Researchers at Lawrence Livermore National Laboratory have documented how diamond melts under extreme pressure, resolving a decades-long scientific mystery and potentially opening the path to triple the energy gain in fusion experiments. In a study published in Nature Physics on August 13, 2026, scientists led by Marius Millot used shock-compression techniques to melt diamond samples at pressures three times greater than the conditions at Earth’s core.
The experiments finally reconciled a stubborn conflict: for nearly 20 years, the measured melting temperature of diamond differed by roughly 20 percent from what quantum-mechanical simulations predicted. “No matter what the theorists did—even with the most advanced computer simulation techniques—they could not reproduce the experiments,” Millot said.
Resolving the Mystery
To address the discrepancy, the LLNL team conducted laser-driven dynamic compression experiments at the University of Rochester’s Laboratory for Laser Energetics using the Omega Laser Facility. The scientists vaporized the outside layer of tiny diamond samples, sending shock waves rocketing through the interior at pressures between 600 gigapascals and 1.8 terapascals—conditions hotter than the sun’s surface and exceeding the pressure at the centers of Neptune and Uranus.
The key innovation was measuring X-ray diffraction data all the way through melting, a feat that had never been accomplished before. “These measurements are extremely difficult because carbon is a small and lightweight atom. It scatters very few X-rays, so the signal we needed to measure was quite faint,” Millot explained. The new measurements put diamond’s melting temperature at around 7,300 Kelvin at 1 terapascal, significantly lower than the old experimental readings and in agreement with theoretical predictions.
The experiments revealed another surprising finding: diamond does not transform into an intermediate crystalline phase called BC8 before melting, as some earlier theories suggested. Instead, the carbon remains locked in its familiar diamond crystal structure all the way until it melts into liquid carbon. “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,” Millot said.
One of the most counterintuitive results is that solid diamond is actually less dense than the liquid carbon it melts into—a property it shares with ice and water. This means diamond could theoretically float in molten carbon at these extreme pressures, an unusual behavior among most materials.
Implications for Fusion Energy
The findings have direct consequences for inertial confinement fusion, the approach used at the National Ignition Facility. In these experiments, powerful lasers generate shock waves that drive a tiny diamond capsule inward, compressing fusion fuel to the extreme pressures and temperatures needed for nuclear reactions. Melting the diamond capsule into a uniform fluid during the initial shock is critical to minimize implosion imperfections that can reduce fusion yield.
Current NIF protocols use relatively strong first shocks to ensure the diamond melts completely. But the new research suggests scientists could use slightly slower initial shocks and still achieve full melting. “This is exciting because such a slower shock would make the fusion fuel more compressible. That in turn increases the maximum energy yield we could obtain with the same laser energy,” Millot said. Computer simulations incorporating the updated carbon model predict that these slower shocks could triple energy gain, provided other degradation mechanisms can be controlled.
The work also extends beyond fusion. The findings provide planetary scientists with a stronger basis for understanding the interiors of ice giant planets like Neptune and Uranus, where carbon may experience similar extreme pressures and could form “diamond rain” deep beneath the surface. The new melting data will help refine models of planetary formation and evolution in these distant worlds.
Sources
- Lawrence Livermore National Laboratory — official announcement and quotes from Marius Millot and Jon Eggert on diamond melting experiments, pressure conditions, and fusion implications
- Nature Physics — peer-reviewed publication of the study “Diamond melting in shock compression experiments at 1 TPa pressures” (August 2026)
- ScienceAlert — detailed explanation of experimental methods, X-ray diffraction measurements, and the absence of BC8 phase transition
- Phys.org — reporting on the resolution of the 20-year temperature discrepancy and fusion energy gain predictions
