Researchers at Lawrence Livermore National Laboratory have successfully shock-compressed diamond samples to pressures higher than the center of Neptune and Uranus and temperatures hotter than the Sun. The findings resolve long-standing discrepancies regarding melting temperature and offer a new path to tripling inertial confinement fusion energy gain.
Recreating Ice-Giant Conditions in the Laboratory
Deep beneath the upper atmospheres of Uranus and Neptune, extreme pressures and temperatures cause carbon to form and fall as diamond rain. Replicating those alien environments on Earth has long challenged planetary scientists, who previously had to rely on test spaces just a few thousandths of an inch thick. Researchers at Lawrence Livermore National Laboratory in northern California have managed to recreate these extreme conditions, recording how diamond behaves deep inside these high-pressure regions.
The study, published in Nature Physics, brings together high-pressure physics research. By pushing carbon samples into the terapascal range—tens of millions of times ordinary atmospheric pressure—the team captured precise data that helps researchers build better atomic-scale models of ice-giant interiors.
Solving a Melting Temperature Discrepancy
The new experiments resolve two long-standing discrepancies that had stumped researchers. About 20 years ago, laboratory scientist Jon Eggert and colleagues pioneered high-pressure melting experiments, observing that diamond actually became denser when it melted—a rare trait shared by liquid water and ice cubes.
That early work created a major puzzle: a roughly 20% difference between observed and predicted melting temperatures that advanced computer simulations failed to reproduce. A separate mystery arose at Sandia National Laboratories, where researchers using the Z machine’s extreme magnetic fields obtained experimental fingerprints suggesting diamond might pass through an intermediate crystalline structure before liquefying.
To settle both questions, the team performed laser-driven dynamic compression experiments at the University of Rochester’s Laboratory for Laser Energetics. Using the Omega Laser Facility, scientists vaporized the outer layer of a tiny sample to send a squeezing shockwave through the diamond interior.
Probing Carbon With Faint X-Ray Diffraction
Collecting data during these tests required immense precision because the high-pressure states lasted for only about a billionth of a second. The team captured X-ray diffraction data to illuminate atomic structure right up to the point of melting.
Enhanced diagnostic tools developed and maintained by the team at the Laboratory for Laser Energetics made those measurements possible for the first time.
The updated melting temperature matched quantum-mechanical simulations almost perfectly. However, the data told a different story regarding intermediate phases. Unlike the results indicated at Sandia, the carbon remained in a diamond structure all the way until it melted, skipping any transitional crystalline phases because a single shock leaves the sample no time to change.
Implications for Inertial Confinement Fusion
Beyond planetary science, these findings carry practical consequences for clean energy research. The inertial confinement fusion process begins with a miniscule diamond capsule holding fuel that is imploded by high-energy lasers.
Maintaining a uniform fluid state during the implosion is essential for fusion reactions to sustain momentum. The Lawrence Livermore National Laboratory team discovered they can achieve this with less power than past tests.
Slower initial shocks make the fusion fuel more compressible, increasing the maximum energy yield obtainable with the same laser energy. Researchers are now looking toward whether slightly slower shocks in future implosions at the National Ignition Facility can unlock triple the energy gain.
