The diamond target is at the end of a diagnostic cone attached to the gold hohlraum.
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 megabars (50 million times
Earth’s atmospheric pressure). By 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-nanosecond, 750-kJ NIF laser pulse within 176 beams to quasi-isentropically compress, or
“ramp-compress,” multiple thicknesses of diamond to nearly four times
the pressure previously achieved in similar experiments by the same team
at the University of Rochester’s OMEGA laser. Use of the timedependent
ramp-wave compression technique was essential to keep the material
relatively cool and solid to higher pressures than could be achieved
with standard near-instantaneous shockphysics experiments.
It is the reduced level of heating made possible by ramp compression that makes these experiments relevant to the study of planetary interiors. The standard technique for
studying material conditions relevant to planetary interiors is through quasi-static compression in a diamond anvil cell. Those platforms, however, are limited to peak
pressures of about three megabars (Mbar), while planetary core pressures range from about 3.5 Mbar (Earth) to about eight Mbar (Neptune), 30 Mbar (extra-solar super-
Earths), 40 Mbar (Saturn) and 70 Mbar (Jupiter).
“The 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. In fact we believe we can build on these initial experiments and go to even higher pressures,” said the experiment’s principal investigator, Ray Smith of LLNL’s Physics Division.
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, while challenging, is key 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. Unraveling interior structure requires determining the
properties of planetary materials under extreme pressure-temperature (P-T) conditions.
Image captured by one of the two VISAR (Velocity Interferometer System for Any Reflector) channels. Data gathered by VISAR provides information for the determination of the equation-ofstate (EOS) of carbon including its many potential structures at extreme pressures. EOS express the thermodynamic relationship between pressure, temperature and volume and are used to generate computational models of material behavior.
determination of the equation of state (EOS) is a fundamental
requirement. Diamond is especially interesting from an astrophysical
standpoint because carbon is the fourth most abundant element in the
universe and is of central importance in planetary science. Ice giant
planets such as Neptune and Uranus contain large quantities of methane
that decompose at high pressure and temperature conditions, possibly
forming diamond-rich layers in their interiors. A number of extra-solar
Neptunes and super-Neptunes have already 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.
Smith said shock experiments have made considerable progress in furthering understanding of the phase diagram of diamond in the vicinity of the melting boundary,
and the pioneering ramp-loading work at the OMEGA laser has investigated the EOS and strength of diamond at pressures up to eight Mbar.
“The behavior of diamond in the solid state at 10 to 100 megabars, however, is wholly unexplored at present,” Smith said. “This represents an exciting opportunity for NIF