Researchers Michael Zarnstorff [left] and Kenneth Hammond at the Princeton Plasma Physics Laboratory run nuclear-fusion reactions in a stellarator built with mostly off-the-shelf parts. For a machine that’s designed to replicate a star, the world’s newest stellarator is a surprisingly humble-looking apparatus. The kitchen-table-size contraption sits atop stacks of bricks in a cinder-block room at the Princeton Plasma Physics Laboratory (PPPL) in Princeton, N.J., its parts hand-labeled in marker...