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Helion Energy, a nuclear fusion startup backed by OpenAI’s Sam Altman, still has to prove it can produce electricity to serve data centers and other customers. But investors seem confident it can deliver.

The Everett, Wash.–based company said it has raised $465 million in a funding round led by Thrive Capital, the investment firm founded by Josh Kushner, a big backer of OpenAI. Helion’s valuation after the investment was $15.5 billion, nearly triple the company’s $5.43 billion valuation during its prior round, announced in January 2025.

Other investors in the round include Alta Park Capital, Anti Fund, BoxGroup, Lux Capital, Peak XV Partners and Bill Ford, the executive chair of the Ford Motor Co. Many of the company’s existing investors, including Lightspeed Venture Partners, SoftBank Vision Fund 2 and Mithril Capital, also participated.

The steep increase in Helion’s valuation reflects the continued clamor among investors to put money into startups in and adjacent to the AI boom. While Helion has signed deals with customers, it has not yet begun to generate revenue from them. Like other clean energy startups, Helion is capitalizing on the explosion in demand for electricity driven in part by the proliferation of data centers.

In Helion’s case, its potential to provide a novel source of reliable, clean energy to AI companies attracted the early personal backing of Altman, who disclosed last month, during a trial in Elon Musk’s lawsuit against OpenAI, that he owned a third of Helion. In March, Altman stepped down from Helion’s board of directors, a move aimed at enabling the fusion company and OpenAI to explore future partnerships together, Helion’s CEO and founder David Kirtley said at the time.

Helion is building a generator that relies on nuclear fusion, the same process that powers the sun and other stars. In fusion, atoms combine under extreme heat and pressure, releasing energy in the process. It is the opposite of fission, the process of splitting atoms that is at the heart of traditional nuclear power plants. Both processes create energy without producing greenhouse gases, but fusion promises to do so without the safety risks of fission, such as meltdowns caused by runaway chain reactions and long-lasting radioactive waste.

In an interview, Kirtley said the new round of capital from investors will allow the company to advance two of its most important goals. The first of those is the completion of a manufacturing facility in Everett, dubbed Omega, to build capacitors, a critical component of Helion’s fusion generators, which store electrical energy.

The second goal is to begin operations at Orion, the company’s first commercial fusion power plant, which Helion has begun building in Malaga, Wash. Helion is aiming to begin producing electricity for customers at the plant in 2028 and has an agreement with Microsoft to ultimately deliver 50 megawatts of power for its data centers from the facility.

“What this means is we get to move faster,” Kirtley said of the new funding round. “This is accelerating what we’re building at Helion.”

The evidence of that building effort was on display on a recent tour of its operations in Everett, a city about a 45-minute drive north from Seattle. At the facility, Kirtley showed off a functional prototype of Helion’s fusion generator, Polaris, which is helping to drive the design of its Orion power plant. A multistory concrete box with walls 2 to 5 feet thick surrounds the generator. Inside the box, the hourglass-shaped generator is surrounded by a seemingly endless number of black power cables.

Kirtley was able to provide a tour of the box because it was a weekend, so Polaris was not operating and the machine was silent. The security and safety protocols for a fusion generator are very different from those for a nuclear fission reactor. Fission reactors use highly radioactive materials such as uranium for fuel. Nuclear power plants require large security perimeters in part to account for related safety risks.

In contrast, Polaris is housed in an unassuming industrial office park formerly occupied by a supplier to Boeing, which has a huge airplane factory nearby. The generator relies for fuel on a far less radioactive isotope, tritium, which doesn’t produce the same long-lived radioactive waste as uranium. On the recent tour of Helion’s generator, this reporter wore a dosimeter, a radiation monitoring device, which showed zero radioactivity present.

Because they pose fewer risks, fusion plants are generally more lightly regulated than traditional nuclear power plants, which have to navigate a thicket of federal rules. In Washington, for example, Helion is licensed by the state’s department of health, which also oversees equipment like the particle accelerators hospitals use in cancer treatments.

A big part of Helion’s physical presence in Everett is devoted to manufacturing the electronics that go into its generator, including its capacitors. Helion uses thousands of the devices to power the machines. One of the company’s biggest priorities is building as many of its capacitors as possible in-house to reduce its reliance on Chinese-made versions.

“We joke sometimes that we’re more of a power electronics company than a fusion company,” Kirtley said during the recent tour.

Still, Helion has to hit some significant milestones in the next couple of years to show it can be a viable long-term business. The company has missed key deadlines in the past. And more broadly, the concept of fusion has been talked about for so long as a source of cheap, clean electricity that there’s an old joke about it: The technology is 20 years away and always will be.

This week, Kirtley said the big increase in Helion’s valuation this week shows its investors believe the probability of the company hitting its goals has grown. “Fusion is going to be here a lot faster than a lot of people think,” he said.