Researchers at Lawrence Livermore National Laboratory have resolved a 20-year scientific mystery regarding how diamond melts under extreme pressures three times greater than Earth’s core. The breakthrough, achieved using laser-driven compression at the University of Rochester, could triple energy gain in laser-driven nuclear fusion and reshape models of ice giant planets.
Resolving a 20-Year High-Pressure Melting Mystery
For two decades, physicists have wrestled with a stubborn discrepancy in high-pressure physics. About 20 years ago, laboratory researchers pioneered experiments compressing diamond and observed that the material’s density actually increased as it melted. While that unusual behavior mirrors how liquid water is denser than ice, it kicked off a persistent scientific puzzle. Theorists using advanced computer simulation techniques could never quite match the experimental numbers.
No matter what approach theorists took, a roughly 20 percent gap persisted between observed and predicted melting temperatures. Adding to the complexity, experiments using extreme magnetic fields at Sandia National Laboratories produced fingerprints hinting that diamond might take an extra step on its way to liquid, transforming into an intermediate crystalline structure first. However, no one could directly capture the atomic structure to verify it.
Laser-Driven Experiments at the Omega Laser Facility
To settle the debate, a research team turned to the University of Rochester’s Laboratory for Laser Energetics. Using the Omega Laser Facility, scientists vaporized the outer layer of a tiny diamond sample, launching a powerful squeezing shockwave directly through its interior.
Capturing the resulting data was an extraordinary technical challenge because the extreme states lasted for only a billionth of a second. Carbon is a lightweight atom that scatters very few X-rays, making the required signal exceptionally faint.
“We were able to take tiny diamond samples and shock compress them to temperatures hotter than the surface of the sun and to pressures higher than the center of Neptune and Uranus — and still measure atomic structure, temperature, density and optical reflectivity.”
Marius Millot, Lawrence Livermore National Laboratory scientist
This marked the first time researchers successfully probed shock-compressed diamond with X-ray diffraction all the way up to melting. The resulting measurements closed the 20-year gap by agreeing almost perfectly with quantum mechanical simulations.
Skipping Intermediate Phases Under Single Shocks
The new X-ray diffraction data also clarified the question of intermediate phase changes. Rather than transforming into another crystalline structure before liquefying, the carbon remained firmly in a diamond structure all the way until it melted.
“We think that is because the sample does not have time to change when it only experiences a single shock. It remains ‘trapped’ in the diamond structure.”
Marius Millot, Lawrence Livermore National Laboratory scientist
This outcome demonstrates that material response depends heavily on exactly how a shock is applied, rather than relying solely on final pressure and temperature metrics. The findings were published in Nature Physics.
Implications for Inertial Confinement Fusion and Planetary Interiors
At Lawrence Livermore National Laboratory, home to the National Ignition Facility, understanding diamond under extreme conditions holds direct operational importance. Tiny diamond capsules are routinely used to encase deuterium-tritium fuel mixtures for inertial confinement fusion. During an implosion, lasers deliver a series of shockwaves to compress the fuel. If the encasing diamond melts unevenly, pressure distortions can disrupt the entire operation and prevent ignition.
To prevent such degradation, the National Ignition Facility has historically relied on a strong first shock guaranteed to melt the diamond uniformly. However, that strong initial shock raises entropy and reduces maximum theoretical compression, ultimately capping energy yield. With the newly refined understanding of diamond’s high-pressure phases, researchers are exploring slower first shocks that allow the fuel to compress more efficiently.
According to supplementary information in the published work, adjusting the first shock downward from 33–34 kilometers per second down to 24.5 kilometers per second could potentially triple energy gain in laser-driven nuclear fusion. Beyond energy applications, the updated melting parameters provide critical benchmarks for quantum simulations, offering fresh insights into the conditions deep inside ice giant planets like Neptune and Uranus.
Next Steps in Shock-Tuning Experiments
With the thermodynamic behavior of diamond now reconciled between theory and experiment, researchers are shifting focus toward practical implementation. The team has begun designing experiments to test slower first shocks using the laser system. While tuning the initial shock is straightforward with current technology, the primary challenge ahead involves mastering hohlraum fluid dynamics and laser-plasma interactions to preserve the spherical symmetry of the fuel implosion over a slightly longer laser pulse duration.
