Taking Hydrogen to the Extreme

Hydrogen at extremely high pressures, upwards of a million times that on the Earth's surface, can now be produced in physics laboratories. Understanding hydrogen's behavior under such extreme conditions answers questions about the interior of Jupiter, provides coveted information on designing optimal fuel pellets for fusion energy, and yields information on aging nuclear weapons without having to test them. Reporting at the Albuquerque meeting, two national labs are producing seemingly contradictory high- pressure data on the universe's most abundant element.

Using Sandia's Z machine, which consumes tremendous amounts of electric current to generate very high magnetic fields, laboratory researchers launch a metal plate that travels at high speeds (up to 28 km/s, making it the fastest gun in the world) towards a target containing low-temperature deuterium molecules (D2). According to Marcus Knudson, who heads the group, the impact of the plate launches a shock wave that compresses D2 to up to megabars of pressure. Deuterium, a neutron-containing isotope of hydrogen, is used because its higher density enables it to be compressed to much higher pressures than ordinary hydrogen.

The Livermore experiments, on the other hand, used the high-power (and recently decommissioned) Nova laser to shock compress liquid D2. According to Livermore's Robert Cauble, his group finds D2 to be much more compressible than do the Sandia researchers. At a million atmospheres, for example, Livermore finds the D2 to be compressed by a factor of 6 while Sandia sees a compression of a factor of 4.

If the Livermore results are correct, then there is more metallic hydrogen in Jupiter's interior than previously thought and it is easier than expected to trigger self-sustaining nuclear fusion in deuterium fuel pellets, since they would be more compressible. If the Sandia results are right, then more traditional assumptions hold. But it's also possible, Cauble says, that both results are right (each group's compression occurs in slightly different time scales). As a final possibility, Cauble and Knudson admit, both results could be wrong (they are both relatively new techniques).

These possibilities are being carefully explored in conjunction with computer simulations of high- pressure hydrogen, which require the fastest available computers in the world. The question is likely to be settled with further experimental research, including more data from Sandia and future laser experiments, possibly occurring at Rochester's Omega facility. The ultimate goal of these experiments is to determine hydrogen's equation of state, the interrelationship between such properties as its pressure and temperature, at these high-pressure conditions. Such information can provide information on such things as the intriguing possibility that gas-giant Jupiter has a solid-rock core.

-Benjamin P. Stein