Fusion device crosses threshold. (Physics).

By sparking thermonuclear reactions, a machine simply called Z has joined the big leagues among potential technologies for producing power from controlled nuclear fusion.

Thermonuclear fusion takes place when matter becomes so hot that violent collisions force atomic nuclei to fuse together. Those reactions unleash a flood of energetic neutrons.

Until now, the only other fusion-energy approaches that achieved thermonuclearfusion reactions in a lab are one in which huge lasers blast tiny, hydrogen-filled capsules and another in which hulking reactors use potent magnetic fields to squeeze hot, ionized hydrogen gas.

Here's how Z goes thermonuclear: About the size of a hockey rink, the machine pumps 19 million amperes of electric current through a bracelet-size, cylindrical array of tungsten wires within about 100-billionths of a second. The wire cage vaporizes, collapsing inward and radiating X-rays toward a sesame-seed-size capsule of heavy hydrogen. This heats and squeezes the hydrogen until some of it fuses into helium and tritium nuclei.

Four years ago, temperatures at Z had already topped 2 million degrees, enough for thermonuclear fusion (SN: 1/23/99, p. 63). Yet both the machine and the hydrogen capsule needed further tweaking, says Thomas A. Mehlhorn, a Z project leader at Sandia National Laboratories in Albuquerque, N.M., where the fusion device is located.

Now the temperature in Z soars past 10 million degrees, and each blast, called a Z-pinch, yields some 10 billion neutrons, Sandia researchers reported April 6 at a meeting of the American Physical Society in Philadelphia. With numbers like that, "we're now credible," Mehlhorn says.--P.W.