The UK's Joint European Torus fired its final experiment before decommissioning and set a fusion energy record on the way out: 0.2 milligrams of deuterium-tritium fuel, less mass than a grain of sand, held together in a hot plasma for 5.2 seconds and released 69 megajoules. That is the most energy JET produced in four decades of operation, and it came from a machine already being dismantled. JET works by bending plasma into a ring inside a doughnut-shaped magnetic cage (a tokamak) so the charged fuel never touches the vessel wall, which would cool it below the temperature fusion needs. Deuterium and tritium nuclei are heavy hydrogen isotopes; fusing them releases energy because the resulting helium nucleus weighs slightly less than the two isotopes combined, and that missing mass comes out as heat. The 69 megajoules is roughly enough to run a household kettle for a week, which sounds unimpressive until you remember the fuel that produced it weighed less than a fifth of a milligram.

The number matters because it is a data point ITER needs and can no longer generate for itself: JET was the closest existing machine to ITER's size and magnetic field strength, so its final shots are the last real-world check on how a deuterium-tritium plasma of that scale behaves before ITER, the international tokamak under construction in southern France, attempts its own first plasma. JET's operator, the UK Atomic Energy Authority, is now moving into the machine's teardown phase at Culham, recovering tritium and cataloguing components for reuse in future reactors. The record stands until ITER runs its own deuterium-tritium campaign, currently not expected before the early 2030s.