LLNL Laser Experiment Melts Diamond at 1 TPa: Diamond Rain, a 20-Year Dispute, and a Potential 3x Fusion Gain

> Note: This is an English translation of a Chinese forum post discussing an LLNL study published in *Nature Physics* around August 20, 2026, led by physicist Marius Millot.

The 'Diamond Rain' Hypothesis: A 20-Year Experimental Gap

Planetary scientists have long speculated that inside ice giants like Uranus and Neptune, methane (CH4) breaks down under extreme pressure and temperature, and carbon crystallizes into diamond at roughly 200 GPa and 5,000 K. If diamond is denser than the surrounding liquid carbon, it sinks toward the planetary core, and friction from this descent generates heat — one of the sources of Neptune's excess energy (the planet radiates about 1.4 times the energy it receives from the Sun).

Until now, no laboratory could reproduce the extreme conditions of ice giant interiors or observe carbon's phase transition on such short timescales.

The Experiment: 1 TPa + Sun-Surface Temperature + One Nanosecond

The experiment was performed at the Omega Laser Facility at the University of Rochester:

• Ultrashort-pulse shock: The laser instantly vaporizes the outer layer of a diamond sample, producing a powerful shock wave.
• Extreme pressure: ~1 TPa — about 3 times the pressure of Earth's core.
• Extreme temperature: Rising to sun-surface levels.
• Extreme duration: All of this lasts about one nanosecond.
• Diagnostics: Ultrafast X-ray diffraction tracks carbon's phase transition in real time.

Finding 1: Resolving the 20-Year Melting Point Dispute

For two decades, quantum-mechanical simulations and laboratory measurements of diamond's melting point disagreed by about 20% — more than 1,000 K in this regime. Multiple research groups attempted to close the gap with different methods, lasers, and diagnostics, without success.

The LLNL team's combination of X-ray diffraction, extreme pressure, and ultrashort pulses achieved precision sufficient for direct comparison with quantum models. The measured melting point agrees with modern quantum-mechanical predictions, resolving the 20-year dispute.

> Why a ~1,000 K gap matters: a 1,000 K difference in diamond's melting point shifts the depth at which diamond melts inside an ice giant, directly changing how much diamond and liquid carbon exist there and how much energy diamond settling releases — enough to yield very different planetary thermal evolution models.

Finding 2: Solid Diamond Floats on Liquid Carbon

Analogous to ice floating on water (but unexpectedly), solid diamond was found to be less dense than liquid metallic carbon, so it floats on a liquid carbon "ocean." Combined with thermal convection and gravity, this density inversion means diamonds would cycle at different depths inside ice giants, with frictional heating contributing to internal heat budgets.

The experiment also found that diamond retains its normal crystal structure until melting — there is no intermediate phase; the solid-to-liquid transition is direct.

Finding 3: A Potential 3x ICF Energy Gain

Because the diamond samples closely resemble the capsule materials used at the National Ignition Facility (NIF), the results carry implications for inertial confinement fusion (ICF). The experiment showed that a gentler, staged shock — rather than an extreme single strike — can uniformly and controllably melt and compress the diamond target into the desired state.

The authors estimate this optimized shock strategy could theoretically triple the fusion energy output per shot. For context, NIF achieved net energy gain in December 2022 (2.05 MJ laser input → 3.15 MJ fusion output), but remains far from engineering break-even (output exceeding total facility energy consumption). A 3x improvement would narrow, though not close, that gap.

What to Watch Over the Next 6–12 Months

1. Reproducibility: Can the 3x gain result be repeated across multiple experiments? 2. NIF integration: Will the target-optimization strategy enter NIF's experimental program? LLNL operates NIF, which affects translation speed. 3. Planetary model updates: Ice giant formation and evolution models will need to incorporate the new melting point and density-inversion data; Neptune's excess-energy budget may be reassessed. 4. Engineering break-even: NIF is still 1–2 orders of magnitude away; 3x is a step, but 10–100x further improvement is needed for engineering viability. 5. Other ICF programs: Whether facilities such as China's Shenguang series can adopt the target-optimization strategy. 6. Gas giants: Whether Jupiter and Saturn, with small internal carbon inventories, exhibit similar carbon behavior.

Conclusion

A single set of experimental data delivered results across three fields: resolving the 20-year diamond melting point dispute in condensed matter physics, providing precise density-inversion data supporting the ice giant diamond rain hypothesis, and suggesting a path to 3x ICF gains. The key test ahead is whether the 3x gain can be reproduced; even if not, the planetary science and condensed matter findings stand on their own.

References

1. Lawrence Livermore National Laboratory, *Scientists crushed diamond beyond Neptune-like pressures—and solved a 20-year mystery*, 2026-08-20 — ScienceDaily 2. LLNL press release, *Diamond rain decoded: Twenty years of disagreement solved*, 2026-08-20 3. Marius Millot et al., *Shock compression of diamond to terapascal pressures and melting*, Nature Physics, 2026-08-20 4. Universe Today, *Scientists Melted A Diamond and Cracked a Secret of Ice Giants*, 2026-08-20 5. Space Nuts & Astronomy Daily, S05E178, 2026-08-27 — https://spacenutspodcast.com/podcast/astronomy-daily-space-astronomy-news/episode/diamond-rain-decoded-twenty-years-of-disagreement-solved 6. NetEase, 2026-08-20 7. Sohu, 2026-08 8. Pravda Report, *How diamond rain occurs inside ice giants like Neptune*, 2026-08-20 9. NIF official website, ICF engineering progress 10. Omega Laser Facility (University of Rochester) capability documentation