US experiment reveals diamond melting behavior under high pressure, potentially advancing inertial confinement fusion research

Researchers at Lawrence Livermore National Laboratory published a new study in Nature Physics documenting the melting process of diamond at pressures approximately three times greater than those at Earth's core. The experiments showed that under high-pressure conditions, diamond floats in liquid metallic carbon, similar to how ice floats in water. This finding helps improve inertial confinement fusion experimental models and provides new evidence for understanding the potential "diamond rain" phenomenon inside ice giant planets such as Neptune and Uranus.

The latest melting experiments confirmed that under high pressure, diamond floats in liquid metallic carbon, just as ice floats in a glass of water. (Concept image: James Wickboldt/LLNL)

The research team stated that they subjected tiny diamond samples to shock compression, reaching temperatures higher than the surface of the Sun and pressures greater than those at the centers of Neptune and Uranus, while simultaneously measuring atomic structure, temperature, density, and optical reflectivity. For a long time, experimentally observed diamond melting points differed from quantum mechanics-based simulations by approximately 20%; this experiment brought the two into basic agreement, bridging a data discrepancy that had persisted for about 20 years.

The experiments were conducted at the Omega Laser Facility at the University of Rochester's Laboratory for Laser Energetics. Scientists used lasers to vaporize the outer layer of tiny samples, generating strong shock waves that traveled through the diamond interior. Since the high-pressure state lasted only on the order of one billionth of a second, researchers had to complete critical diagnostics within an extremely short timeframe, including acquiring X-ray diffraction data revealing the atomic structure.

The researchers stated that this was the first time X-ray diffraction was used to probe shock-compressed diamond all the way to melting. Because carbon atoms are small and lightweight, the X-ray signals they scatter are very weak, making measurements highly challenging. Improved diagnostic tools helped the team redetermine the melting point data and directly confirm the previously inferred melting phenomenon.

The study also found that under single-shock compression conditions, carbon maintained its diamond structure throughout until melting, without passing through any intermediate crystalline phases. The team believes this may be because the sample did not have sufficient time to undergo structural transitions during a single shock pass, remaining "trapped" in the diamond structure. This result indicates that a material's response under extreme conditions depends not only on pressure and temperature but is also closely related to the shock loading method.

This finding has direct reference value for inertial confinement fusion research. In related experiments, intense laser-driven shock waves cause tiny diamond capsules to implode inward, compressing the fusion fuel within to the high-temperature, high-pressure conditions required for fusion. The researchers stated that if the initial shock can completely melt the diamond target into a more uniform, smooth fluid, it could reduce implosion defects and improve experimental stability.

According to these results, even with slightly slower initial shock velocities, implosion experiments at the National Ignition Facility could still achieve complete diamond melting. Slower shocks help improve the compressibility of the fusion fuel, enhancing maximum energy output under the same laser energy conditions. The study predicts that, provided other influencing factors are controlled, such slower shocks could further increase energy gain.

Beyond fusion research, these findings also serve planetary science. The interiors of ice giant planets such as Neptune and Uranus are difficult to probe directly, so scientists typically rely on extreme-condition laboratory experiments and models for inference. Some studies suggest that carbon crystallization may occur deep within ice giant planets, forming "diamond rain" that settles toward the interior. The high-pressure diamond melting data from this study will provide a more reliable experimental basis for related planetary formation and evolution models.

The research team plans to continue using the National Ignition Facility to explore diamond behavior under even more extreme conditions, focusing on the response of diamond capsules during later stages of implosion, as well as the stability limits of the diamond structure under multiple shock wave passes.