Blasting materials to magnetic extremes.

Researchers in the laboratory don't often get a chance to glimpse the behavior of matter at the high pressures, temperatures, and magnetic fields attained in the interiors of stars and planets. As a step in that direction, an explosive series of experiments now under way at the Los Alamos (N.M.) National Laboratory is yielding data on the electric activity of various materials in extremely strong magnetic fields.

"Nobody has ever had magnetic fields this high," says Johndale C. Solem of the high-energy-density physics program at Los Alamos. "We're doing things that have never been tried before. Nothing in this experimental series is in any sense a small step from where we've been."

The current project involves researchers from Australia, Russia, Japan, and the United States, representing more than a dozen universities, laboratories, and research institutes. They apply compression technology originally developed at Los Alamos and in Russia for research on nuclear weapons.

To obtain magnetic fields in excess of 1,000 teslas, the scientists use explosive-driven generators designed and built in Russia. A household magnet typically produces a field of less than 0.01 tesla.

In the Russian device, a layer of high explosive surrounds a cylinder, a few centimeters across, made up of numerous fine, parallel wires embedded in epoxy. An initial electric pulse sends current through the wires, generating a magnetic field. When the explosive is detonated, the resulting shock wave penetrates the cylinder, shoving the wires together and turning them into an electricity-carrying cylinder, which greatly compresses the magnetic field. As the shock wave propagates inward, it compresses two additional wire-epoxy cylinders, further concentrating the accompanying magnetic field.

Consequently, samples of various materials packed into the center of this layered device experience very large magnetic fields, Solem says.

Using the Russian generator and a device designed at Los Alamos, researchers have investigated the electric behavior of superconductors, semiconductors, two-dimensional metals, and other materials.

Scientists know, for example, that strong magnetic fields cause conventional superconductors to lose their ability to conduct electric current without resistance. Theorists have predicted that when an applied magnetic field gets considerably stronger, the material can again become a superconductor.

Tentative results using the explosive generator indicate that the electric resistance of the materials tested did decrease. "But the data need further analysis," Solem cautions.

In separate experiments in Russia at the once-secret weapons development site Arzamas 16, researchers are using a similar technique to compress argon gas into a purely metallic state (SN: 4/20/96, p. 250). Data obtained at a peak pressure of 7 million times atmospheric pressure indicate that this goal hasn't yet been reached.