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William H. Goldstein

William H. Goldstein
Associate Director of Physics and Advanced Technologies

A New Era in Plutonium Research

NEXT month marks the first anniversary of the first “hot” shock experiment at the Joint Actinide Shock Physics Experimental Research (JASPER) Facility at the Nevada Test Site. Last July, a team of engineers and physicists hunched intently over digital displays, watching the arrival of data so accurate and precise that they realized a new era of plutonium research had begun. “JAS-021” was the culmination of years of design, construction, and regulatory hurdles. Since last July, JASPER has fired six shots that have begun to experimentally map out the behavior of plutonium at the high pressures obtained when a nuclear weapon goes off.
JASPER, a two-stage gas gun that largely duplicates a facility at Livermore, fires projectiles at up to 8 kilometers per second into precisely fabricated plutonium targets, producing pressures up to 600 gigapascals. At these extreme conditions, experiments at the JASPER Facility achieve unprecedented accuracy, better than 1 percent. A suite of diagnostics measures the response of the target, directly yielding fundamental physics data—the equation of state—at these pressures for the most mysterious and complicated element.
Shock physics is a fundamental core competency at Livermore. The Shock Physics Group has made important contributions to a wide range of Laboratory programs since its formation in the 1960s. Over the years, the group has produced its share of scientific breakthroughs, including the first observation in 1994 of the elusive metallic phase of hydrogen, and most recently, determination of the melting point for iron in Earth’s core, as reported in Nature. Today, this capability is key to science-based stockpile stewardship. JASPER’s mission now includes a central role in the National Nuclear Security Administration’s strategy for evaluating the effects of aging on the nuclear stockpile. Understanding the behavior of plutonium at extreme conditions, particularly its equation of state, is one of the program’s highest priorities.
The success of JASPER is a striking reminder of the important role of experimental science and facilities aimed at basic physics in maintaining confidence in the nation’s nuclear stockpile. It sometimes seems that stockpile stewardship is primarily a simulation activity enabled by increasingly powerful computers, such as the Advanced Simulation and Computing (ASC) machines. However, the massive simulations of nuclear weapons require the properties of plutonium as input, and precise measurements remain the essential source for these data.
Exploring high-pressure physics using gas guns is nothing new. It’s a well-established, time-tested technique; in fact, it is just the thing to rely on when it comes to ensuring the nuclear stockpile. The arrival of JASPER allows us to routinely apply this technique to plutonium, a material never short on surprises.
The JASPER Facility is one element of a suite of new capabilities that together mark a renaissance in plutonium research under the auspices of stockpile stewardship. In addition to the shock data from JASPER, at least three additional milestones have been posted during this past year. At the Advanced Photon Source at Argonne National Laboratory, a consortium of researchers from Lawrence Livermore, Argonne, the University of Nevada, and the Carnegie Institute of Washington completed and commissioned a beam line devoted to static high-pressure physics. Shortly after “first light,” Livermore scientists found the first evidence of a new high-pressure structure of plutonium that had been long predicted. Another team of Livermore researchers working at the European Synchrotron Radiation Facility in Grenoble made the first measurements of phonons in plutonium, providing a unique and crucial constraint on the interatomic potentials that underlie computer simulations of equations of state. (See S&TR, January/February 2004, A First Look at Plutonium’s Phonons.) And those simulations took a major step forward this past year when Livermore physicists performed the first ab initio, fully quantum-mechanical molecular dynamics simulation for any actinide metal, in this case uranium, using the Q machine at Los Alamos National Laboratory.

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UCRL-52000-04-6 | June 4, 2004