Though a few billion dollars is not a lot of money in the fusion world, it’s still a lot for private investors to pony up for tech that may or may not work. So they went back to the drawing board and create the SPARC (which stands for “smallest possible ARC”) fusion reactor. It would cost a few hundred million dollars, and, if it works, would be the world’s first nuclear-fusion reactor to show net-positive energy gain.

The biggest story in the fusion field these days is the growing amount of money pouring in from private investors. It’s a high-risk, high-reward strategy, says Malcolm Handley, who runs Strong Atomics, a US-based fund that only invests in fusion startups. But he insists it’s not foolish. A fusion startup today only needs to show that it has a reactor able to achieve net-positive energy gain—it doesn’t even need to build a full-fledged power plant—to prove itself. The demonstration plant, he thinks, will take somewhere between $50 million and $500 million to build.

Once it succeeds, the startup can go public or sell itself to a large company like General Electric in the US or France’s EDF Energy, which have the know-how to build complete nuclear power plants. The whole process of iterating on the science and building a demonstration plant may only take 10 years, Handley thinks.

“Building a net-energy gain tokamak today is more like the Apollo Project than the war on cancer,” says Mumgaard. That is, he believes it’s more of an engineering challenge rather than a physics problem—there are no fundamental-science hurdles to cross.

Mumgaard makes a compelling case. That’s helped him raise $75 million so far, including $50 million from the Italian oil company Eni. Though it’s not enough to build SPARC, it is plenty to do all the work to ensure that the high-temperature superconductors SPARC relies on are up to scratch.

Breakthrough Energy Ventures won’t share how much it’s actually invested in CFS. But the firm’s meaningful contribution to the latest round of investment in the startup significantly raises CFS’s chance of raising the hundreds of millions of dollars it will need to build SPARC. Mumgaard now has realistic hopes that, by 2025, his company will have built a proof-of-concept facility that will show net-energy gain is possible. Once SPARC proves that the high-temperature superconductors can do their job at scale, CFS’s plan is to build an ARC fusion reactor that will put electricity on to the grid. He thinks it can happen in as little as 15 years from today.

MIT’s C-Mod reactor with CFS’s proposed SPARC and ARC reactors.
MIT’s C-Mod reactor with CFS’s proposed SPARC and ARC reactors.
Image: Commonwealth Fusion Systems

Many in the fusion community are pessimistic about the future of tokamaks. “The pessimists say we’ve simply spent the most time and money on tokamaks and we’re still not there,” says Handley. Among the 20 or so fusion startups, only three are building off the tokamak design.

Moreover, SPARC and ARC rely on the success of YBCO high-temperature superconductors, which are expensive to produce and understudied. “We think that the MIT projection of 15 years to a power plant is very ambitious, if not overly ambitious,” Tim Luce, ITER’s head of operations and science told IEEE. “But we will celebrate any success, and we share the dream of making energy from fusion.”

Handley, who is interested in investing CFS, is less pessimistic. He says Mumgaard has built one of the best nuclear-fusion teams in the world, and he notes that, unlike many other fusion startups, CFS engages deeply with the fusion community and it is genuinely open to criticism and debate. That, he says, is a good sign for anyone trying to solve a tough problem.

Carmichael Roberts, the head of investing at Breakthrough Energy Ventures, was upfront. “We didn’t compare the different types of nuclear-fusion reactors in a bake-off competition before deciding on investing in CFS,” he says. “We knew the team; they had a promising idea; they needed our help.”

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