ON 445 ACRES in southern France, construction is under way on a hugely ambitious experiment: a doughnut-shaped vacuum chamber surrounded by 11,000 tons of magnets, built to submillimeter precision by a multicountry consortium trying to harness nuclear fusion, the power source of the stars.
On paper, nuclear fusion is an energy dream: abundant, with no meltdowns, planet-baking carbon emissions, or long-lived radioactive waste. The in-progress reactor is called ITER, Latin for “the way.” It’s designed to coax hydrogen nuclei to fuse into helium, which will heat the reactor’s walls. In future reactors, this heat could boil water to drive electric steam turbines.
Getting fusion to work, however, has been an engineering nightmare. In a device called a tokamak (right), igniting fusion within a magnetically confined plasma requires temperatures of 270…