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 The electrically conducting structure of metallic water occurs at a more accessible part of the water phase diagram than formerly thought. Here, a snapshot from a first-principles computer simulation demonstrates the atomic disorder. Red spheres are hydrogen atoms, white spheres are oxygen atoms, and the electron density from a partially occupied electron state responsible for the conductivity is shown as gold. Pic: www.sandia.gov
The Sandia theoretical 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 to 4,000 kelvin and from 250 to 100 gigapascals. (A phase boundary describes conditions at which
materials change state — think water changing to steam
or ice, or in the present instance, water — in its pure
state an electrical insulator — becoming a conductor.)
The lowered boundary is sure to revise astronomers’ calculations
of the strength of the magnetic cores of gas-giant planets
like Neptune. Because the planet’s temperatures
and pressures lie partly in the revised sector, its electrically
conducting water probably contributes to its magnetic field,
formerly thought to be generated only by the planet’s
core.
The calculations agree with experimental measurements
in research led by Peter Celliers of Lawrence Livermore
National Laboratory.
Surprising results were not the intent of Sandia co-investigators
Thomas Mattsson and Mike Desjarlais. “We were trying to understand conditions at [a powerful
Sandia accelerator known as] Z,” says Mattsson, a
theoretical physicist, “but the problems are so advanced
that they hopscotched to another branch of science.”
In July 2007, Z is undergoing an extensive renovation
that will increase the machine’s pulse from 20 to
26 million amps — a 30 percent rise. The question
to researchers: How will water behave, subjected to these
more extreme conditions?
The power Z emits in X-rays when it fires is equivalent
to many times the entire world’s generation of electricity — but
only for a few nanoseconds. The machine creates high temperatures
and pressures in water because of the 20-million-amp electrical
pulses it sends through a row of water switches. First,
the water acts as an insulator, restraining the incoming
electric charge. Then, overcome by the buildup, water
transmits the pulse, shortening it from microseconds to
approximately 100 nanoseconds. This compression in time
is a key element of what makes the Z accelerator so powerful.
It is known that so much electricity passing through water
vaporizes it, causing surrounding water pressures to rise
as the shock wave from vaporization travels outward. But
how much is the increase? How big a 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 [Z Refurbishment] or its
successors might go beyond the ability of a water switch
to function as designed and carry the required current,” says
Keith Matzen, director of Sandia’s Pulsed Power Sciences
Center. “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?”
One aspect of this knowledge is to model water to get
a better understanding of its behavior under these extreme
conditions, he says. Mattsson and Desjarlais first found the standard water-phase
diagram out of whack when they ran an advanced quantum
molecular simulation program on Sandia’s Thunderbird
supercomputer that included “warm” electrons
instead of unrealistic cold ones, says Desjarlais.
The molecular modeling code VASP (Vienna Ab-initio Simulation
Package), based on density functional theory (DFT), was
written in Austria. Desjarlais extended it to model electrical
conductivity and Mattsson 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. Desjarlais’ 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.
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.
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.
Source: Sandia National Laboratories
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