Researchers at Lawrence Livermore National Laboratory have determined that diamond melts at much lower pressures and temperatures than theoretical models previously suggested. Using the National Ignition Facility's laser-driven shock waves, the team observed the phase transition from solid to liquid diamond at pressures between 600 and 1,000 gigapascals (GPa). The experiments, which used x-ray diffraction to probe the material's atomic structure on nanosecond timescales, showed melting occurred at temperatures approximately 8,000 Kelvin. This finding challenges long-held assumptions about carbon's equation of state under extreme conditions relevant to both inertial confinement fusion and planetary science. Source: LLNL / NIF

The result has direct consequences for target design in inertial confinement fusion experiments. At the National Ignition Facility, a high-density carbon (diamond) capsule serves as the ablator, encasing the deuterium-tritium fuel. For successful ignition, this capsule must remain solid during the initial stages of compression to maintain a smooth, spherically symmetric implosion. Premature melting of the ablator can seed hydrodynamic instabilities, such as the Rayleigh-Taylor instability, which disrupt the compression and prevent the fuel from reaching the required density and temperature for fusion. The new, lower melting curve suggests that current ICF implosion models may need significant revision, as capsules could be transitioning to a liquid state earlier than anticipated. Source: LLNL / NIF

The result has direct consequences for target design in [inertial confinement fusion](/glossary/inertial-confinement-fusion) experiments.

The experimental campaign leveraged NIF's unique capabilities to generate and diagnose matter at extreme states. By precisely timing and shaping laser pulses, scientists drove a shock wave through a diamond sample. A separate set of laser beams generated a bright, short-lived x-ray source that diffracted off the diamond's crystal lattice. The resulting diffraction pattern, captured by a detector, provided a direct measurement of the material's atomic structure. The disappearance of the sharp diffraction peaks characteristic of a solid crystal, and the appearance of a broad, diffuse signal, marked the transition to a liquid state. This technique provided unambiguous evidence of melting under dynamic compression, resolving discrepancies in prior static compression experiments and theoretical calculations. Source: LLNL / NIF

Beyond its implications for fusion energy, this research provides critical data for understanding the interior structure of ice giant planets like Uranus and Neptune. Prevailing models suggest that deep within these planets, extreme pressures and temperatures could cause hydrocarbons to break down, leading to the formation of solid diamond which then sinks toward the core in a phenomenon described as 'diamond rain.' The experimentally verified melting curve for carbon helps constrain the conditions under which this process occurs, refining models of planetary formation and internal dynamics. The new data suggests that a liquid carbon layer may exist atop a solid diamond core inside these planets, a feature not accounted for in previous models. Source: LLNL / NIF

With this revised understanding of diamond's behavior, ICF target designers can now re-evaluate and optimize implosion strategies. The source suggests that modifying the laser pulse shape to keep the ablator on a lower-temperature compression path (isentrope) could prevent premature melting and mitigate instabilities. This could potentially lead to more stable implosions and higher fusion yields. The LLNL team states that successfully managing the ablator's phase state could enable a threefold increase in fusion energy gain. Future experiments will likely focus on testing these modified pulse shapes and further refining the equation of state for capsule materials. Source: LLNL / NIF