The diamond target sits at the end of a diagnostic cone attached to a gold hohlraum. Image: Lawrence Livermore National Laboratory
the first university-based planetary science experiment at the National
Ignition Facility (NIF), researchers have gradually compressed a diamond sample
to a record pressure of 50 Mbar (50 million times Earth’s atmospheric
replicating the conditions believed to exist in the cores of several recently
discovered “super-Earths”—extra-solar planets three to 20 times more
massive than Earth—the experiments could provide clues to the formation and
structure of these and other giant planets, as well as the exotic behavior of
materials at ultrahigh densities.
team of scientists from University of California-Berkeley, Princeton University,
and Lawrence Livermore National Laboratory (LLNL) used a carefully shaped
20-nsec, 750-kiJ NIF laser pulse with 176 beams to ramp-compress multiple
thicknesses of diamond. The properties of compressed diamond were then measured
at more than six times the pressure previously achieved in similar experiments
by the same team at the University
of Rochester’s OMEGA
ramps give a much “softer” compression than that provided by a shock
front, so they cause less heating. Use of the time-dependent ramp-wave
compression technique is key to keeping the material relatively cool and solid
at higher pressures than could be achieved with standard near-instantaneous
reduced level of heating by ramp compression makes these experiments relevant
to the study of planetary interiors. The standard technique for studying
material conditions relevant to the inside of planets is through quasi-static
compression in a diamond anvil cell. Those platforms, however, are limited to
peak pressures of about 3 Mbar, while planetary core pressures range from about
3.5 Mbar (Earth) to about 8 Mbar (Neptune) to 30 Mbar (extra-solar
super-Earths) to 40 Mbar (Saturn) and 70 Mbar (Jupiter).
Data gathered by VISAR (Velocity Interferometer System for Any Reflector) provides information for the determination of the equation-of-state (EOS) of carbon, including its many potential structures at extreme pressures. Image: Lawrence Livermore National Laboratory
only way to recreate the interior conditions of large planets is through ramp
compression—and currently, as we’ve just demonstrated, NIF is the highest
pressure platform by some way,” says Ray Smith, the experiment’s principal
investigator and scientist in LLNL’s Physics Division. “In fact, we
believe we can build on these initial experiments and go to even higher pressures.”
many as 30% of main sequence stars are estimated to have planetary companions
and more than 500 exoplanets have been discovered to date. Determining the
interior structure and composition of exoplanets is challenging but is
important to understanding the diversity and evolution of planetary systems.
Planetary surface conditions are closely coupled to interior processes and both
have an important influence on the habitability of planetary environments.
key to unraveling interior structure is determination of the properties of
planetary materials under extreme pressure-temperature (P-T) conditions.
Accurate determination of the equation of state (EOS) is a fundamental
is especially interesting from an astrophysical standpoint because carbon
(which diamond is formed from) is the fourth most abundant element in the
universe and is important in planetary science. Ice giant planets such as
Neptune and Uranus contain large quantities of methane that decompose at high
pressures and temperatures, possibly forming diamond-rich layers in their
interiors. A number of extra-solar Neptunes and super-Neptunes already have
been discovered, and this class of planet is expected to be quite abundant. In
addition, for certain protoplanetary disks with high carbon content—the chemistry
of planetary formation—may lead to the formation of carbon-rich planets dominated
by carbides and diamond.
overall technical and scientific importance and its potential use in laser
compression experiments as an ablator or target make this material one of the
most important for immediate study,” Smith says.