AS THE old joke has it, fusion is the power of the future—and always will be. The sales pitch is irresistible: the principal fuel, a heavy isotope of hydrogen called deuterium, can be extracted from water. In effect, therefore, it is in limitless supply. Nor, unlike fusion's cousin, nuclear fission, does the process produce much in the way of radioactive waste. It does not release carbon dioxide, either. Which all sounds too good to be true. And it is. For there is the little matter of building a reactor that can run for long enough to turn out a meaningful amount of electricity. Since the first attempt to do so, a machine called Zeta that was constructed in Britain in the 1950s, no one has even come close.

At the moment, the main bet being placed by fusion enthusiasts is on ITER, the International Thermonuclear Experimental Reactor, a research machine that can hold 840 cubic metres of hot, gaseous fuel. It is being bolted together at a projected cost of €15 billion ($22 billion) in the south of France. ITER is what is known as a tokamak, a doughnut-shaped device invented in Russia at about the same time Zeta was active. Deuterium (along with an even heavier hydrogen isotope called tritium, which is made by bombarding either deuterium or lithium with neutrons) is injected into the doughnut, heated to the point at which its electrons break free and it forms a plasma, and squeezed by magnetic fields.

If the speed of the nuclei (a consequence of their temperature) and their density (a consequence of the magnetic squeezing) can both be made high enough, that will overcome the mutual repulsion of the nuclei's positive electric charges. This allows a short-range phenomenon called the strong nuclear force to take over and causes the nuclei to merge and form helium. The fusion of deuterium and tritium into helium in this way releases energy—enough of it, in theory, both to power the reactor and to yield a surplus that can be converted into electricity. It also releases neutrons, which engineers hope to use to make tritium and thus close the fuel cycle.

Unfortunately, there is a fundamental snag. The shape of the reactor means that the magnetic field which does the squeezing (and thus also keeps the superhot plasma away from the walls) produces different forces in the inner and outer parts of the doughnut. That would result in a turbulent release of plasma if it were not counteracted by a second magnetic field created by an electric current induced in the plasma itself.

The problem is that sustaining this second current is hard, and if its level varies too much, the system breaks down. That means the reactor is constantly starting and stopping. This is not a tenable arrangement for a commercial power station. One of ITER's goals is to get the length of individual runs up to 50 minutes. (In ITER's predecessor, the Joint European Torus, runs lasted for a matter of seconds.) Even that, though, is not really satisfactory. Hence the interest in another reactor design, the stellarator, a rival to the tokamak which fell behind in the 1960s but which is now making a comeback.

Twist and shout

A stellarator is a tokamak with twists in it. The consequence of its Daliesque geometry is that every particle inside the machine experiences the same forces as it travels around. A stellarator therefore needs only one magnetic field to manage the plasma, and can be run indefinitely rather than just for a few minutes.

The reason stellarators fell out of fashion was that their magnetic fields used to leak plasma faster than those of a tokamak. That, however, is no longer the case. The latest stellarators have their magnetic-field-inducing coils sculpted into complex shapes, so as to ensure that forces are uniformly distributed. Unlike their predecessors from the 1960s, modern computers can handle the complex calculations required to come up with the right shapes.

The Wendelstein 7-AS, a tiddler with a fuel capacity of but a single cubic metre, was built by the Max Planck Institute for Plasma Physics at Garching, Germany, and operated from 1988 to 2002. An analysis of its performance showed its containment capacity did, indeed, match a tokamak's. As a result, the 7-AS is being followed by a larger machine, the Wendelstein 7-X, which is being built (see photograph on previous page) by the Max Planck Institute in Greifswald and has a capacity of 30 cubic metres.

The 7-X will cost €377m, to be provided by the German government, the local state government, the European Union—and, since July, by America's Department of Energy, which agreed to supply $7.5m-worth of magnets, wall cladding and measuring instruments as its contribution. Although nowhere near the size of ITER, the Wendelstein 7-X is still a substantial machine. It should show whether stellarators can be scaled up to a useful size. If they can then, just possibly, the old quip will be shown to be wrong—and the future of fusion might actually arrive.