A 40-year-old nuclear-fusion experiment bows out in style

THE HORSE archers of ancient Iran had a trick known as the Parthian shot. When retreating after a charge they were able to twist their bodies around in the saddle and loose a final salvo. That is not a bad description of the announcement made on February 8th by the controllers of the Joint European Torus (JET), a fusion experiment at Culham, England. After 40 years of operation, JET shut up shop in December. But the result of one of its final runs, conducted on October 3rd of last year, makes an excellent Parthian shot.

Fusion reactors are sometimes said to mimic the process that keeps the sun shining. That is not quite accurate. The raw materials of solar fusion are protons, the nuclei of the lightest and most abundant form of hydrogen. The process that turns them into a helium nucleus (two protons and two neutrons) has several steps. The approach used in JET, a type of reactor called a tokamak, creates helium in a single step by reacting together the nuclei of two heavier types of hydrogen: deuterium, with a proton and a neutron, and tritium, with a proton and two neutrons. Both the solar and the terrestrial transmutations release lots of energy, but they have different subatomic by-products—two positrons and two neutrinos in the case of the former, and a neutron in the latter.