Crystal structure and equation of state of Fe-Si alloys at super-Earth core conditions

The high-pressure behavior of Fe alloys governs the interior structure and dynamics of super-Earths, rocky extrasolar planets that could be as much as 10 times more massive than Earth. In experiments reaching up to 1300 GPa, we combine laser-driven dynamic ramp compression with in situ x-ray diffraction to study the effect of composition on the crystal structure and density of Fe-Si alloys, a potential constituent of super-Earth cores. We find that Fe-Si alloy with 7 weight % (wt %) Si adopts the hexagonal close-packed structure over the measured pressure range, whereas Fe-15wt%Si is observed in a body-centered cubic structure. This study represents the first experimental determination of the density and crystal structure of Fe-Si alloys at pressures corresponding to the center of a ~3–Earth mass terrestrial planet. Our results allow for direct determination of the effects of light elements on core radius, density, and pressures for these planets

Crystal structure and equation of state of Fe-Si alloys at super-Earth core conditions

The high-pressure behavior of Fe alloys governs the interior structure and dynamics of super-Earths, rocky extrasolar planets that could be as much as 10 times more massive than Earth. In experiments reaching up to 1300 GPa, we combine laser-driven dynamic ramp compression with in situ x-ray diffraction to study the effect of composition on the crystal structure and density of Fe-Si alloys, a potential constituent of super-Earth cores. We find that Fe-Si alloy with 7 weight % (wt %) Si adopts the hexagonal close-packed structure over the measured pressure range, whereas Fe-15wt%Si is observed in a body-centered cubic structure. This study represents the first experimental determination of the density and crystal structure of Fe-Si alloys at pressures corresponding to the center of a ~3–Earth mass terrestrial planet. Our results allow for direct determination of the effects of light elements on core radius, density, and pressures for these planets

Bayesian inference of fundamental physics at extreme conditions

Thesis (Ph. D.)--University of Rochester. Department of Physics and Astronomy, 2021.Much of modern physics has been built on studying phenomena in isolated systems. These seemingly simple mechanisms, when applied to larger collection of system, become more complicated. The tools used to understand these systems were developed primarily at conditions relevant to the surface of the Earth and therefore comparison to experiments have been readily available. A similar addition of complexity exists when transitioning to extreme thermodynamic states. In these conditions, temperatures in the tens of thousands to many millions of Kelvin and densities from miligrams per cubic centimeter to kilograms per cubic centimeter, there exists complex collective behaviour that leads a wealth of interesting new phenomena not present in isolated atomic interaction or under standard thermodynamic conditions. These phenomena no doubt are critical to the evolution of astrophysical bodies, such as planets and stars. The modeling efforts have largely outpaced the experimental capabilities with regards to these systems at high energy density (HED, equivalently high pressure) and the laboratory landscape is moving towards measurements that are challenging to analyze independent of advanced modeling techniques. This work aims to present a systematic method for designing, executing, and analyzing HED experiments that enforces a self-consistent picture of the physics present in the system. This is done by creating a Bayesian inference framework, similar to those that exist in other fields of physics, that uses all relevant experimental data to constrain the integrated physicals model used to understand the experiments. The ultimate goal is to move away from 'benchmarking' techniques and towards proper statistical constraint of the models used to describe physics under extreme conditions

Exploring the off-Hugoniot phase diagrams of carbon dioxide and magnesium oxide utilizing static and dynamic precompression techniques

Thesis (Ph. D.)--University of Rochester. Department of Physics and Astronomy, 2021.Compressing matter with shock waves produced by high-power lasers is a robust method for achieving extreme pressures in sample materials of interest. However, a shock can only access a single line in phase space, referred to as the Hugoniot, which is subject to certain conservation relations. In order to explore off-Hugoniot states while still utilizing shock waves, this thesis employs static (diamond-anvil cell) and dynamic (double-shock) precompression techniques to study the phase diagrams of carbon dioxide and magnesium oxide, respectively. Carbon dioxide samples were precompressed in diamond-anvil cells to tune the initial densities from 1.35 g/cm3 (liquid) to 1.74 g/cm3 (solid) at room temperature and were then shock compressed up to 1 TPa and 93,000 K. Variation in initial density was leveraged to infer thermodynamic derivatives including specific heat and Gruneisen coefficient, exposing a complex bonded and moderately ionized state at the most extreme conditions studied. Magnesium oxide was precompressed with a 150- to 250-GPa transparent shock wave prior to further compression with a 1- to 2- TPa second shock. A plateau in temperature between 1218 and 1950 GPa was observed in the second shock states due to latent heat of melting. This is the highest pressure to which any material’s melt curve has been studied experimentally

Hugoniot measurements of silicon and radiance transition in shocked silica aerogel

Thesis (Ph. D.)--University of Rochester. Department of Physics and Astronomy, 2022This dissertation describes the study of crystalline silicon (Si) and silica (SiO2) aerogel shock-compressed to extreme conditions. Shock waves were generated by laser irradiation and recorded using time-resolved optical diagnostics. The behavior of these materials at high pressure is important to understanding the structure and evolution of terrestrial planets, as well as the performance of ICF capsule designs. Principal Hugoniot and sound speed measurements were performed on silicon to 2100 GPa using high-intensity laser drivers and impedance matching techniques. A change in the shock velocity versus particle velocity (us-up) slope was detected along the fluid silicon principal Hugoniot at 200 GPa. Density functional theory-based quantum molecular dynamics simulations suggest that an increase in ionic coordination and average ionization are coincident with the observed change in slope. Thermodynamic behavior of shock-compressed silica aerogel was studied to test its viability as a bright optical source for high energy density physics experiments. Radiance, reflectance, and shock velocity measurements were performed on singly-shocked SiO2 aerogel at initial densities of 0.3, 0.2, and 0.1 g/cm3 . A change in brightness temperature versus pressure slope is observed along the aerogel Hugoniot, which could be due to radiative, conductive, or microstructure effects. A shock front radiance model is presented to enhance predictive capabilities for shock physics experiments using silica photon sources

Effects of fuel-capsule shimming and drive asymmetry on inertial-confinement-fusion symmetry and yield

Three orthogonal proton emission imaging cameras were used to study the 3D effects of low-mode drive asymmetries and target asymmetries on nuclear burn symmetry and yield in direct-drive, inertial-confinement-fusion experiments. The fusion yield decreased quickly as the burn region became asymmetric due to either drive or capsule asymmetry. Measurements and analytic scaling are used to predict how intentionally asymmetric capsule shells could improve performance by compensating for drive asymmetry when it cannot be avoided (such as with indirect drive or with polar direct drive).United States. Department of Energy (Grant DE-NA0002726)United States. Department of Energy (Grant DE-NA0002949

Response to Comment on "Insulator-metal transition in dense fluid deuterium"

International audienceIn their comment, Desjarlais et al. claim that a small temperature drop occurs after isentropic compression of fluid deuterium through the first-order insulator-metal transition. We show that their calculations do not correspond to the experimental thermodynamic path, and that thermodynamic integrations with parameters from first-principles calculations produce results in agreement with our original estimate of the temperature drop