Diamond melting study could triple fusion energy gain

Researchers at Lawrence Livermore National Laboratory have resolved a 20-year discrepancy in understanding how diamond behaves under extreme pressure, with findings that could triple energy gain in fusion experiments and reshape models of ice giant planet interiors.

Closing a Two-Decade Gap

The study, published Thursday in Nature Physics, documents how diamond melts under pressures of one terapascal — roughly three times greater than conditions at Earth's core. Using laser-driven shock compression at the University of Rochester's Omega Laser Facility, the team compressed tiny diamond samples to temperatures exceeding the surface of the sun while simultaneously measuring atomic structure, temperature, density and optical reflectivity.phys+1

"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 centers of Neptune and Uranus — and still measure atomic structure, temperature, density and optical reflectivity," said LLNL scientist Marius Millot, the study's lead author.phys

The experiments produced a new measurement of diamond's melting temperature near 7,300 Kelvin that matched quantum mechanics-based simulations almost perfectly, closing a longstanding gap of roughly 20% between observed and predicted melting temperatures. The work also showed that diamond under a single shock remains in its crystalline structure all the way until it melts, skipping any intermediate phases — contradicting earlier results from Sandia National Laboratories' Z machine experiments.nature+2

Implications for Fusion Energy

The findings carry direct consequences for inertial confinement fusion at LLNL's National Ignition Facility, where powerful lasers drive diamond capsules inward to compress fusion fuel. Melting the diamond into a uniform fluid during the initial shock is critical to minimizing implosion imperfections.phys

"Our work indicates that we could use slightly slower initial shocks and still achieve full melting of the diamond in our NIF implosions," Millot said. "This is exciting because such a slower shock would make the fusion fuel more compressible. That in turn increases the maximum energy yield we could obtain with the same laser energy."phys

Predictions suggest these slower shocks could triple energy gain, provided other degradation mechanisms can be controlled.llnl+1

Diamond Rain on Ice Giants

The results also give planetary scientists new data for modeling the interiors of Neptune and Uranus, where carbon is believed to crystallize and fall as "diamond rain." Because the experiments probed pressures exceeding those inside ice giants, the melting data provides a stronger basis for planetary formation and evolution models. The team plans further experiments at NIF to study diamond behavior under conditions even more extreme than those achieved so far.newswise+1