Washington

Fusion researchers at Sandia National Laboratory in New Mexico have shown that an X-ray machine, known as the Z-machine, can be used to compress a small plastic pellet uniformly, bringing hopes of achieving controlled nuclear fusion a step closer.

The researchers want to develop the Z-pinch, as the machine's technique is known, to produce fusion energy. The technique already generates the world's most powerful laboratory X-rays. But until last month, the researchers hadn't been able to observe directly that the X-ray flash produced by their machine crushes its target pellet evenly.

Laser gazer: part of the Beamlet, which is used to capture images of material compressed by X-rays. Credit: SANDIA NATIONAL LABORATORIES

They have now confirmed this by reconstructing the powerful Beamlet laser and using it to obtain images of the compressed pellet. Beamlet was first built at the Lawrence Livermore National Laboratory in California as a prototype for the National Ignition Facility now under construction there.

The laser image shows that the pellet was compressed uniformly, by a factor of two. “Pellets have been compressed before by more than that, and they will have to be compressed a lot more to achieve nuclear fusion,” says Stephen Dean, president of Fusion Power Associates, a Maryland-based group that promotes fusion energy. But he says the finding is “a real advance” for the New Mexico laboratory.

The Sandia researchers now want to increase the laser energy during compression so that the temperatures and pressures exerted on the deuterium-filled pellet are closer to those at which fusion will occur.

Seen from above, Sandia's Z-machine looks like a wagon wheel, some 40 metres across. Its rim is surrounded by huge power capacitors, and the spokes consist of 36 aluminium and steel conductors that take power to the centre. There it is fed into extremely fine tungsten wires strung along the sides of a small cylinder.

When fired up, the capacitors send a current of 20 million amps through the conduits and into the tungsten wire. In the instant before the wires evaporate, the electrical current produces an extraordinarily powerful magnetic field, which in turn generates X-rays. The project's researchers think these X-rays could be channelled to produce controlled fusion, just as X-rays produce fusion in a hydrogen bomb.

Controlled nuclear fusion would require compression by a factor of about 20, says John Porter, manager of the Sandia project. He says he is confident that the Z-machine can achieve a factor of at least 10.

Sandia researchers spent $13 million reconstructing the Beamlet for the Z-machine. Having used it to photograph the pellet, they want to shorten its laser pulse by a factor of 1,000 and use it to inject extra heat into the pellet. This project would cost a further $30 million, the researchers say.

But even with better compression and extra heat, the existing equipment would probably fall short of achieving the self-sustaining fusion reaction that fusion researchers call ignition. “Theoretically [ignition] is within the realms of possibility,” says Porter. “But we'd be doing this more as a proof of concept than an attempt to start a real fusion burn. We are excited by the possibilities, yes, but we do have to temper that with reality.”