How to harness the power of the stars on a table top

A few years ago, Todd Ditmire's on-the-cheap fusion-energy experiment would have drawn smirks from high-powered research teams that work with lasers big enough to fill large halls.

Now, entering what some are calling a new realm of physics, Dr. Ditmire and his colleagues at the Lawrence Livermore National Laboratory in California, have achieved nuclear fusion - the process that lights the stars - by using a table-top laser to zap tiny puffs of gas.

Their device will never power a Sony Play Station or light up a 60-watt bulb. That's the long-term goal of researchers trying to design and build large-scale fusion reactors that would turn water into steam to generate electricity.

Ditmire's approach, however, could accelerate fusion research by reducing the cost to conduct such studies, allowing more scientists to take part, experts say.

Moreover, table-top fusion "reactors" eventually could play an important behind-the-scenes role as cheap, environmentally friendly sources of neutrons - subatomic particles that are increasingly valuable tools in looking for oil, maintaining aircraft, and developing new materials for products ranging from pharmaceuticals to pocket calculators.

Such reactors "would give you neutrons without fission," says Greg Moses, associate dean for research at the University of Wisconsin's College of Engineering.

Fission - splitting atoms' nuclei and tapping the energy released - is the basis for today's nuclear power plants and research reactors. It yields significant amounts of radioactive waste. Fusion energy, by contrast, is released when atomic nuclei slam together with sufficient energy to merge - a relatively clean process.

Ditmire explains that his device grew out of work using lasers to probe the properties of matter. For several years, physicists and chemists have been interested in how matter responds to extremely intense, rapid pulses of laser light. When such light hits individual atoms, for example, it strips them of their electrons, turning them into ions.

When such lasers are focused on groups of 10 to 10,000 atoms, though, these clusters produce extremely hot, microscopic balls of electrically charged gas, or plasma. The plasma explodes, ejecting ions at energy levels that correspond to temperatures as high as 1 billion degrees Celsius.

When Ditmire's team aimed its laser at a jet of gas containing clusters of deuterium, a form of hydrogen, the exploding plasma ejected deuterium ions with sufficient energy to fuse those that collided. The energy level of neutrons emerging from the fused nuclei provided the "smoking gun," establishing the process as a fusion reaction.

Each shot of the laser produced 10,000 of these neutrons, compared with 1 billion to 100 billion per shot with larger, more powerful fusion-research lasers, according to a presentation Ditmire made at a recent meeting of the American Physical Society. (His team is publishing more-detailed results this week in the journal Nature.)

Still, Ditmire says, it should be possible to beef up his table-top laser-fusion device to the point where it could serve as an economical neutron source.

While Ditmire and his team pursue small-scale laser fusion, other researchers are trying to harness sound waves to generate fusion reactions, according to Andrew Szeri, associate professor of mechanical engineering at the University of California at Berkeley.

By aiming intense, pure, high-pitched sound at bubbles rising in water, researchers have been able to force the bubbles to compress so rapidly and to such a small volume that the gas inside fleetingly reaches temperatures several times that of the surface of the sun.

At the point of maximum scrunch, the bubble's diameter is 1/100th the diameter of a human hair. In addition, such bubbles emit pulses of light as they expand and compress with the passing sound waves, leading researchers to dub the phenomenon sonoluminescence.

Dr. Szeri notes that researchers worldwide are trying several techniques to raise temperatures and pressures inside the bubbles another 100-fold to reach fusion-friendly levels. Some hope to achieve fusion by giving bubbles an additional sharp boost in pressure as they compress. Others are reducing the pitch, or frequency, of the sound used. Others are modifying the sound by using more than one pure tone. So far, Szeri says, no neutrons.

THE extent to which table-top fusion reactors become even modest commercial successes remains to be seen. Brookhaven National Laboratory physicist Steve Shapiro says the size of table-top devices will have to increase to provide enough neutrons for sophisticated studies, making the reactors anything but table top.

But to those pursuing table-top fusion, part of the value of their efforts lies in the chase. "If we can develop commercial applications early, the public gets used to fusion, we generate some revenues, and we learn some physics along the way," says Gerald Kulcinski, director of the University of Wisconsin's Fusion Technology Institute. Such efforts, he suggests, could help build public support for the ultimate goal: using power plants that harness the energy source of the stars to meet humanity's energy needs.