For long-time followers of fusion energy, Tokamak Hall is the high-water mark after decades of ups and downs. From an observation deck on the side, the surgically-clean, hospital-white room feels both like a cathedral and yet somehow too tiny for its lofty purpose. Commonwealth Fusion Systems, the company building it, says the room will soon hold a tokamak, the central donut-shaped device necessary for one approach to nuclear fusion. In the tokamak, scientists will heat up deuterium and tritium