Seattle-area company Helion Energy gets $425 million from investors including Sam Altman and Softbank to pursue clean energy's "Holy Grail."

Helion’s nuclear fusion promises have garnered the attention of Silicon Valley—and raised concerns amongst scientists wary of its aggressive timeline.

A US start-up whose backers include tech billionaires Sam Altman and Peter Thiel has raised $425mn to keep it on track to achieve its target of producing electricity from nuclear fusion in 2028. Helion has the most ambitious timeline among start-ups racing to develop nuclear fusion,