Researchers at Lawrence Livermore National Laboratory have successfully shock-compressed diamond samples to pressures higher than the centers of Uranus and Neptune, resolving a melting temperature mystery. The findings, published in Nature Physics, reveal that carbon remains in a diamond structure until it melts and could triple fusion energy gain.
Deep beneath the crushing atmospheres of our solar system’s ice giants, extreme heat and pressure turn carbon into something extraordinary. Under these conditions, researchers believe it literally rains diamonds. Replicating those forces on Earth has long challenged physicists, but a new study out of northern California has pushed experimental boundaries far beyond previous limits.
Scientists working at the Lawrence Livermore National Laboratory (LLNL) have managed to recreate the conditions deep within Neptune and Uranus. The team utilized high-energy lasers to generate shock waves hotter than the surface of the sun.
Recreating Ice Giant Interiors in the Lab
The extreme middle atmospheres of Uranus and Neptune exist in states so intense that replicating them required test spaces measuring less than a few thousandths of an inch thick. Past experiments managed this feat nearly a decade ago, but the LLNL team targeted even higher pressures stretching into the terapascals—tens of millions of times ordinary atmospheric pressure.
To achieve this, researchers traveled to the University of Rochester’s Laboratory for Laser Energetics. Using the Omega Laser Facility, scientists vaporized the outer layer of tiny diamond samples. That action launched a squeezing shockwave through the carbon interior.
Solving a Melting Mystery
The experiments finally settled a long-standing debate regarding how diamond behaves when melting under pressure. LLNL laboratory scientist Jon Eggert pioneered high-pressure melting experiments, observing an unusual trait: diamond actually became denser when it melted.
Liquid water shares this property, making ice cubes float. Eggert’s finding meant diamond would similarly float in liquid carbon at high pressures. Yet that breakthrough created a puzzle: laboratory melting temperatures differed by roughly 20% from theoretical computer simulations, and no advanced model could reproduce the experimental results.
A separate mystery arose at Sandia National Laboratories, where researchers used the Z machine’s extreme magnetic fields to shock compress diamond. Those tests suggested diamond might pass through an intermediate crystalline structure before melting completely into liquid. However, direct observation of the atomic structure remained out of reach.
The new LLNL tests resolved the temperature discrepancy by probing shock-compressed diamond with X-ray diffraction all the way up to melting. Because carbon is a lightweight atom that scatters very few X-rays, the signals were faint and difficult to capture during conditions that lasted for only a billionth of a second. The updated diagnostics yielded an improved melting temperature that matched quantum mechanics simulations almost perfectly.
Interestingly, the experiments did not confirm the intermediate crystalline phase suggested at Sandia. The carbon remained locked in its diamond structure right until melting began, likely because a single shock leaves the sample no time to change phases.
Implications for Inertial Confinement Fusion
Beyond modeling ice giants, the findings offer practical benefits for clean energy research. Inertial confinement fusion relies on tiny diamond capsules encasing fuel, which are imploded by high-energy laser shock waves.
LLNL has pursued this technology since construction began on the National Ignition Facility in 1997, a project that achieved a major milestone in 2022 by successfully producing more energy than its ignition lasers put in. Millot’s group discovered that using slightly slower initial shocks can still achieve full melting of the diamond ablator in NIF implosions.
This slower shock makes the fusion fuel more compressible, which could potentially triple the maximum energy yield obtained from the same laser energy. Planetary scientists and energy researchers alike now possess atomic-scale benchmarks to refine both quantum simulations and fusion reactor designs.
