Equation of State Determination of Lead to 800 GPa at the National Ignition Facility *

By compressing materials over 10s to 100s of nanoseconds, ramp compression equation of state (EOS) experiments provide access to the highest stress and density conditions in solids. In highly compressible materials, the rapidly increasing sound speeds with compression make ramp EOS measurements difficult to obtain due to shock formation and the associated sample heating. Furthermore, the presence of multiple solid phases may complicate the analysis and interpretation of these experiments. In this work, we ramp compressed lead, a model highly compressible post-transition metal with multiple accessible solid phases, to test the pulse shaping and design capabilities/limitations of the National Ignition Facility for EOS development. Using a 168-beam indirect hohlraum drive to deliver precisely shaped input stress pulses, we successfully ramp-compressed and obtained EOS data on lead up to ~800 GPa peak longitudinal stress — twice the previously achieved maximum stress under ramp compression. Although highly incompressible materials, such as diamond, have been successfully ramp compressed to ~5 terapascals using the National Ignition Facility, this work demonstrates the feasibility of studying highly compressible solids up to near terapascal conditions. Since no features of potential phase transformations were observed in the data, this work also demonstrates the potential limited effect of small-volume phase transformations on EOS measurements. (LLNL-ABS-824485)