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Heat spikes: fusers chill, rocketeers cheer.

Heat spikes: Fusers chill, rocketeers cheer

A phenomenon that puzzled and hindered a team of laser-fusion scientists may give a new boost to rocket research. Propellants two to four times as energy-rich as any used today could emerge from the mystery's recent solution, a NASA program manager says, allowing engineers to develop significantly smaller and less expensive rockets to carry hefty payloads.

The discovery that has fired rocketeers' interest comes from Lawrence Livermore (Calif.) National Laboratory, where researchers have been trying to develop new targets for inertial-confinement fusion. In the course of that work, they found surges of heat within frozen hydrogen targets that they had hoped to use as fusion fuel sources.

In inertial-confinement fusion, laser or ion beams bombard a tiny hydrogen pellet from all sides simultaneously to implode it. This can create enough heat and pressure to force the hydrogen nuclei to fuse and form helium nuclei, a process that releases enormous amounts of energy. So far, however, attempts at inertial-confinement fusion have failed to yield more energy than they consume.

Gilbert W. Collins, P. Clark Souers and their colleagues were seeking to improve their mixtures of the frozen hydrogen isotopes deuterium and tritium by aligning the nuclear spins of the molecular hydrogen lattice in a special arrangement. Theoretically, nuclei in a "spinpolarized" lattice would fuse at only half the laser power required for other targets.

The researchers tried harnessing freeroaming hydrogen atoms within the lattice to influence the spins of their paired molecular cousins, but the troublesome heat bursts continually interfered. Sometimes the spikes arose spontaneously; at other times they followed modest heating or a rapid change in the magnetic field used to align spins.

"The heat spikes were driving us up a wall," Collins says.

A NASA scientist had reported similar energy surges 14 years ago, and two theorists had proposed that the heat resulted when free hydrogen atoms suddenly combined en masse into hydrogen molecules. That work was largely forgotten, Collins says, but it ultimately provided the explanation for the heat spikes detected at Livermore. Subsequent tests by the Livermore group provided the first verification of the 14-year-old theory, Collins says.

In the July 23 PHYSICAL REVIEW LETTERS, the Livermore team reports successfully suppressing the heat surges by capping hydrogen samples with a layer of liquid helium. But the researchers say the bursts may find a welcome at another stage of the fusion process. Maintaining near-perfect beam uniformity is crucial during target compression, and heat bursts might provide a way to make the targets more uniform. When the spike occurs, the sample explodes and then recondenses evenly on the inner walls of its container, Collins says.

And far bigger heat bursts, if properly controlled, might show promise for propulsion, the researchers suggest. Since the 1950s, NASA and the Air Force have sought ways to store free atoms within hydrogen lattices until their energy is needed. Already, rocket experts have responded to the new report with enthusiasm, says Bryan Palaszewski, a program manager at NASA's Lewis Research Center in Cleveland.

Collins and his colleagues have made the "first real attempt to understand how energy is released [from atomic hydrogen]," Palaszewski says. "If we can understand how it is released, we can understand how to store it."

Even so, he adds, atomic-hydrogen fuels won't see the launch pad for 30 to 50 years.
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Title Annotation:nuclear fusion research may lead to new rocket engines
Author:Weiss, Peter L.
Publication:Science News
Date:Aug 4, 1990
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