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AN OUTSIDER in the race towards efficient fusion power is fast gaining on more established rivals. Researchers have passed a key milestone in the development of a relatively new approach called the Z-pinch, although achieving fusion is still a long way off.

Nuclear fusion promises unlimited clean power: make a mix of hydrogen isotopes hot enough for their nuclei to fuse, and they’ll yield colossal amounts of energy. But getting this to work on a practical scale remains fraught with problems.

Physicists have had some success using powerful magnetic fields to trap a hot deuterium-tritium plasma long enough for fusion to begin, but this is proving incredibly expensive. Another method, called inertial confinement, is to blast pea-sized pellets of frozen hydrogen with radiation. This produces a mini hydrogen bomb in which the exploding outer layers squash the atoms in the core to make them hot and dense enough for fusion to start. In principle, a bank of powerful lasers could provide the radiation, but with current laser technologies this probably won’t generate much power compared to what’s put in.

Now researchers are turning their attention to a relatively new and more efficient inertial confinement approach, using a machine called a Z-pinch. Currently being tested at Sandia National Laboratories in Albuquerque, New Mexico, it uses a magnetic field generated by a powerful pulse of electricity to crush and vaporise, or “pinch”, a set of tungsten wires a centimetre tall standing in a ring. The resulting plasma produces X-rays, which fill the surrounding cavity.

About 15 per cent of the electrical power that’s fed in gets turned into X-rays, “which in this field is just enormous,” says John Porter of Sandia. The method has already created impressively powerful pulses of electricity. In 1998, it produced a fleeting pulse of 290 trillion watts, around 80 times the output of all the world’s power stations.

But to have a chance of igniting fusion on a large scale, the Z-pinch must create radiation that is uniform in all directions, so that it crushes a fuel pellet completely evenly. This is one of the toughest challenges for the new machine, researchers believe.

Now Porter and colleagues at Sandia and other labs have measured the intensity of X-rays produced by vertically stacked pairs of Z-pinches, and found that the radiation is indeed symmetrical. Next they plan to test the machine with actual fuel pellets, to see whether they can get them to implode evenly. If that happens, Porter is confident that a larger pulsed-power machine will produce high fusion yields.

The Z-pinch is sometimes overlooked because it appeared relatively late in the game, says Stephen Dean, president of the non-profit research group Fusion Power Associates. But, he says, given its potentially low cost, the system is starting to look like a very good option.

There are still a number of technical problems with the Z-pinch, though. For example, the machine is currently incapable of firing rapidly many times in a row.