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America's Underdog Fusion Experiment Is Closing In on the Nuclear Future

Sandia Labs' Z-Machine is one step closer to delivering commercial fusion.

by Michael Byrne
Oct 12 2014, 12:00pm

The fusion-chasing Z-Machine has to be the most sci-fi-looking experiment in a field of research that's so often regarded as just that: science-fiction. Nonetheless, researchers working on the Z-Machine experiment, based at New Mexico's Sandia National Laboratories, are reporting a crucial advance in their quest for energy's holy grail—the detection of "significant numbers" of neutrons, a byproduct of fusion reactions.

Fusion is nothing less than the promise of free energy, at least relatively speaking, a version of nuclear power that releases no waste products and is based on a self-limiting process. So: no meltdowns, by definition, and no radioactive leftovers needing to be shoved under the carpet while politicians play hot potato with their long-term storage responsibilities. It figures then that fusion would be really, really hard to do.

It's challenging enough that the world at large has essentially pooled its resources in a megaproject located in the south of France called ITER. At $50 billion, it's the most expensive science project every pursued by humans, beating out the Large Hadron Collider several times over. ITER is not the only game in town, however, and smaller-scale projects like Z-Machine continue to chase the dream for a whole lot less money. In this case, they're also doing it in a radically different way.

The fundamental problem in fusion is that the nuclei of atoms repel each other just due to everyday electromagnetic forces: like charges repel, opposite charges attract. Those everyday electromagnetic forces happen to be extremely powerful at close range and in order to get atomic nuclei near enough to fuse—in which the electromagnetic force gives up to the attractive strong nuclear force—they need to collide at speeds upwards of 1,000 kilometers per second.

These sorts of collisions can be accomplished via massive amounts of heat, requiring temperatures of up to 50 million degrees Celsius.

That sort of heat isn't free. It takes lots and lots of energy. So, to create a fusion reaction, we're already investing massive amounts of power and successful fusion becomes a question of whether or not we can harvest more power than we expend to facilitate the reaction in the first place. As fusion research stands right now, it's like we have a free liter of gas waiting for us just x number of kilometers away. So far, that x is large enough such that we're using more than a liter to get to where the "free" gas is, resulting in a net loss of energy. Fusion then just remains a neat nuclear trick.

The ITER project is built around what's known as a Tokamak reactor. This is basically a pool of superheated plasma shaped like a doughnut that contains the fusion reaction's fuel. The hotter something gets, the more its atoms bounce around and crash into each other. Those crashes may eventually happen with enough energy for the atoms to fuse.

An alternative approach to fusion involves blasting tiny bits of hydrogen with super-high energy laser beam pulses. The idea is that the beam will compress the hydrogen enough such that a fusion reaction results. This is the approach being taken at California's National Ignition Facility. Neither the Tokamak nor the laser approach have resulted in a reaction delivering more energy than it took to create.

The Sandia fusion technique is a bit of column A and a bit of column B. "It crushes fuel in a fast pulse, as in laser fusion, but not as fast and not to such high density," Daniel Clery reports in Science magazine. "Known as magnetized liner inertial fusion (MagLIF), the approach involves putting some fusion fuel (a gas of the hydrogen isotope deuterium) inside a tiny metal can 5 millimeters across and 7.5 mm tall. Researchers then use the Z machine to pass a huge current pulse of 19 million amps, lasting just 100 nanoseconds, through the can from top to bottom. This creates a powerful magnetic field that crushes the can inward at a speed of 70 km/s."

X-ray emissions indicated that the reaction produced "a hot fuel region" lasting about 2 nanoseconds.

Meanwhile, the fuel is preheated with another, gentler laser beam and a magnetic field is applied to hold it in place. So, we wind up with a heated plasma that is then blasted with a laser pulse. Instead of flying off into space, the magnetic field constrains the plasma so that the laser acts to compress it instead. And then, fusion.

The latest Sandia results were published last month in the journal Physical Review Letters. The physicists reported a reaction at 35 million degrees Celsius with each pulse delivering around 2 trillion neutrons; neutrons are one product of a reaction fusing two nuclei of deuterium atoms (along with either helium-3 or tritium). X-ray emissions indicated that the reaction produced "a hot fuel region" lasting about 2 nanoseconds.

As Clery notes, these latest results offer 100 times the number of neutrons produced by the experiment a year ago, yet the project will still need to beat out the current number 10,000 times over to reach energy break-even. That seems extreme, but simulations have suggested that Sandia's maximum power output, 27 million amps, is sufficient to produce a reaction at the break-even point.

"On a future facility, trapped alpha particles would further self-heat the plasma and increase the fusion rate, a process required for break-even fusion or better," the current paper's lead author, Adam Sefkow, noted in a Sandia news release.

An upgrade would boost that to 60 million amps, enough to push the project's fusion scheme well into or at least very close to the realm of commercial viability. A 2012 paper presenting results for simulated Z-Machine fusion conditions at 60 million amps concluded, "For a drive current of 60 MA the simulated gain exceeds 100, which is more than adequate for fusion energy applications."

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