Devens startup raises another $1 billion to finish its first fusion energy machine

Commonwealth Fusion Systems, one of the leading startups racing to perfect a new clean energy technology, said on Thursday it raised $1 billion in new funding to help complete its first demonstration machine.

The largest venture capital deal in Massachusetts in almost three years, the funding will go towards finishing SPARC, Commonwealth’s proof-of-concept fusion machine in Devens, expected to come online sometime next year, as well as supporting the planning for the company’s first commercial device, dubbed ARC, at a site in Virginia.

“What we used to think was impossible now is feeling more and more inevitable,” cofounder and chief executive Bob Mumgaard said during a Zoom call with reporters.

Mumgaard declined to disclose the identity of the investors or the company’s value based on the new capital infusion.

The fusion process joins hydrogen atoms together to generate heat that can be used to produce electricity. Unlike a nuclear plant’s fission reactor, which generates heat from splitting atoms apart, fusion produces no long-lasting hazardous radioactive waste and cannot trigger a runaway reaction or meltdown.

But no one has yet demonstrated that a fusion reaction can generate more energy than is required to keep it going for more than a fraction of a second. Commonwealth and its rivals have competing designs and are using different materials to build their fusion machines.

Commonwealth has now raised a total of $4 billion and will need to raise billions more, but “not tens of billions,” to complete its commercial plant in Virginia in the mid-2030s, Mumgaard said.

“I’d love to do fusion for $10 million in my garage if that was doable, if the laws of nature allowed that to happen,” he said. “However, we’re just not in that universe.”

Commonwealth’s latest funding round was the largest by a Massachusetts-based company since health care tech company Devoted Health in Waltham raised $1.2 billion in September 2023, according to data from research firm PitchBook. Commonwealth holds the record for the largest-ever VC deal in the state, thanks to a $1.8 billion round it raised in 2021.

The fusion company also announced it had hired Lorence Kim as chief financial officer. A former investment banker at Goldman Sachs, Kim was the CFO of Moderna from 2014 to 2020, including when the drug maker went public in 2018.

On a call with reporters, Kim was quick to quiet speculation that joining Commonwealth was a sign the fusion company was going public soon. “Don’t take my arrival as an indication that this is IPO prep,” he said.

Aaron Pressman can be reached at aaron.pressman@globe.com. Follow him @ampressman.