Fusion roundup: Helion sets temperature record; Inertia raises $450M

Fundraising details: Inertia’s $450 million fundraising round was led by Bessemer Venture Partners, with participation from Google Ventures, Modern Capital, and other venture capital firms.

Inertia Enterprises was founded in August 2025 with the stated goal of capitalizing on and commercializing the fusion advancements made in recent years by the Department of Energy’s Lawrence Livermore National Laboratory. In December 2022, LLNL’s National Ignition Facility (NIF) conducted the first controlled fusion experiment to achieve fusion ignition.

Inertia was cofounded by Andrea “Annie” Kritcher, a member of the NIF team that conducted that experiment, along with Mike Dunne, a professor of photon science at Stanford University and associate lab director of the SLAC National Accelerator Laboratory, and Jeff Lawson, founder and CEO of the tech platform Twilio.

At the time of its founding, Inertia said it aimed to develop a new generation of low-cost lasers and fuel targets that would build on NIF’s success with laser-induced inertial confinement fusion. To that end, the company said it had partnered with LLNL and planned to spend the next four years refining its laser, target, and plant designs before beginning construction on a demonstration power plant.

In its fundraising announcement, Inertia provided some new details on its forthcoming plans. The company has dubbed the technology at the heart of their power plant design the “Thunderwall,” described as “the world’s first grid-scale fusion laser beamline.” According to Inertia, it will be “50 times as powerful (measured in average power) as any prior laser of its type.”

“In just three years, we’ve gone from the first experiment to ever produce more fusion energy than was delivered to the target, to repeating that result many times and pushing the target gain higher,” Kritcher said. “We’re now focused on translating physics we know works into a pathway toward commercial-scale fusion energy, and the real benefits it can deliver for people and the planet.”

Record temperature: Helion’s 7th-generation prototype, Polaris, entered operation at the end of 2024. In January, the company said, Polaris “became the first and currently only private fusion energy machine to use deuterium-tritium fuel.” In addition to that milestone, Polaris also achieved plasma temperatures of 150MºC.

This new temperature record breaks Helion’s own commercial fusion industry record of 100MºC, which it set with Trenta, its 6th-generation prototype. According to the company, 100MºC is broadly considered “the threshold plasma temperature for a commercially relevant fusion machine.”

Trenta used deuterium-helium-3 fuel, as opposed to the deuterium-tritium fuel currently being used by Polaris. While the successful utilization of deuterium-tritium fuel demonstrates “the company’s ability to operate and show scaling across multiple fuels,” Helion will now work to continue increasing plasma temperatures in Polaris “to demonstrate that it can reliably operate with deuterium-helium-3, which will be relevant for future Helion commercial operations.”

“We believe the surest path to commercializing fusion is building, learning, and iterating as quickly as possible,” said David Kirtley, cofounder and CEO of Helion. “We’ve built and operated seven prototypes, setting and exceeding more ambitious technical and engineering goals each time.”

While this new latest announcement did not reconfirm any commercialization timelines, Helion is tentatively committed to bringing a power plant on line by 2028 that generates 50 MW or more after a one-year ramp-up period. This plant will be supported through a power purchase agreement with Microsoft which was announced in 2023. Constellation is also attached to that deal, serving as the power marketer and transmission manager.

Helion began construction on this first commercial site—which it has named Orion—in Malaga, Wash., in July 2025.