The most intriguing questions Whyte and his students were exploring had to do with how tokamaks could produce lots of electricity without being gigantic and expensive. MIT’s tokamak, which still sits in a two-story tall, garage-like room in a former Nabisco cookie warehouse, generated a magnetic field by running electricity through copper coils that surrounded a round metal chamber. In that chamber, plasma would be heated with microwaves and other methods to millions of degrees. On one of its last runs, it set a new record for plasma pressure while hitting 35 million Celsius.

Just outside the chamber, the vital measurement isn’t heat, but cold. The magnets that squeeze the plasma in place have to be kept well below minus-200 Celsius, or else their performance will degrade from a buildup of electrical resistance.

Particle accelerator. Photo: Monty Rakusen/Getty

It was a graduate student who suggested that the MIT team see what would happen if they made magnets out of a newly developed superconducting tape. A superconductor conducts electricity so well that it doesn’t build up electrical resistance and this new tape maintains that property, even at slightly higher temperatures than other superconductors do.

Using less energy on cooling could make a tokamak cheaper to run. But that benefit was minor compared to the other things Whyte’s group figured out. As they plotted out ways of winding the tape into coils in a tokamak, they realized this method could double the strength of the magnetic field they could exert on a plasma. Increasing the field strength is crucial because plasma is wild. It’s unstable and evasive, and only overwhelming force can keep it from spreading out and cooling too much.

Perhaps best of all: using this tape instead of rigid superconductors could make the machine 10 times smaller.

That led them to another problem with traditional tokamaks. If you need to replace parts of the machine, you have to take the whole thing apart and put it back together. That’s unacceptable for a power plant in regular use. And again, one of Whyte’s graduate students had a great idea. If you apply the superconducting tape in sections, with joints, the magnets can be snapped on and off for quick and easy repairs or upgrades.

“This was the beginning of the ‘aha!’ moment,” Whyte says. “The people who are in CFS were in that class.”

Other big ideas kept coming. One of the great things about fusion is its inherent safety. It’s impossible for this tiny star to slip out and cause trouble, because the plasma’s weird physical state can’t be sustained outside of the magnetic field. Still, the plasma does send something out that you’ve got to deal with: neutrons.

Fusion projects generally aim to fuse two forms of hydrogen: deuterium and tritium. Deuterium is readily available in seawater, but tritium is very rare, so you have to make it. (More on that in a minute.) In this version of fusion, 80 percent of the energy that is released comes out in the form of neutrons. These are subatomic particles that have no electric charge, so they’re not contained by the magnetic field. They come flying out like angry spittle.

In fusion experiments measured in seconds or less, flying neutrons aren’t a big problem. But over time, they can be nasty. These particles jump a foot and a half from the plasma and have enough energy to rearrange the atoms in the tokamak’s inner wall, eventually degrading it. What to do about that in a power plant that needs to run for long stretches?

Whyte describes the answer with a wry smile. “We turned the problem around,” he says.

In essence, the MIT plan takes a ride on the neutrons by catching them in a liquid. Neutrons wreck solid materials by scrambling the order of their atoms, but liquids are already disordered, by definition. In the design that CFS is developing, the neutrons pass through an inch or two of steel and then barrel into a liquified salt, which they essentially just heat up. Then, that molten salt can be pumped around a power station to generate electricity. By the way, there’s lithium in the molten salt, and when neutrons hit lithium, they create tritium, which you can take out and use to fuel the fusion reactor.

Thetatron Experiment designed to study the ionisation and compression produced in deuterium plasma, 1964. Photo: Fox Photos/Getty

This setup isn’t perfect, however. Blanketing the tokamak’s steel wall with molten salt will lessen, but not eliminate, the damage that the neutrons would otherwise cause to the metal. It will have to be replaced every so often. Just how often? That’s a crucial question for the cost of a power plant.

For now, Whyte says, the metal barrier should last a year or two. That’s not great, so materials that better withstand neutrons have to be developed, to extend the lifespan of that wall. That looks doable; reducing the erosion of the wall in fusion reactors is a long-standing field of research.

But the issue is nonetheless significant enough that General Fusion, the company that intends to compress plasma with pistons, plans to keep a solid metal case relatively far away. It will directly surround the plasma with liquid metal that gets pumped off to convert its heat to electricity. There will be lithium in that liquid, too, to breed tritium.

Even if the MIT team manages to extend the life of the barrier, there’s another issue: The neutron bombardment will eventually render the metal radioactive.

Is that a big problem? Well, one of the novel things about a fusion company called TAE Technologies, which has raised $600 million from Google, the late Microsoft founder Paul Allen, and other luminaries, is that it plans to fuse hydrogen protons with boron, a reasonably abundant element, because that reaction emits hardly any neutrons. TAE’s co-founder and CEO, Michl Binderbauer, says that because of its cleaner profile, hydrogen-boron fusion is “the single shining opportunity for mankind.”

But since we’re talking about fusion, of course there’s a catch. Hydrogen-boron fusion is much harder to pull off: The plasma has to get to billions of degrees, not millions. And the “reaction rate” is much lower, which means less fusion happens. TAE is going to start with deuterium-tritium fusion before trying to work its way up.

In the meantime, Whyte and just about everyone else in fusion thinks deuterium-tritium fusion is well worth gunning for. Any radioactive components in MIT’s design will be relatively small and have a short half-life. The material would be nowhere near as problematic as the stuff that comes out of nuclear power plants today. If fusion plant operators have to replace the inner wall from the reactor Whyte envisions, they’d “put it in a swimming pool for 10 years,” he says. “And then you can walk up beside it.”