Few fusion startups have been as closely watched as Helion. The 12-year-old company is backed by Sam Altman, rumored to be in talks with OpenAI, and has a deal to supply Microsoft with electricity by 2028 — years earlier than its competitors. The company’s unorthodox approach to fusion power and relative secrecy has earned it plenty of fans — and critics. But don’t count its investors among the naysayers. Helion announced Tuesday a $425 million Series F raise that pushed its valuation $5.245 bil...