Magnets might be the future of nuclear fusion
For both scientists and dreamers, the best place to find bountiful energy in the future is east of San Francisco, in a winery-coated vale.
This is the National Ignition Facility, (NIF), in California’s Lawrence Livermore National Laboratory . Scientists are trying to create nuclear fusion using the same physics as the sun. NIF scientists came close to reaching a crucial milestone in the quest for nuclear fusion a year ago: creating more energy than was consumed.
Unfortunately–but in a familiar outcome to those familiar with fusion–that world would have to wait. NIF scientists weren’t able to replicate their feat in the months that followed.
But they aren’t giving up. And a recent paper, published in the journal Physics Review Letters on November 4, might bring them one step closer to cracking a problem that has confounded energy-seekers for decades. Their latest trick is to ignite fusion within strong magnetic fields.
Fusion power is a way to imitate the sun’s inner workings. You can make hydrogen atoms stick together by smashing them together and making them stick. This will give you helium and a lot of energy. However, making the atoms stick together takes very high temperatures. This, in turn, means that fusion-operators must spend enormous amounts of energy.
To make a fusion power plant feasible, you must create more energy than what you have already put in. The tipping point, which plasma physicists refer to as ignition, has been the goal of fusion for a long time.
The NIF’s preferred container is a cylinder with gold plating. It is smaller than a fingernail. Scientists call this a hohlraum, as it contains a pellet of hydrogen fuel in peppercorn size.
At fusion time, scientists fire finely tuned laser beams at the hohlraum–in NIF’s case, 192 beams in all–energizing the cylinder enough to evoke violent X-rays within. Those X-rays then wash over the pellet, squeezing it and battering it until it forms an implosion that fuses hydrogen atoms together. At least that is the hope.
NIF used this method to achieve its smashing result in late 2021: creating some 70 percent of the energy put in, far and away the record at the time. It was a siren call for plasma physicists. “It has breathed new life into the community,” says Matt Zepf , a physicist from the Helmholtz Institute Jena, Germany. Fusion-folk wondered if NIF could do it again.
They would have to wait. The subsequent laser shots didn’t succeed in getting even close to the original. Scientists are unable to predict what a shot will do, despite all their knowledge and capabilities.
“NIF implosions are currently showing significant fluctuations in their performance, which is caused by slight variations in the target quality and laser quality,” says John Moody, a physicist at NIF. “The targets are very,very good, but small imperfections can have a huge effect .”
Physicists can continue to fine-tune their laser or tweak their fuel pullet. However, there may be a third option to improve performance: bathing the hohlraum with its fuel pellet in magnetic fields.
Tests with other lasers, like OMEGA in Rochester, New York, and the Z-machine in Sandia, New Mexico–had shown that this method could prove fruitful. Computer simulations of NIF’s laser showed that a magnetic field could double NIF’s best-performing shots.
“Premagnetized fuel will allow us get good performance even when targets or laser delivery is a little off the mark,” Moody, one if the paper’s authors, said.
So NIF scientists decided that they would try it.
They had to first replace the hohlraum. Pure gold would not work well as a magnet field such as theirs would cause electric currents to build up in the cylinder walls and tear it apart. The scientists created a new cylinder from an alloy of tantalum and gold, which is a rare metal used in some electronics.
The scientists then filled their new hohlraums with a hydrogen pellet and switched on the magnetic fields. Finally, they lined up a shot.
As it turned out, the magnetic field did indeed make a difference. The energy was three times higher than similar magnetless shots. Although it was a low-powered test shot to begin with, scientists now have a new hope. Zepf, who wasn’t the author of the report, said that “the paper marks a major accomplishment.”
Moody warns that the results are still early days and that “essentially we are learning to walk before running.” Next, scientists from NIF will attempt to replicate the experiment using other laser setups. If they are able to do so, they will be able to add a magnetic field for a wide range shots.
As with all things in this murky plane of physics it won’t solve all the problems of fusion. Even if NIF achieves ignition, phase two is what physicists refer to as “gain”. This is a more difficult quest, Zepf says, because NIF’s small size makes it even more difficult.
However, the fusion world’s eyes will be on them. Zepf believes that NIF’s results could be used to teach other facilities around the globe how to get the best from their laser shots.
Obtaining a sufficient gain is a prerequisite to a phase that’s even farther into the future: actually converting the heat from fusion power into a practical power plant design. This is a new step for particle physicists, and it’s something that the fusion community has already begun to work on.
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