Researchers from Lawrence Livermore National Laboratory and the University of Rochester’s Laboratory for Laser Energetics have revised the high-pressure melting curve of diamond, a critical material used in inertial confinement fusion (ICF) targets. Using the OMEGA laser, the team subjected single-crystal diamond samples to pressures up to 1,400 GPa, or 14 Mbar, observing a transition to a body-centered cubic solid phase (BC8) at temperatures lower than predicted by previous models. This discovery fundamentally alters the equation of state for carbon under extreme conditions, with direct implications for the design and performance of hohlraum targets used at facilities like the National Ignition Facility. The findings suggest that current ICF implosion strategies may be operating on flawed assumptions about the ablator material's phase behavior. Source: DOE Fusion

The experiments employed laser-driven shock compression on the OMEGA laser system to generate immense pressures, while time-resolved X-ray diffraction was used to probe the atomic structure of the diamond sample in situ. Previous models predicted that diamond would melt at these pressures, but the diffraction data provided clear evidence of the solid BC8 crystal structure persisting before melting occurred. This indicates that diamond remains solid for longer during the initial compression stages of an ICF implosion than previously understood. This stability is crucial, as a solid ablator is less susceptible to hydrodynamic instabilities, such as the Rayleigh-Taylor instability, which can disrupt the symmetric compression required to achieve ignition. Source: DOE Fusion

This indicates that diamond remains solid for longer during the initial compression stages of an ICF implosion than previously understood.

The practical consequence for inertial confinement fusion is a revised strategy for shock timing and laser pulse shaping. By designing implosions that keep the diamond ablator in its solid BC8 phase, it is possible to achieve a more stable and efficient compression of the D-T fuel capsule. According to the research team, simulations incorporating this updated physics predict a significant performance improvement. The enhanced stability of the ablator allows for a more aggressive implosion, transferring laser energy to the fuel more effectively. This could lead to higher fuel densities and temperatures at stagnation, key components of the Lawson criterion for fusion ignition. Source: DOE Fusion

Simulations incorporating the new diamond equation of state project a potential tripling of energy gain from the fusion reaction. This substantial increase highlights the sensitivity of ICF performance to the precise material properties of the target components. While this 3x gain is a computational prediction and has not yet been demonstrated experimentally, it provides a clear and immediate path for improving target designs. The results underscore the importance of fundamental high-energy-density physics research in advancing the goals of the fusion energy community. These findings will likely inform the next generation of experiments at major ICF facilities. Source: DOE Fusion

Beyond its direct application to fusion energy, this work also has significant implications for planetary science and astrophysics. The extreme conditions created in the lab are analogous to those found in the interiors of carbon-rich exoplanets. A more accurate model of carbon's phase diagram at terapascal pressures will allow for more precise modeling of the internal structure, evolution, and magnetic fields of these celestial bodies. The collaboration between Lawrence Livermore National Laboratory and the University of Rochester’s Laboratory for Laser Energetics demonstrates the dual-use nature of research in high-energy-density physics, advancing both terrestrial energy goals and our understanding of the cosmos. Source: DOE Fusion