Measurement of the sound speed in dense fluid deuterium along the cryogenic liquid Hugoniot

Abstract

Many experimental studies, spanning several decades of research and using various dynamic compression schemes, have been conducted to investigate cryogenic liquid deuterium under strong shock compression. The consensus emerging from these studies is that of a progressive dissociation of the D2 molecules into an electrically conducting, atomic plasma, when subjected to shock pressures exceeding ∼50 GPa. While state-of-the-art numerical simulations based on density-functional-theory or quantum Monte-Carlo techniques capture this behavior quite well, subtle differences subsist between these simulations and the available experimental data regarding the pressure-density compressibility. Here, leveraging a recently developed analysis method for high-resolution Doppler interferometric velocity data, we present Eulerian sound speed measurements in compressed deuterium to shock pressures between 50 and 200 GPa. These results, extracted from laser-driven shockwave experiments, are found to agree with several of the most accurate equation of state models for deuterium at those conditions up to ∼150 GPa. However, the data indicate that these models fail to reproduce the experimentally observed sound speed at higher pressures, approaching 200 GPa. In particular, we unveil a discrepancy between the experimental results and the equation of state model that is most commonly used in inertial confinement fusion at the National Ignition Facility.