Helion, the Sam Altman-backed fusion startup, raises $465M to build a power plant for Microsoft

Helion, the fusion startup backed by Sam Altman, announced on Thursday that it had raised $465 million in a new funding round that values the company at $15.5 billion.

The startup last raised $425 million in January 2025. Altogether, Helion said it has raised $1.5 billion.

The new round, a Series G, was led by Thrive Capital with a long list of participants, including new investors Alta Park Capital, Anti Fund, BoxGroup, Lux Capital, Peak XV Partners, and Bill Ford, along with existing investors, which include Capricorn Technology Impact Funds, Lightspeed Venture Partners, Mithril Capital, Dustin Moskovitz through Good Ventures Foundation, SoftBank Vision Fund 2, and a university endowment fund.

Helion’s approach to fusion power differs from many of its peers. Some use magnets to contain the superheated plasma required for fusion conditions, while others use lasers to compress fusion fuel until it reacts. In both cases, the majority of startups plan to use steam turbines to transform the intense heat into electricity.

But Helion, which uses magnets to compress the fuel, intends to harvest electricity straight from the magnets themselves. When fusion occurs in the plasma inside the reactor, it expands, pushing against the magnetic fields. That force can be drawn off the magnets as electricity, similar to how an electric vehicle can reverse its motors to provide braking force and recharge the battery.

Such a configuration would dramatically improve the efficiency of a fusion power plant. But some fusion experts are skeptical it could work. That’s in part because Helion, unlike many of its competitors, doesn’t frequently publish in peer-reviewed journals, so physicists haven’t been able to poke at the theoretical underpinnings. David Kirtley, Helion’s CEO, argues that eventual results from the company’s fusion devices should be sufficient. “We don’t want to theorize about fusion,” he told me last year. “We just want to go build it.”

Helion isn’t alone in attracting fresh funding. The fusion sector has become an investor darling in recent months. Focused Energy and Thea Energy both announced new rounds last week: Focused for $240 million, Thea for $100 million. In February, Inertia Energy emerged from stealth with a $450 million Series A, and the month before, Type One Energy said it was in the process of raising $250 million for a Series B.

The investments have poured in despite fusion’s lengthy timeline. Though several companies have made progress in recent months on milestones they say pave the way to a viable power plant, most predict they won’t b