OpenAI CEO Sam Altman. Josh Edelson / AFP via Getty Images Sam Altman said he owns about a third of Helion Energy, a nuclear fusion power startup. An exhibit in his trial against Elon Musk gave unusual clarity to his finances. Altman is a billionaire, but not from shares in OpenAI. Sam Altman on Tuesday said his investment in nuclear energy startup Helion, which is deep in talks on a power purchasing agreement with OpenAI, has climbed to over $1.6 billion in value. That's up from the $375 million he invested in the company in November 2021, leading its Series E round. While testifying at the trial over OpenAI's for-profit ambitions , Altman was presented with a document he'd signed on April 24. The exhibit gave an unusually clear picture of Altman's financial standing, showing the value of his holdings in a slate of buzzy startups. Altman confirmed on the witness stand that he had a stake in Helion worth more than $1.6 billion as of the end of 2025.…