirect drive, the laser’s multiple beams are focused at the openings in a pencil-eraser-sized gold cylinder called a hohlraum, blasting the insides to create X-rays.The X-rays then heat and squeeze the fuel pellet inside the hohlraum to produce fusion. But unexpectedly turbulent interactions between the laser light and the plasma inside the hohlraum sap energy from the beams. That could wipe out any gains as NIF managers ramp up the laser energy to the threshold needed for ignition.