A factor of two
By Michael Banks, news editor of PhysicsWorld
“A factor of two is not a small thing, it is quite a challenge,” says Robert McCory from the University of Rochester in New York.
McCory was speaking about the latest in laser-based fusion research (known as inertial confinement fusion) at the 2013 AAAS conference.
The National Ignition Facility (NIF), which began full operation in 2009, is based at the Lawrence Livermore National Laboratory in the US and is currently the world’s leading laser-based fusion device.
It uses a 1.8 MJ laser to create X-rays that act to rapidly heat and compress a capsule containing deuterium and tritium, which cause the hydrogen isotopes to fuse. Yet so far the facility has failed to achieve “ignition” - the point at which the fusion reactions generate enough heat to become self-sustaining. Indeed, the best shot so far at NIF has fallen short of the pressure needed to compress the fuel enough to produce ignition by a factor of two or three.
This has led to some to suggest that NIF needs to change course. The current method - indirect drive - uses X-rays rather than the light itself to heat the fuel capsule. But some say that direct drive should now be employed - where the laser light is shone directly onto the fuel capsule.
However, McCory, who runs a direct-drive experiment at Rochester called OMEGA, has done simulations showing how results on his machine could scale to NIF conditions.
“If we take our best OMEGA shot and extrapolate it to NIF then we would still be a factor of two or three away in required pressure from ignition,” says McCory. “And there is still physics that does not scale well, so there are added uncertainties.”
Researchers working on both direct and indirect methods are now looking at ways to get around the factor of two problem, which include trying to remove impurities in the plastic shell that holds the fusion fuel.
NIF scientists still remain confident, however, that they will overcome these barriers. Hopefully, policy-makers will have the same amount of patience.
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