Laser experiments offer insight into evolution of "gas giants" (Forwarded)

Public Affairs
Lawrence Livermore National Laboratory

Contact: Anne M. Stark
Phone: (925) 422-9799

FOR IMMEDIATE RELEASE: April 25, 2008

NR-08-04-06

Laser experiments offer insight into evolution of "gas giants"

By shooting the high-energy Omega laser onto precompressed samples of
planetary fluids, scientists are gaining a better understanding of the
evolution and internal structure of Jupiter, Saturn and extrasolar giant
planets.

The properties of dense helium (He) -- which happens to be a principal
constituent of giant gas planets like Jupiter -- at thermodynamic conditions
between those of condensed matter and high-temperature plasmas are
theoretically challenging and unexplored experimentally.

Laboratory scientists collaborating with researchers at the Laboratory for
Laser Energetics, CEA France and UC Berkeley were able to determine the
equation of state (EOS) for fluid He at pressures above 100 GPa (one million
times more pressure than the Earth's atmosphere -- one GPa (gigapascal)
equals 10,000 atmospheres).

The only previous high temperature and pressure He EOS data available for
constraining planetary models was performed at LLNL by Bill Nellis and his
team using a two-stage gas gun. However, those earlier experiments used
cryogenic techniques at ambient pressure so their densities were
significantly lower than those achieved with the precompressed samples.
Also, the final pressures, 16 GPa for a single shock, were significantly
lower than the new laser shock data.

Theoretical research points out that material deep within a planet's
interior could exhibit unusual characteristics, such as high-temperature
superconductivity, superfluidity and Wigner crystallization.

"The state of materials in the center of a giant planet are difficult to
observe and challenging to create or predict," said Gilbert Collins of the
Physical Sciences Directorate. "Defining the equation of state of helium at
these pressures is a first step to deepen our understanding of these massive
objects."

Jupiter is thought to contain matter to near 100 Mbar (100 million
atmospheres of pressure.)

The LLNL team of Jon Eggert, Peter Celliers, Damien Hicks and Collins,
together with several university collaborators from UC Berkeley, the
Carnegie Geophysical Institute, CEA, Princeton, Washington State and the
University of Michigan, plan to conduct experiments at the National Ignition
Facility. There they will be able to recreate and characterize the core
states of solar and extrasolar giants, as well as terrestrial planets, such
as the recently discovered "superEarths," to better understand the evolution
of such planets throughout the universe.

Using the Omega laser at the Laboratory for Laser Energetics at the
University of Rochester, the team launched strong shocks in He that was
already compressed to an initial high state of pressure and density in a
diamond anvil cell. Precompression allows researchers to tune the sample's
initial density and the final states that can be achieved with strong
shocks.

Quartz was used as a reference material, allowing shock velocities to be
determined just before and after the shock crossed the quartz-He interface.
This technique reduced the measurement uncertainty as compared to previous
studies.

"By applying a strong shock to a precompressed sample," Collins said, "we
can re-create the deep interior states of solar and extrasolar giant
planets."

The diamond anvil's thickness determines the initial precompressed pressure.
To prevent the sample from being heated before the shock, a preheat barrier
was used to absorb the high-energy X-rays. An ultrafast diagnostic called
VISAR (Velocity Interferometer System for Any Reflector), which works like a
speedometer for shocks, recorded the shock velocity of the sample and
reference material. From these data, the team determined the density and
pressure of the shocked precompressed helium.

A pre-compressed helium sample is shown prior to shot in diamond anvil cell.
The square is quartz reference, the circle is a gasket containing
high-pressure fluid helium. After the shot, all that remains is a 2 mm hole
in the target.

By applying laser-driven shocks to statically compressed samples, equation
of state data for fluid He have been obtained with sufficient accuracy in
the 100 GPa pressure range to test theoretical predictions.

They also discovered that near 100 GPa, the shock-compressed He transformed
to an electronically conductive state and the shock front reflects the
532-nanometer probe laser beam of the VISAR.

The research also has other applications in the national security arena
because the extreme conditions in a planet's deep interior also occur during
a nuclear weapon detonation. Plans are under way to significantly extend
these research results with experiments at the National Ignition Facility.

The research appeared in the March 28 edition of Physical Review Letters.

Founded in 1952, Lawrence Livermore National Laboratory has a mission to
ensure national security and to apply science and technology to the
important issues of our time. Lawrence Livermore National Laboratory is
managed by Lawrence Livermore National Security, LLC, for the U.S.
Department of Energy's National Nuclear Security Administration.

More Information:

* OMEGA Laser (Laboratory for Laser Energetics)
http://www.lle.rochester.edu/

IMAGE CAPTIONS:

[Image 1:
https://publicaffairs.llnl.gov/news/..._omega_big.jpg
(2.5MB)]
Shown is a time-integrated photo of one of the Omega laser experiments where
the research team discovered ultra high compressibility of helium at the
metal insulator transition.

[Image 2:
https://publicaffairs.llnl.gov/news/...ic457x261s.jpg
(28KB)]
Jupiter is thought to contain matter to near 100 Mbar (100 million
atmospheres of pressure).

[Image 3:
https://publicaffairs.llnl.gov/news/...sample_big.jpg
(152KB)]
A pre-compressed helium sample is shown prior to shot in diamond anvil cell.
The square is quartz reference, the circle is a gasket containing
high-pressure fluid helium. After the shot, all that remains is a 2 mm hole
in the target.