The Sciences

The Year in Science: Physics 1997

Cluster Bombs

By Jeffrey WintersJan 1, 1998 6:00 AM


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One promising method for creating nuclear fusion is to zap pellets of hydrogen fuel with lasers. To achieve temperatures and pressures high enough to force the hydrogen to fuse into helium, however, scientists have had to build big lasers at considerable expense. Last March a group of physicists at Imperial College in London announced a way to do fusion research with a laser small enough to fit on a tabletop. The trick was to take advantage of the peculiar physics of atomic clusters.

Whereas big laser-fusion devices work by forming a tightly compressed plasma of about a hundred million trillion highly energetic hydrogen atoms, Todd Ditmire and his colleagues squirted an extremely fine mist of xenon into a vacuum chamber, where the xenon atoms hung in clusters—microscopic droplets—of about 2,500 atoms each. Then the researchers fired a sharply focused laser beam into the mist for less than a trillionth of a second. Any clusters that crossed the beam were shredded into highly energized ions and electrons. When Ditmire measured the speed of the shrapnel-like particles and thus their temperature, he found that they had reached over 940 million degrees Fahrenheit—more than 30 times hotter than the core of the sun. We expected to see high-energy ions, Ditmire says, but we certainly didn’t expect them to be that high.

How can a small laser unleash so much energy? Since the beam is tightly focused, it packs enough energy into a small enough area to trigger some fancy physics inside the xenon clusters. The intense laser comes in and rips all the electrons off this ball of atoms so that there’s a cloud of electrons within the ball, Ditmire says. As the cloud heats, it swells until it gets to a critical size at which the cloud begins to resonate. The cloud sloshes back and forth around the xenon ions, and when it’s in resonance with the frequency of the laser, you can put a lot of energy into the cloud of electrons. These energized electrons then bash the xenon ions until they themselves are superheated.

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