Discovery: The Puzzle of the Plasma Doughnut

Nuclear fusion is the power source of the sun and the stars. Physicists and engineers are trying to build reactors than can harness fusion power to provide limitless clean energy. Dr Adam Rutherford and Dr Hannah Fry are joined by Dr Melanie Windridge, plasma physicist and CEO of Fusion Energy Insights, who explains why the fourth state of matter - plasma - helps get fusion going, and why a Russian doughnut was a key breakthrough on the path to fusion power