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Unlike the $14 billion ITER project under way in France, General Fusion's approach doesn't rely on expensive superconducting magnets--called tokamaks--to contain the superheated plasma necessary to achieve and sustain a fusion reaction. Nor does the company require powerful lasers, such as those within the National Ignition Facility at Lawrence Livermore National Laboratory, to confine a plasma target and compress it to extreme temperatures until fusion occurs.
Instead, General Fusion says it can achieve "net gain"--that is, create a fusion reaction that gives off more energy than is needed to trigger it--using relatively low-tech, mechanical brute force and advanced digital control technologies that scientists could only dream of 30 years ago.
It may seem implausible, but some top U.S. fusion experts say General Fusion's approach, which is a variation on what the industry calls magnetized target fusion, is scientifically sound and could actually work. It's a long shot, they say, but well worth a try.
The prototype reactor will be composed of a metal sphere about three meters in diameter containing a liquid mixture of lithium and lead. The liquid is spun to create a vortex inside the sphere that forms a vertical cavity in the middle. At this point, two donut-shaped plasma rings held together by self-generated magnetic fields, called spheromaks, are injected into the cavity from the top and bottom of the sphere and come together to create a target in the center. "Think about it as blowing smoke rings at each other," says Doug Richardson, chief executive of General Fusion.
On the outside of the metal sphere are 220 pneumatically controlled pistons, each programmed to simultaneously ram the surface of the sphere at 100 meters a second. The force of the pistons sends an acoustic wave through the lead-lithium mixture, and that accelerates into a shock wave as it reaches the plasma, which is made of the hydrogen isotopes deuterium and tritium.
If everything works as planned, the plasma will compress instantly and the isotopes will fuse into helium, releasing a burst of energy-packed neutrons that are captured by the lead-lithium liquid. The rapid heat buildup in the liquid will be extracted through a heat exchanger, with half used to create steam that spins a turbine for power generation, and the rest used to recharge the pistons for the next "shot."
The ultimate goal is to inject a new plasma target and fire the pistons every second, creating pulses of fusion reactions as part of a self-sustaining process. This contrasts with ITER, which aims to create a single fusion reaction that can sustain itself. "One of the big risks to the project is nobody has compressed spheromaks to fusion-relevant conditions before," says Richardson. "There's no reason why it won't work, but nobody has ever proven it."
General Fusion's basic approach isn't entirely new. It builds on work done during the 1980s by the U.S. Naval Research Laboratory, based on a concept called Linus. The problem was that scientists couldn't figure out a fast-enough way to compress the plasma before it lost its donut-shaped magnetic confinement, a window of opportunity measured in milliseconds. Just like smoke rings, the plasma rings maintain their shape only momentarily before dispersing.
Nuclear-research giant General Atomics later came up with the idea of rapidly compressing the plasma using a mechanical ramming process that creates acoustic waves. But the company never followed through--likely because the technology to precisely control the speed and simultaneous triggering of the compressed-air pistons simply didn't exist two decades ago.
Richardson says that high-speed digital processing is readily available today, and General Fusion's mission over the next two to four years is to prove it can do the job. Before building a fully functional reactor with 220 pistons on a metal sphere, the company will first verify that smaller rings of 24 pistons can be synchronized to strike an outer metal shell.