Researchers at Lawrence Livermore National Laboratory (LLNL) have resolved two long-standing discrepancies in the scientific understanding of how diamond melts under extreme conditions.
Resolving Long-Standing Discrepancies in Diamond Melting
For decades, researchers grappled with a roughly 20% difference between observed and predicted melting temperatures of diamond. According to LLNL scientist Marius Millot, theorists using advanced computer simulation techniques could not reproduce earlier experimental findings. Furthermore, experiments at Sandia National Laboratories using the Z machine’s extreme magnetic fields suggested that diamond might take an intermediate step on its way to melting, transforming into another crystalline structure before liquefying.
To address these uncertainties, the LLNL team conducted laser-driven dynamic compression experiments at the University of Rochester’s Laboratory for Laser Energetics (LLE). Utilizing the Omega Laser Facility, scientists vaporized the outside layer of a tiny diamond sample to send a powerful shockwave through its interior.
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,
Millot said.
Capturing Data in a Billionth of a Second
Making precise measurements during the compression process presented significant technical hurdles. The extreme high-pressure states lasted for only a billionth of a second, requiring the team to capture critical information—including X-ray diffraction data illuminating the atomic structure—within that microscopic timeframe.
This was the first time that shock-compressed diamond was probed with X-ray diffraction all the way up to melting,
Millot noted, explaining that carbon’s lightweight atoms scatter very few X-rays, leaving a faint signal to measure.
The results demonstrated that carbon remains in a diamond structure all the way until it melts, skipping intermediate phases entirely. Millot explained that the sample remains trapped in the diamond structure because it does not have time to change when experiencing a single shock. This revelation indicates that material response depends on the exact method of shock application rather than solely on pressure and temperature.
Jon Eggert, an LLNL scientist who pioneered high-pressure melting experiments approximately 20 years ago, observed that while it was initially frustrating to discover past temperature measurements were off by more than 1,000 Kelvin, the new diagnostics yielded a dramatic improvement in data quality.
Implications for Nuclear Fusion Energy Gain
At LLNL, home to the National Ignition Facility (NIF), understanding diamond under extreme conditions is closely tied to inertial confinement fusion (ICF). In ICF, tiny diamond capsules encase a deuterium-tritium fuel mixture. Lasers deliver a series of shockwaves to compress the fuel, triggering a fusion-driven implosion.
If the diamond ablator melts unevenly, it distorts the pressure applied to the fuel. These distortions can be amplified by hydrodynamic instabilities, potentially disrupting the implosion and preventing the fuel from reaching the temperature and compression needed for ignition. To prevent this degradation, NIF traditionally employed a strong first shock—around 12 Mbar or 33–34 km/s—guaranteeing that the diamond melts while allowing for a shorter, more controlled laser pulse.
However, a stronger first shock raises entropy and reduces maximum theoretical compression, which limits maximum energy yield. Using a slower first shock would make the fuel more compressible and allow a larger fraction of it to burn.
Armed with their new understanding of diamond’s phase change, the research team is designing experiments to test slower first shocks, supporting a reduction to 24.5 km/s. While tuning the first shock is straightforward using NIF’s laser system, challenges remain in understanding hohlraum flows and laser-plasma interactions to maintain spherical symmetry during a slightly longer laser pulse.
Reshaping Models of Planetary Interiors
Beyond nuclear fusion, the findings have implications for planetary science. Scientists believe that carbon diamonds rain down deep inside ice giant planets like Neptune and Uranus, where materials experience immense pressures.
Two decades ago, Eggert’s high-pressure melting experiments revealed that diamond’s density increases when melting, meaning solid diamond would float in liquid carbon at high pressures. The new study provides definitive atomic-scale benchmarks for quantum simulations of condensed matter under extreme conditions, helping reshape theoretical models of planetary interiors.