The most powerful pulsed superconducting magnet system ever built is now complete, and will soon be part of the world’s grandest energy experiment. The system is the Central Solenoid of the International Thermonuclear Experimental Reactor (ITER), a towering magnet core built and tested in the United States and destined for southern France, where the international project is assembling its gigantic tokamak. The magnet system will serve as the electromagnetic “heart” of the reactor, strong enough—according to ITER—to lift a freaking aircraft carrier. For the uninitiated: Tokamaks are doughnut-shaped vessels that contain superheated plasma for nuclear fusion, the energetic reaction that…