Sandia LabNews

Phase diagram of water revised by Sandia researchers

Two Sandia researchers have significantly altered the theoretical diagram universally used by scientists to chart the phases of water. The new model also expands the known range of water’s electrical conductivity under extreme temperatures and pressures.

“We were trying to understand conditions at [Sandia’s] Z [accelerator] when its flash goes through water,” says Thomas Mattsson (1674), a theoretical physicist, “but the problems are so advanced that they hopscotched to another branch of science and apply to the outside world to a high degree.”

The intent of Thomas and Mike Desjarlais (also 1674) was to produce more accurate information on the changing state of water in Sandia’s Z machine as its extreme amperage passes through a water bath insulating Z’s transmission lines, as well as through water switches placed along those lines to first restrain and then transmit Z’s electrical pulse.

The researchers first found the standard water-phase diagram out of whack when, on Sandia’s Thunderbird supercomputer, they ran an advanced quantum molecular simulation program able to include “warm” electrons instead of unrealistic cold ones, says Mike.

The work showed that phase boundaries for “metallic water” — water with its electrons able to migrate like a metal’s — should be lowered from 7,000 kelvin and 250 gigapascals to 4,000 K and 100 GPa.

This new range — far beyond that which Z routinely enters — is nevertheless useful because it is sure to revise astronomers’ calculations of the strength of the magnetic cores of gas-giant planets like Neptune. Because the characteristics of Neptune’s interior water partly lie in this electrically conducting sector, the water probably contributes to a magnetic field formerly thought to be generated only by the planet’s core.

Diagram confirmed experimentally

The new calculations agree with experimental measurements in research led by physicist Peter Celliers of Lawrence Livermore National Laboratory.

The computational work, paid for by Sandia’s internal Laboratory Directed Research and Development program, is part of a broad front of research to understand conditions that will prevail when the current upgrade of Z to ZR is completed in July ’07. With new giant capacitors around Z’s circular rim replacing ones 20 years old, the expected amperage sent through the machine’s 36 “spokes” to a target placed at its hub is expected to rise from 20 million to 26 million amps.

A key question for Sandia designers is to determine what characteristics of water can be expected as greater amounts of electricity pour through the machine’s switches. These switches not only rely on water’s insulating properties to momentarily restrain the current but, in water’s ionized state, to pass the pulse forward at a time interval reduced from micro- to nanoseconds.

So much electricity passing through water vaporizes it, causing pressures to rise in surrounding regions as the shock wave travels outward. But how much is the increase? How big a transmitting cavity does the ionized region form to transmit what amounts to a giant spark? And what are the best sizes for these channels, and for the switches themselves, to optimize the transmission of electrical pulses in future upgrades?

“The concern was that ZR or its successors might go beyond the ability of a water switch

to function and carry the current we want it to carry,” says Keith Matzen, director of Sandia’s Pulsed Power Sciences Center (1600). “The

concern is that more efficient, larger machines may run into a limit and their switches not meet design requirements. So the question is, how does a water switch really work from first principles?”

Understanding water’s phases

One aspect of this knowledge is to model water to get a finer understanding of its phases, he says.

The molecular modeling code, VASP (Vienna Ab-initio Simulation Package), based on density functional theory (DFT), was written in Austria and initially used at Sandia by Peter Feibelman (1114). Mike extended it to model electrical conductivity and Thomas developed a model for ionic conductivity based on calculations of hydrogen diffusion. An accurate description of water requires this combined treatment of electronic and ionic conductivity.

The adaptation of VASP to high-energy-density physics (HEDP) work at Sandia was motivated by earlier experimental measurements of the conductivity of exploding wires by Alan DeSilva at the University of Maryland. DeSilva found a considerable disparity between his data and theoretical models of materials in the region of phase space called warm dense matter. Mike’s early VASP conductivity calculations immediately resolved the discrepancy. In recent years, a team of Sandia researchers has been extending one of Sandia’s own DFT codes (Socorro) to go beyond the capabilities of VASP for HEDP applications.

“Mike was the first to pioneer this capability for warm dense matter,” says Tom Mehlhorn, manager of 1674, “and Thomas has come on to be a near-perfect complement as the work enters more complex areas.”

Information vital for ZR

Sandia’s ability to calculate electrical properties and phase diagrams “has rapidly progressed from simple metals like aluminum for Z’s flyer plates — a critical breakthrough by Mike, the need for which was driven by experimental-theoretical discrepancies,” says Tom, “to alloys like stainless steel for Z’s wire arrays and ZR’s structural conductors (with Thomas’ help), and now to water, with Thomas leading the way. This six-year history gives us a unique ability to model the extreme conditions of high-energy-density environments.”

As it turns out, the newly discovered regime will not adversely affect Sandia’s water switches on ZR. But water switches not yet designed for future upgrades may require the more accurate understanding of the phases of water discovered by the Sandia researchers, says Larry Warne (1152).

Because of Z’s success in provoking fusion neutrons from deuterium pellets, it is thought of as a possible (if dark-horse) contender in the race for high-yield controlled nuclear fusion, which would provide essentially unlimited power to humanity.

Compression of Z’s amperage in time is the cause of its huge power, equivalent to 50 times the electrical production of all the generating plants on Earth for a few nanoseconds.

The work on water phases was initially published July 7 in Physical Review Letters and most recently reported at the 12th International Workshop on the Physics of Non-Ideal Plasmas, held in Darmstadt, Germany, Sept. 4-8.