Twenty-five years ago, as a young physicist, I worked on research linked to fusion energy. Nuclear fusion powers the sun and stars through reactions that turn hydrogen nuclei into helium nuclei, and if we could master the process on Earth, we'd have a safe and virtually limitless source of clean energy. At conferences every year, scientists from around the world gathered to discuss the achingly slow progress then being made. Government and university labs have now been trying unsuccessfully for more than 50 years.

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