Nuclear Fusion Flares on a Tabletop.

The dream of sparking tabletop nuclear fusion has become a reality, promising not commercial energy but a promising scientific payoff.

The feat took place on a lab bench only about 1 meter wide and 3.5 meters long. There, scientists zapped clusters of atoms of deuterium, or heavy hydrogen, with brief but extremely powerful laser pulses.

By focusing flickers of energy lasting much less than a trillionth of second into an extremely small volume, the laser beam heats atom clusters to tens of millions of degrees Celsius, report Todd Ditmire and his colleagues at Lawrence Livermore (Calif.) National Laboratory.

The laser pulse can strip atoms of their electrons, creating a cigar-shaped speck of hot, ionized gas, or plasma. The superheated clusters explode, bashing deuterium ions together with high enough velocity to fuse into helium ions. For each helium ion formed, the fusion reaction also spits out a neutron with a characteristic energy.

The experimenters detected emission of approximately 10,000 neutrons with the predicted energy per laser pulse. Ditmire described the experiment on March 23 to a crowded session at the Centennial Meeting of the American Physical Society in Atlanta.

"It's a great achievement," comments Gerard A. Mourou of the University of Michigan at Ann Arbor. Mourou invented a means to amplify laser pulses to extremely high intensities, thus paving the way for lasers such as the one used by the Livermore scientists (SN: 2/10/96, p. 95).

The new fusion scheme is no energy panacea, researchers insist. The laser dumps about 10 million times more energy into the plasma than the neutrons carry back out. The lost energy goes into heating ions and electrons and producing photons that make the plasma glow.

When might the new technique yield more energy than it consumes? "The answer is never," says Ditmire. For a fusion reactor to produce an energy gain, it must confine its fuel long enough, usually no less than a few nanoseconds, to ignite a self-sustaining thermonuclear burn. The plasma in the tabletop device disperses too quickly--after only 200 trillionths of a second.

Fusion energy powers the sun, the stars, and thermonuclear bombs. Scientists have labored since the 1950s to tame fusion reactions for commercial power, but practical reactors remain decades away, at best.

Sustained fusion burn is one of the goals of the stadium-size laser, known as the National Ignition Facility, that Livermore is currently building (SN: 10/19/96, p. 254). Expected to be the world's largest laser, it will hurl a mighty bolt of nearly 2 million joules of energy at a fuel capsule. By contrast, the tabletop fusion laser puts out just a tenth of a joule.

Ditmire says, however, that his team's technique will permit new types of laser fusion experiments and may lead to compact neutron sources for studying materials.