Diamond melting curve data points to softer NIF shocks

Diamond becomes denser as it melts, meaning the solid floats in liquid carbon at extreme pressure, a behaviour LLNL’s new shock compression data confirms via X-ray diffraction

(Image courtesy of Lawrence Livermore National Laboratory, Photo: James Wickboldt)

Diamond keeps its crystal structure under shock compression all the way to melting at roughly 1 terapascal, pressures LLNL describes as three times those found at Earth’s core, according to new Lawrence Livermore National Laboratory research published in Nature Physics. The result closes a 20-year gap between experiment and theory on diamond’s melting curve and could inform how NIF approaches the diamond capsules used to compress inertial confinement fusion fuel.

The 20-year diamond melting curve problem

LLNL scientist Jon Eggert’s original high-pressure melting experiments, run roughly two decades ago, found something unusual: diamond becomes denser as it melts, meaning solid diamond would float in liquid carbon rather than sink, much as ice floats in water. That result held up, but the measured melting temperatures differed from theoretical predictions by roughly 20%. LLNL scientist Marius Millot said no amount of refinement to the computer simulations, even using the most advanced techniques available, closed that gap.

A second unresolved question came from Sandia National Laboratories, where researchers used the Z machine’s magnetic fields to shock-compress diamond samples. Sandia’s data hinted that diamond might pass through an intermediate crystalline phase before fully melting, a result their simulations supported. No experiment had directly measured the atomic structure closely enough to confirm it.

Resolving the diamond melting curve with X-ray diffraction

To investigate both questions, the LLNL team ran laser-driven shock compression experiments at the University of Rochester’s Laboratory for Laser Energetics, using the Omega Laser Facility to vaporise the outer layer of a diamond sample and drive a shockwave through its interior. The team combined this with X-ray diffraction, probing shock-compressed diamond’s atomic structure all the way to melting for the first time. Carbon’s light, small atoms scatter X-rays weakly, so the diffraction signal at these pressures was faint and difficult to isolate.

The resulting melting-temperature measurement agrees closely with simulations, resolving the roughly 20% discrepancy. Eggert said the original measurements had been off by more than 1,000 degrees, and that the new data directly confirms, via X-ray diffraction, what his earlier work had only inferred. On the phase question, the team found no evidence of the intermediate phase Sandia’s data had hinted at, within the resolution of this single-shock experiment. Diamond appeared diamond-structured until it liquefied, which the researchers suggest is because a single shock does not give the sample time to reorganise into another phase before melting.

Why this diamond melting result matters for ICF ablator design

The findings carry implications for capsule design at the National Ignition Facility, where diamond ablators are shocked to compress fusion fuel. NIF’s current approach uses a relatively strong first shock, aiming to ensure the diamond melts into a smooth, uniform fluid, since imperfections in that melt can degrade the implosion. The new data indicates a slightly slower first shock could still achieve full melting while leaving the fuel more compressible, a change LLNL projects, in its own modelled figures, could triple energy gain, provided other degradation mechanisms in the implosion can be controlled.

The same dataset extends beyond fusion applications. Because the experiments reached pressures beyond those inside Neptune and Uranus, the results give planetary scientists a stronger experimental basis for modelling “diamond rain,” the process by which carbon is thought to crystallise and sink through ice giant interiors.

What the LLNL team plans to test next

The LLNL team intends to use NIF’s own capabilities to push diamond into conditions harder to reach than this experiment achieved, examining how diamond capsules behave in the later stages of an implosion and how long the diamond structure holds up under repeated, sequential shocks rather than a single one.

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