Diamond is famous as one of the hardest materials on Earth, but under conditions almost impossible to encounter naturally on our planet, even its exceptionally strong crystal structure eventually breaks down. A new experiment led by scientists at Lawrence Livermore National Laboratory has tracked diamond melting at pressures approaching one terapascal – roughly three times the pressure at Earth’s core. The results resolve a debate that has lasted for nearly two decades over how carbon behaves under extreme conditions and could have implications both for nuclear fusion research and for understanding the interiors of giant planets. Researchers used synthetic microcrystalline diamond samples and powerful ultraviolet lasers at the University of Rochester’s OMEGA Laser Facility. The laser pulses generated shock waves that compressed the samples for only a few nanoseconds. During this extraordinarily brief interval, scientists simultaneously measured shock velocity, temperature, reflectivity and the material’s atomic structure using X-ray diffraction. Combining these measurements allowed the researchers to observe how crystalline diamond approached and entered the liquid state. The results indicate melting at temperatures close to 7,300 kelvin at pressures of roughly 750 to 1,000 gigapascals. Even more unusually, the melting temperature decreases slightly as pressure rises through this region. The behaviour indicates that under these extreme conditions liquid carbon can be denser than solid diamond. The experiment also addresses another long-running question. Previous work had been interpreted as evidence that diamond might transform into a theoretically predicted crystal structure known as BC8 before melting at extreme pressure. The new X-ray measurements instead show that under the rapid shock-compression conditions used in these experiments, the familiar diamond structure persists essentially until melting. This does not necessarily mean that BC8 can never form – compression pathway and timescale may be important – but the researchers found no convincing evidence for a substantial BC8 phase in these measurements.
The potential practical significance lies in inertial confinement fusion. At the National Ignition Facility, tiny diamond capsules contain frozen fuel made from the hydrogen isotopes deuterium and tritium. Powerful lasers strike the capsule, producing an extraordinarily rapid implosion that compresses and heats the fuel until fusion reactions can occur. The behaviour of the diamond shell is therefore critical. Current designs use a strong initial shock partly to ensure that the diamond melts completely, producing a smoother fluid shell during the implosion. The new measurements indicate that complete melting could occur with a weaker initial shock than previously assumed. That could provide a lower-entropy compression pathway, allowing the fusion fuel to become more compressible and potentially reach greater density using the same amount of laser energy. Models considered by the researchers suggest that, if other performance-degrading effects can also be controlled, such optimisation could substantially increase fusion yield, with some scenarios indicating the possibility of roughly tripling the released energy. That figure is a modelled possibility, not an experimental achievement. The discovery also matters far beyond fusion laboratories. Pressures of hundreds of gigapascals occur inside giant planets, and the behaviour of carbon under these conditions is relevant to models of Uranus, Neptune and some carbon-rich exoplanets. Researchers now have a much more accurate experimental benchmark for the boundary between solid diamond and liquid carbon against which quantum-mechanical simulations can be tested. The experiment therefore does not mean scientists have suddenly discovered a way to triple fusion-energy production. Its importance is more fundamental: researchers now understand the behaviour of a crucial material during extreme compression much more accurately. Improvements of this kind in the underlying physics could eventually help engineers design more efficient fusion implosions.
