US-based fusion company Helion Energy has raised $465m in a Series G funding round. The round values the Washington-based nuclear fusion startup at a $15.5bn post-money valuation, nearly tripling its previous valuation. This massive funding injection brings Helion’s total capital raised to date to $1.5bn.
The financing round was led by Thrive Capital. New Investors included Lux Capital, Peak XV Partners, BoxGroup, Anti Fund, Alta Park Capital, and Ford Motor Company Executive Chairman Bill Ford. Returning Investors included Lightspeed Venture Partners, SoftBank Vision Fund 2, Mithril Capital, and Dustin Moskovitz’s Good Ventures Foundation.
Unlike traditional tokamak designs, Helion Helio was built on the Field-Reversed Configuration (FRC) and Magneto-Inertial Fusion (MIF) that shoots pulses of plasma together and directly recovers electrical energy using magnetic forces instead of traditional steam turbines.
Helion’s approach of rapid iteration and testing has enabled the company to make some progress toward a commercial fusion machine. Its 7th-generation prototype, Polaris, is intended to demonstrate the first electricity produced from fusion. With its previous prototype, Trenta, Helion was the first private company to achieve a fuel temperature of 100m degrees Celsius, which is generally considered the required operating temperature for a commercial fusion power plant. Polaris became the first privately funded fusion machine to operate with deuterium-tritium fuel and reached internal record plasma temperatures exceeding 150m degrees C.
In July 2025, Helion announced plans to build its eighth prototype, Orion, a 50 MW fusion plant at property owned by the public utility district operators of Rock Island Dam near Malaga, in Chelan County, Washington. The company said the plant is intended to be operational by 2028, supplying power to nearby Microsoft data centres. In 2023, Helion had announced the world’s first fusion power purchase agreement (PPA) to supply energy from Orion Microsoft by 2028, with Constellation Energy serving as power marketer. Helion also announced a 500 MW development deal with steelmaker Nucor.
Helion ultimately wants Orion to run on deuterium and helium-3, which would produce fewer higher-energy neutrons and maximise its direct capture of electricity. But helium-3, produced by the radioactive decay of tritium, is exceedingly scarce, and fusion reactions are harder to achieve, requiring temperatures of about 200m degrees C. Polaris is also intended to commercially produce helium-3 “for the first time ever here on Earth”, according to David Kirtley, Helion’s co-founder and CEO.
Helion will use the $465m to build out its physical infrastructure across three primary areas:
- Accelerating the construction of Orion;
- Expanding US Manufacturing by boosting high-volume manufacturing at its Omega capacitor facility in Everett, Washington, to mass-produce critical electrical components;
- Fulfilling its power purchase agreement with Microsoft to deliver at least 50 MW of electricity by 2028.
“Fusion is no longer a future idea, but a path to clean, reliable, affordable always-on electricity at scale. This funding accelerates our ability to deliver on that promise,” said Kirtley. “This support of new and existing investors is a strong signal they believe, as we do, that Helion is best positioned to generate electricity from fusion for customers this decade, not the next, and that we have the right technology and strategy to build the commercial fusion market over the long term.”
According to the scientific and energy consensus, Helion’s timeline to deliver grid electricity by 2028 is highly improbable. While the company has hit major plasma engineering milestones, prominent plasma physicists characterise its commercial timeline as “audacious” and borderline “science fiction” given the unsolved physical constraints.
Kirtley said in 2014 that Helion could get a fusion reactor up and running in three years. In 2023, he said that the company would be able to generate fusion power and “go after commercially installed power generation” by 2024.
Dr Troy Carter, a plasma physicist and director of the Plasma Science and Technology Institute at UCLA noted that Helion’s plan to bypass steam turbines via direct energy capture is conceptually brilliant and presents a genuine technical edge. However, he said the operational execution of this approach is a massive “if”, since the plasma conditions required are extraordinarily difficult to stabilise.
Sam Altman, billionaire CEO of OpenAI was leading the Y Combinator incubator in 2014 when the Helium’s founders joined it. He served as the Chairman of Helion’s Board of Directors for 11 years from 2015 and was instrumental in pushing Helion’s engineering team away from slow-moving academic timelines toward aggressive, commercial milestones Kirtley credited Altman with forcing the company to “dream big” and “build with urgency”.
Court documents from a May 2026 legal hearing revealed that Altman’s personal stake in Helion had ballooned to over $1.6bn, giving him ownership of roughly one-third of the company. In March 2026, Altman stepped down from the Board in face of a conflict of interest after OpenAI began exploring partnerships to purchase electricity from Helion although he retains his financial ownership stake in Helion.
Dr John Slough, the chief visionary behind Helion’s core technology and a co-founder of the company, left Helion in May 2018 in view of fundamental scientific and ethical disagreements regarding the company’s engineering direction. Slough spent four decades developing MIF at the University of Washington and his private research lab. In 2013, he teamed up Kirtley to spin his patents out into Helion Energy
The primary rift between Slough and Helion’s executive leadership involves the choice of nuclear fusion fuel. Leadership pivoted the commercial strategy toward a deuterium-helium-3 (D-He3). Slough strongly objected to this, pointing out that a standard deuterium-tritium (D-T) fuel reaction has an energy gain 250 times higher than D-He3. He believed chasing a D-He3 fuel cycle was an impractical and highly inefficient path for commercial power.
Slough has publicly targeted the technical viability of Helion’s scaling timeline. He argued that Helion’s executive leadership was glossing over the historically intractable problem of plasma confinement. In plasma physics, keeping an FRC stable as temperatures and pressures scale up is extraordinarily difficult.
He implicitly questioned the ethics of making aggressive commercial grid promises to investors before proving the underlying confinement physics works. As Helion moved from a lean, government-funded research outfit into a venture-backed startup optimised for massive funding rounds, the corporate culture shifted. Slough stepped away as the company leaned heavily into aggressive PR, corporate timelines, and massive capital acquisition under Altman and Kirtley.
Slough subsequently published alternative fusion reactor blueprints that completely abandon Helion’s D-He3 roadmap, opting instead for a compact staged magnetic-compression reactor utilising standard D-T fuel and a lithium-bearing blanket.
