Sun on Earth

January 2013
Filed under: Sandia

There’ve been two primary technical routes to achieving fusion.

The first has been to heat the fuel and contain it for extended periods not with a vessel but with magnetic fields (which don’t melt). This technique, called magnetic confinement, is the one used in Tokamak-type experimental fusion reactors.

The second approach, inertial confinement fusion, opts for speed over containment. It heats a small amount of dense fuel super-quickly, usually with a laser, with the goal of extracting energy before the heated fuel expands and cools. This approach is being tested at Lawrence Livermore National Laboratory’s (LLNL) National Ignition Facility, using the world’s most powerful laser source.

To date, neither approach has produced the fusion Holy Grail of break-even – producing an amount of energy equal to that put into the system to spark fusion.

A third way to fusion

MagLIF is a newcomer, third-way approach that combines the magnetism of magnetic confinement fusion and the rapid heating of inertial confinement fusion.

“MagLIF is a new regime and it’s driven entirely by the fact that we’ve got a certain kind of machine at Sandia,” says Slutz, whose extensive MagLIF computational simulations are the prompt and guide for current experiments.

That machine is Z, the world’s most powerful pulsed-power accelerator. For several nanoseconds Z can zap a target with more energy than that being produced elsewhere on the entire Earth.

But with MagLIF, the Z machine is used not for its radiation, as is usually the case, but for the massive magnetic currents it generates.

“MagLIF involves two magnetic fields,” explains Slutz, to both heat the fuel and contain that heat.

The first case, as demonstrated in simulations on Sandia’s Red Sky and Unity supercomputers, applies a magnetic current along the length of a pencil eraser-sized cylindrical fuel capsule, or liner. The beryllium-sided fuel liner contains a mix of gaseous deuterium fuel at about air density.

“The fuel is confined by the walls of a tube,” Slutz explains. “What the magnetic field does is lower the loss of heat out of that plasma. It doesn’t actually physically hold the fuel there.”

Then, in lock-step timing with a laser preheating the fuel, the Z machine delivers the fusion coup de grace, a Z pinch: a hundred-nanoseconds-quick, 25 million-ampere magnetic pulse that acts like a Vise-Grip to implode the capsule. The implosion heats the fuel to three times the temperature of the solar core.

In detailed two-dimensional simulations using Lasnex and Hydra, two codes developed and maintained by LLNL’s George Zimmerman and Marty Marinak, MagLIF appears to be a highly energy-efficient fusion possibility. After Slutz’s pre-Christmas revelation, Sandia officials have decided the approach is worth putting to the experimental test.

Still, after three decades of working to get fusion fired up on Earth, Slutz mixes large doses of patience and caution with his optimism.

“We’re not going to have break-even (energy output) in 2013 – not even in our wildest dreams do we think that’s going to happen,” Slutz says of the ongoing MagLIF experiments. “What we’ve talked about is the much more modest idea that we could perhaps get the same fusion yield out of the fuel as the energy that we put into the fuel.”

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About the Author

Jacob Berkowitz is a science writer and author. His latest book is The Stardust Revolution: The New Story of Our Origin in the Stars.

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