Scientists Recreate Melting Diamond Rain of Neptune and Uranus to Help Fusion Power

Deep beneath the thin upper atmospheres of Uranus and Neptune, extreme temperatures and pressures create an environment where carbon rains diamonds toward the planetary cores. Replicating those crushing conditions in a terrestrial laboratory has long challenged physicists. Because these high-pressure states last for only a fraction of a second, capturing precise measurements of carbon under such duress requires extraordinarily rapid diagnostic tools.

Laser-Driven Dynamic Compression at the University of Rochester

To recreate the interior conditions of the solar system’s ice giants, a research team led by scientists at Lawrence Livermore National Laboratory turned to the Omega Laser Facility at the University of Rochester’s Laboratory for Laser Energetics. Researchers vaporized the outer layer of a microscopic diamond sample using intense laser energy, which sent a powerful squeezing shockwave rocketing through the carbon interior.

Capturing the physical changes during this rapid compression proved exceptionally difficult. The extreme pressures lasted for roughly a billionth of a second. Within that fleeting window, the team deployed advanced X-ray diffraction tools to record the atomic structure.

Resolving a Melting Temperature Discrepancy

The new dynamic compression experiments directly addressed a debate that has persisted in high-pressure physics. About twenty years ago, laboratory scientist Jon Eggert and colleagues pioneered high-pressure melting experiments and observed an unusual property: diamond became denser when it melted.

Liquid water is denser than ice, which makes ice cubes float. Jon’s finding means that diamond would float in liquid carbon at high pressures, explained Marius Millot regarding the counterintuitive behavior.

That initial milestone created a puzzle. Laboratory-measured melting temperatures differed by approximately 20% from predictions generated by advanced theoretical computer simulations. Furthermore, independent experiments at Sandia National Laboratories using the Z machine’s extreme magnetic fields suggested that diamond might pass through an intermediate crystalline structure before liquefying entirely.

The latest trials at the Omega Laser Facility settled the temperature discrepancy. The updated melting measurements matched quantum mechanics-based simulations almost perfectly. However, the data revealed a different reality regarding intermediate phases.

The carbon remained locked in its original diamond structure right up until melting began, skipping any transitional crystalline phases detailed in Nature Physics. Researchers attribute this to the speed of a single shock wave, which leaves the sample no time to rearrange its lattice before turning to liquid.

Implications for Inertial Confinement Fusion Energy Gain

Beyond planetary science, these findings offer practical applications for clean energy research on Earth. Inertial confinement fusion experiments rely on tiny diamond capsules to encase fuel, which are then imploded by high-energy lasers. Maintaining a uniform fluid state in the imploding diamond is essential for sustaining the fusion reaction.

Since construction began on the National Ignition Facility in 1997, scientists have worked to refine these implosions. Following milestone experiments in 2022 that successfully produced more energy than the ignition lasers put in, researchers are now looking to increase overall yields.

The new melting data indicates that researchers can utilize slower initial shocks while still achieving full diamond melting during implosions. That operational adjustment makes the fusion fuel more compressible, potentially tripling the maximum energy yield obtained from the same laser energy input.