HYDROGEN is the simplest and most abundant of elements. Composed of one proton and one electron, it makes up 90% of our universe (by number of atoms). On Earth, hydrogen is commonly found as a diatomic molecular gas. But on Jupiter, where interior pressure is millions of times greater than that at our planet's surface, the hydrogen molecule is theorized to exist as a superhot liquid metal.![]() ![]() ![]() ![]() |
How Our Gas Gun Works
Our shock compression studies use a 20-meter-long, two-stage light-gas gun built by General Motors in the mid-1960s for ballistic missile studies; the gun has been in operation at the Laboratory since 1972.
The gun consists of a first-stage breech containing up to 3.5 kilograms of gunpowder and a pump tube filled with 60 grams of hydrogen, helium, or nitrogen gas; and a second-stage evacuated barrel for guiding the high-velocity impactor to its target.
Hot gases from the burning gunpowder drive a heavy (4.5- to 6.8-kilograms) piston down the pump tube, compressing the gas. At sufficiently high pressures, the gas eventually breaks a rupture valve and enters the narrow barrel, propelling a 20-gram impactor housed in the barrel toward the target.
When the impactor hits the target, it produces a high-pressure shock wave. In a fraction of a microsecond, the shock wave reverberates through the target. Diagnostic equipment, triggered by the initial wave, measures the properties of the shocked material inside the target during this extremely brief period.
Projectile velocity can range from 1 to 8 kilometers per second (up to 18,000 mph). The preferred velocity is achieved by selecting the appropriate type and amount of gunpowder, driving gas (hydrogen for velocities at or above 4 kilometers per second, helium and nitrogen for lower velocities), pressure required to open the rupture valve, diameter of the barrel, and the metal and mass of the impactor.
The velocity of the shock wave, when combined with the initial conditions (impactor velocity, known densities, equation of state of the projectile and target materials) yields a precise measure of the pressure, density, and energy attained.
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Quest for Metallic Hydrogen![]() ![]() ![]()
Our Approach |
Our Results![]() ![]() ![]() ![]() ![]() |
Implications/Future Research![]() ![]() ![]() ![]() ![]() |
Key Words: gas gun; hydrogen--fluid, liquid, metallic; Jupiter; National Ignition Facility; shock compression tests; stockpile stewardship.
References
1. S. T. Weir, A. C. Mitchell, and W. J. Nellis, "Metallization of Fluid Molecular Hydrogen," Physical Review Letters 76, 1860 (1996).
2. R. S. Hawke, et al., "Observation of Electrical Conductivity of Isentropically Compressed Hydrogen at Mbar Pressures," Physical Review Letters 41, 994 (1978).
3. "The Diamond Anvil Cell: Probing the Behavior of Metals under Ultrahigh Pressures," Science & Technology Review, UCRL-52000-3-96 (March 1996), pp. 17-27.
4. R. J. Hemley, et al., "Synchrontron Infrared Spectroscopy to 0.15 eV of H2 and D2 at Megabar Pressures," Physical Review Letters 76, 1667 (1996) and H. N. Chen, et al., "Extended Infrared Studies of High Pressure Hydrogen," Physical Review Letters 76, 1663 (1996).
5. W. J. Nellis, et al., "Electronic Energy Gap of Molecular Hydrogen from Electrical Conductivity Measurements at High Shock Pressures," Physical Review Letters 68, 2937 (1992).
6. W. J. Nellis, M. Ross, and N. C. Holmes, "Temperature Measurements of Shock-Compressed Liquid Hydrogen: Implications for the Interior of Jupiter," Science 269, 1249 (1995).
7. W. J. Nellis, S. T. Weir, and A. C. Mitchell, "Metallization and Electrical Conductivity of Hydrogen in Jupiter," Science (in press).
Physicist WILLIAM NELLIS joined the Laboratory in 1973. His specialty is the investigation of condensed matter both during and after high-pressure shock compression. The highlight of this work is the observation of the metallization of fluid hydrogen at 1.4 megabars pressure and nine-fold compression. He has delivered invited talks at 44 professional conferences since 1979 and is the author or co-author of more than 100 papers. A fellow of the American Physical Society's Division of Condensed Matter Physics, Nellis holds M.S. and Ph.D. degrees in physics from Iowa State University. He received his B.S. in physics from Loyola University of Chicago.