A hohlraum at the Lawrence Livermore National Laboratory.

Magnets could be the future of nuclear fusion

For scientists and dreamers, one of the greatest hopes for a vibrant future is nestled in a vineyard-covered valley east of San Francisco.

Here is the National Ignition Facility (NIF) at California’s Lawrence Livermore National Laboratory. Inside the square walls of the NIF, scientists work to create nuclear fusion, the same physics that powers the sun. About a year ago, NIF scientists got closer than anyone to a key checkpoint in the quest for fusion: creating more energy than has been put in place.

Sadly, but in an outcome familiar to those familiar with fusion, that world would have to wait. In the months following the feat, NIF scientists were unable to replicate their feat.

But they didn’t give up. And a recent article, published in the journal Physics Review Letters November 4, could bring them one step closer to solving a problem that has baffled energy seekers for decades. Their latest trick: to ignite fusion in the flow of a strong magnetic field.

Fusion power, to put it simply, aims to ape the interior of the sun. By breaking some hydrogen atoms together and making them stick together you get helium and a plot of energy. The catch is that to stick the atoms together requires very high temperatures, which, in turn, forces fusion operators to expend incredible amounts of energy in the first place.

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Before you even think about creating a feasible fusion power plant, you need to somehow create more energy than you put in.

The NIF’s container of choice is a gold-plated cylinder, smaller than a human fingernail. Scientists call this cylinder a hohlraum; it houses a hydrogen pellet the size of a peppercorn.

Upon melting, scientists fire finely tuned laser beams at the hohlraum – in the case of NIF, 192 beams in all – energizing the cylinder enough to conjure violent X-rays within. In turn, these X-rays pass over the pellet, squeezing and beating it into an implosion that fuses the hydrogen atoms. At least that’s the hope.

The NIF used this method to obtain its earth-shattering result at the end of 2021: to create around 70% of the energy invested, by far the record at the time. For plasma physicists, it was a siren call. “It has instilled new enthusiasm in the community,” says Matt Zepf, a physicist at the Helmholtz Institute in Jena in Germany. Fusion-folk wondered: Could NIF do it again?

In this case, they should wait. Subsequent laser shots failed to even come close to this original. Part of the problem is that, even with all the knowledge and abilities at their disposal, scientists have a very hard time predicting exactly what a shot will do.

[Related: Nuclear power’s biggest problem could have a small solution]

“NIF implosions currently exhibit large fluctuations in their performance, which are caused by slight variations in target quality and laser quality,” says NIF physicist John Moody. “The targets are very, very good, but slight imperfections can have a big effect.”

Physicists could keep refining their laser or tinkering with their fuel pullet. But there could be a third way to improve this performance: bathe the hohlraum and its fuel pellet in a magnetic field.

Tests with other lasers, such as OMEGA in Rochester, New York, and the Z machine in Sandia, New Mexico, had shown that this method could prove successful. Additionally, computer simulations of NIF’s own laser have suggested that a magnetic field could double the energy of NIF’s most successful shots.

“The pre-magnetized fuel will allow us to get good performance even with targets or laser delivery that is a bit outside of what we want,” says Moody, one of the paper’s authors.

So the NIF scientists decided to try it out themselves.

They had to exchange the hohlraum first. Pure gold wouldn’t do well – putting the metal under a magnetic field like theirs would create electric currents in the walls of the cylinder, tearing it apart. Scientists therefore designed a new cylinder, forged from an alloy of gold and tantalum, a rare metal found in some electronic devices.

Then the scientists stuffed their new hohlraum with a hydrogen pellet, turned on the magnetic field and lined up a shot.

In this case, the magnetic field did indeed make a difference. Compared to similar magnetless shots, the energy has tripled. It was an underpowered test, of course, but the results give scientists a new ray of hope. “The document marks a major achievement,” says Zepf, who was not one of the report’s authors.

Still, the results are still early, “essentially learning to walk before running,” warns Moody. Next, NIF scientists will try to replicate the experiment with other laser setups. If they can do that, they’ll know they can add a magnetic field to a wide range of shots.

As with everything in this hazy plane of physics, this alone won’t be enough to solve all the fusion problems. Even if the NIF manages to turn on, next comes phase two: being able to create a lot more energy than you put in, what physicists call “gain”. Especially for a laser of NIF’s limited size, Zepf says, it’s an even more worrying quest.

Nevertheless, the eyes of the fusion world will be watching. Zepf says the NIF results can teach similar facilities around the world how to get the most out of their laser shots.

Achieving a high enough gain is a prerequisite for an even more distant phase: transforming the heat of fusion power into a workable power plant design. It’s yet another milestone for particle physicists, and it’s something the fusion community is already working on.

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