Fusion factory

The world’s largest nuclear-fusion experiment has a new lease of life. Joint European Torus (JET), at the Culham Science Centre near Oxford, has restarted after an 18-month shutdown to replace the entire reactor lining and is now collecting data that will be vital to the construction and operation of its successor, ITER, currently being built in southern France.

ITER - the name is not an abbreviation, but is Latin for ’next’ - is intended to be the first fusion reactor to achieve self-sustaining operation and to generate net energy; that is, more energy than it takes to operate it. Like JET, it will be a tokamak: a toroidal (doughnut-shaped) vessel that constrains a plasma made from hydrogen atoms using powerful magnetic fields, while heating it by pumping in microwaves, pushing an electric current around the ring and injecting fast streams of more hydrogen atoms. Eventually, the magnetic pressure and the heat overcome the tendency of the hydrogen nuclei in the plasma - which have the same electric charge - to repel each other and they are forced together to form helium nuclei, releasing a burst of energy and a high-energy neutron.