The US government has put its weight behind efforts to create an economically viable fusion reactor, endorsing a new category of designs that could become the most efficient and viable yet.

Test cell of the NSTX-U.
Image credits Elle Starkman / PPPL Office of Communications.

Re-creating the atom fusing processes that sustain the sun on Earth has long been one of the holy grails of modern physics. Hydrogen fusion has been powering out Sun for the past 4.5 billion years now, and it’s still going strong — a machine that could safely and stably harvest these processes would offer humanity safe, clean, and virtually endless energy.

But, at the risk of stating the obvious, making a star isn’t easy. Physicists have seen some progress in this field, but a viable fusion reactor still remains out of their grasp. We’re inching forward, however, and in an effort to promote progress the US government has just backed plans for physicists to build a new kind of nuclear fusion device that could be the most efficient design yet.

Harnessing the atom…again

Our nuclear plants today rely on nuclear fission — the splitting of an atom into tinier atoms and neutrons — to produce energy, and they’re really good at it. Per unit of mass, nuclear fission releases millions of times more energy than coal-burning. The downside is that you have to deal with the resulting radioactive waste, which is really costly and really hard to get right.

But merging atoms, in nuclear fusion, produces no radioactive waste. If you heat up the nuclei of two lighter atoms to a high enough temperature, they merge into a heavier one releasing massive amounts of energy, with the only reaction product being the fused atom. It’s an incredibly efficient process, one that sustains all the stars in the Universe, our sun included.

So there’s understandably a lot of interest into taking that process, scaling it down, and harvesting it to power our lives. Physicists have been trying to do just that for the past 60 years and still haven’t succeeded, a testament to how hard it can be to put “a star in a jar.” The biggest issue, as you might have guessed, is that stars are incredibly hot.

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While fission can be performed at temperatures just a few hundred degrees Celsius, fusion takes place at star-core temperatures of several millions of degrees. And because our would-be reactors have to jump-start the reaction from scratch, they need to generate temperatures in excess of that. A successful reactor should be able to resist at least 100 million degrees Celsius. Which is a lot.

“During the process of nuclear fusion, atoms’ electrons are separated from their nuclei, thereby creating a super-hot cloud of electrons and ions (the nuclei minus their electrons) known as plasma,” Daniel Oberhaus said for the Motherboard.

“The problem with this energy-rich plasma is figuring out how to contain it, since it exists at extremely high temperatures (up to 150 million degrees Celsius, or 10 times the temperature at the Sun’s core). Any material you can find on Earth isn’t going to make a very good jar.”

So what scientists usually do to keep the plasma from vaporizing the device is to contain it through the use of magnetic fields. So far, the closest anyone’s gotten to sustainable fusion is a team of physicists at the Wendelstein 7-X stellarator in Greifswald, Germany, and researchers at China’s Experimental Advanced Superconducting Tokamak (EAST) – both of which have been trying to hold onto the super-heated plasma that results from the fusion reaction.

The German device managed to heat hydrogen gas to 80 million degrees Celsius and sustain a cloud of hydrogen plasma for a quarter of a second last year. That doesn’t sound like a lot but it was a huge milestone in the world of physics. Back in February, the Chinese team reported that it successfully generated hydrogen plasma at 49.999 million degrees Celsius, and held onto it for 102 seconds. Neither of these devices has proved that fusion can produce energy — just that it is possible in a controlled environment.

Physicists at the US Department of Energy’s Princeton Plasma Physics Laboratory (PPPL) think that progress has been so slow because we’ve been working with the wrong jar. They plan to redesign the fusion reactor to incorporate better materials and a more efficient shape — instead of using the traditional tokamak to contain the plasma in a doughnut-like shape, they suggest employing spherical tokamaks, more akin to a cored apple. The team writes that this spherical design halves the size of the hole in the doughnut, meaning we can use much lower energy magnetic fields to keep the plasma in place.

Traditional tokamak.
Image credits Matthias W. Hirsch / Wikimedia.

The smaller hole could also allow for the production of tritium – a rare isotope of hydrogen – which can fuse with another isotope of hydrogen, called deuterium, to produce fusion reactions.

They’ve also set their sights on replacing the huge copper magnets employed in today tokamak designs with high-temperature superconducting magnets that are far more efficient because electricity can flow through them with zero resistance.

To save development time, the team will be applying these improvements to two existing spherical tokamaks – UK’s Mega Ampere Spherical Tokamak (MAST), which is in the final stages of construction, and the PPPL’s National Spherical Torus Experiment Upgrade (NSTX-U), which came online last year.

“We are opening up new options for future plants,” one of the researchers behind the study, NSTX-U program director Jonathan Menard, said in a statement.

“[These facilities] will push the physics frontier, expand our knowledge of high temperature plasmas, and, if successful, lay the scientific foundation for fusion development paths based on more compact designs,” added PPPL director Stewart Prager.

Right now, all we can do is wait and see the results. But if this works, we’ll be one step closer to creating stars right here on Earth — then plugging them right into the grid to power our smartphones.

The full paper titled “Fusion nuclear science facilities and pilot plants based on the spherical tokamak” has been published in Nuclear Fusion.