A 40-year-old nuclear fusion experiment is emerging in style

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THE HORSE ancient Iranian archers had a trick called the Parthian shot. When they were returning after a charge they could turn their bodies around in the blanket and release a final salvo. That’s not a bad description of the announcement made on February 8 by the management of the European Community (JET), a fusion experiment at Culham, England. After 40 years of work, JET shop closed in December. But the result of one of his finals, which was held on October 3 last year, gives Parthian a good outlook.

Fusion reactors are sometimes said to mimic the process that keeps the sun shining. That’s not quite right. Protons are the raw materials of solar fusion, the nuclei of the lightest and most abundant form of hydrogen. There are several steps in the process that turn them into a helium nucleus (two protons and two neutrons). The method used in JET, a type of reactor called a tokamak, creates helium in one 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 terrestrial motions emit large amounts of energy, but they have different subatomic by-products – two positrons and two neutrinos in the former, and a neutron in the latter.

JETthe reaction vessel was, as the name suggests, toroidal. Magnetic fields confined and heated the deuterium and tritium within this doughnut-shaped cavity to form plasma (a type of gas-like substance consisting of atomic nuclei and separate electrons) with a measure- volume of 90 cubic meters. At a temperature of 150m kelvin, nuclei collide in a plasma thus coalescing.

JET he never assumed it was a prototype for commercial power production. He was there to study the physics of malleable plasmas, without emitting more energy than he consumed. But when its operators turned the dial up to 11 last October, they managed to achieve a single 5.2 seconds of continuous rotation, producing 69 megajoules of energy, for an average power output of 13 megawatt. Although small by the gigawatt standards of today’s power stations, that is a record for a tokamak and therefore a good step in the right direction. What comes next, however, is much debated.

Some look to ITER, an experimental tokamak being built in France by a coalition of governments. This, when it opens on a yet to be confirmed date, will be nine times the size of plasma JET. Meanwhile, on the other side of the Atlantic, in Massachusetts, a private enterprise in the form of Commonwealth Fusion Systems is building SPARKtokamak which he hopes to launch next year. SPARK plasma volume will be less than JET but, thanks to design advances using superconducting magnets, twice as much energy should be emitted as it is. The company hopes that this will be the last step before a fully commercialized device.

In addition to these projects, a second private reactor is planned in Culham itself by a company called Tokamak Energy. Other companies in both North America and Britain, using different approaches to tokamaks, are also joining the fray. And the British government is hoping to get involved too, with a so-called tokamak WINTER. All these initiatives, however, are helped by JETand results over the decades. Welcome and farewell, then, to one of the workhorses of fusion research.

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