Tokamak research entered a new and important phase last month. Shortly before midnight on 9 December, the 11‐year‐old Tokamak Fusion Test Reactor at the Princeton Plasma Physics Laboratory was fired for the first time with a 50:50 mixture of deuterium and tritium. That's the mixture of hydrogen isotopes envisioned for the first generation of fusion reactors, because DT fusion can be harnessed at much lower temperatures than DD orD3Hefusion, and the cross section for the proton‐proton reaction that powers the Sun is hopelessly small. But until now most tokamak research has been done, for practical reasons, with pure deuterium plasmas.