Session GO7: HED: Materials Under Extreme Conditions

Fundamental science at the extremes on NIF

The universe abounds with extreme phenomena and conditions. Examples include planetary core properties at extraordinary pressures and densities; star formation dynamics; stellar nucleosynthesis; supernova explosions launching powerful shocks that ripple throughout the universe for centuries to millennia, generating magnetic fields and accelerating particles; and relativistic shocks resulting from the most powerful explosions in the universe, namely, gamma-ray bursts (GRBs). Laboratory experiments on the National Ignition Facility (NIF) are improving our understanding of these extreme phenomena through the NIF Discovery Science program. I will discuss results from NIF on (1) high pressure equations of state and detailed plasma characterization at planetary core and brown dwarf interior conditions; (2) nuclear reactivities and astrophysical S-factor measurements at stellar core conditions; (3) hydrodynamic instabilities relevant to supernova explosions and stellar and planetary formation dynamics; (4) astrophysical collisionless shocks, magnetic field generation, and particle acceleration; and (5) relativistic pair plasma generation using the NIF ARC laser, relevant to aspects of GRB dynamics.   

Temperature Measurements of Solid-Density Germanium Plasmas Created with an X-Ray Free-Electron-Laser

We have used the focused femtosecond x-ray output from LCLS at photon energies of 1400 eV and intensities of order 10$^{17}$ Wcm$^{-2}$ to isochorically heat sub-micron thick foils of Ge to temperatures between 150-200 eV. L-shell X-Rays emitted from the solid-density plasma were recorded using a Bragg crystal spectrometer. An analysis of the bound-free recombination emission, including both its slope and relative intensity as a function of total energy in the FEL beam, allows for an accurate determination of the peak temperature of the plasma. Simulations using atomic-kinetics calculations show that the system cools due to the emission of blackbody radiation on a timescale of several picoseconds, and that the time-integrated recombination emission is heavily gated towards the peak temperature within the system, rapidly decaying on timescales shorter than the estimated inertial confinement time.   

Optimizing low-adiabat ramped megabar drives for material strength experiments

Reducing the heat load on material samples undergoing ramped compression to megabar pressures was the objective of a series of shots at the National Ignition Facility, utilizing improved simulation tools and combinations of ultra-low-density foams to optimize an existing `reservoir release' platform. VISAR data from these shots will be presented which demonstrate the effects of modifying the structure of the reservoir, and the future development of this technique will be discussed.   

Experimental studies of material strength in the high energy density regime

A solid material can be placed in the high energy density regime by compressing it to pressures \textgreater 1 Mbar using a laser driven plasma piston drive. We create a ramped laser drive that keeps the material in the solid state during compression without shock melting. Understanding plastic deformation dynamics of materials under these extreme conditions is of high interest to a number of fields, including meteor impact dynamics and advanced inertial confinement fusion. We infer the strength of Ta, Pb [1,2] and Fe at high pressures (upto 8 Mbar), high strain rates (\textasciitilde 10$^{\mathrm{7}}$ s$^{\mathrm{-1}})$ and high strains (\textgreater 30{\%}) by measuring the growth of Rayleigh-Taylor instabilities (RTI) under ramped compression. We find that the RTI growth for materials in the solid state, compressed under high pressure and high strain rates, is reduced compared to the no-strength case. We will describe the experimental results from NIF and compare them to various strength models. [1] H. -S. Park et al., Phys. Rev. Lett. 114, 065502 (2015). [2] A. Krygier et al., Phys. Rev. Lett., submitted (2019).   

Liquid Cu Rayleigh Taylor Experiments

The deviation of Rayleigh Taylor growth from the predicted classical (liquid) growth has been used to infer the strength of solid materials. Some of the most recent work has been done using laser platforms which achieve high pressures and high strain rates where the deviation from liquid growth is assumed to be correct. We will present experimental and simulation results comparing the growth of shock-melted, liquid Cu samples at similar time scales (10's of ns) and strain rates (\textgreater 10\textasciicircum 6/s) for recent experiments carried out at the Omega laser facility.   

Shock Compression of Multiple Materials up to 120 Mbar at the National Ignition Facility

We present shock Hugoniot data for quartz, molybdenum (Mo), boron (B), boron carbide (B$_4$C), beryllium oxide (BeO), and boron nitride (BN) at ~100 Mbar pressures, far exceeding the pressure, temperature conditions attained in previous studies on these materials. Impedance-match data were obtained relative to a diamond standard for up to 4 materials during single shots at the National Ignition Facility. Elements and compounds with similar densities (B, C, B$_4$C, BeO, and BN) were chosen to systematically test our ability to model the equation of state of both pure and mixed low-Z matter in the warm dense matter/plasma regime. New data on quartz and Mo help develop them as material standards to higher pressures than previously characterized.   

Forward modeling of boron equation of state (EOS) measurements at the National Ignition Facility (NIF)

A convergently-driven absolute EOS platform capable of accessing pressures up to 1 Gbar has been developed at the NIF. At these high-temperature, high-pressure regimes, atomic shell structure may affect the compressibility of materials. Using this NIF platform, we can conduct experimental measurements needed to validate existing EOS models. For this work, we will discuss boron EOS measurements at pressures of hundreds of Mbars. Boron is a candidate for experiments at the NIF that require low areal-density, high strength ablators. In our absolute EOS measurements, we use streaked x-ray radiography to determine shock Hugoniot states. We will examine forward modeling of radiographs of different boron experiments. Coupling radiation-hydrodynamic calculations to simulated radiographs allows us to explore sensitivities of the platform.   

Wide-range equations of state of carbon and boron materials from first principles

Using several independent approaches (path integral Monte Carlo, density functional theory, and activity expansion), we performed extensive investigation providing the theoretical benchmark for the equations of state (EOS) of a series of low-Z materials (CHx, B, BN, and B4C) over a wide range of temperatures (0.1-1e4 eV) and densities (0.01-100 g/cc). Across the warm-dense regime, our predictions show remarkable consistency with experimental data and constrain the EOS to better than 4{\%}, with the largest uncertainties occurring at 1e6 K and 1 Gbar where K shell ionization occurs. Constrained by our first-principles data, we made improved EOS models to be used for the design and interpretation of high-energy-density and inertial confinement fusion experiments. We also discuss the strengths and weaknesses of empirical approaches such as the ideal-mixing approximation and the Arrhenius relation, as well as structural complexities during shock compression. (LLNL-ABS-780064)   

X-Ray Diffraction of Diamond on the Double-Shock Hugoniot

The high-pressure equation of state of carbon is integral to inertial confinement fusion and the modeling of gas giants such as Uranus and Neptune. Numerous studies have investigated the compression and melting curve along the principal Hugoniot, where the liquid--solid coexistence regime exists between 6 and 10 Mbar. Here we explore the secondary Hugoniot of dynamically compressed diamond using velocity interferometry and optical pyrometry. Simultaneous x-ray diffraction measurements are performed to determine the crystal structure and to detect the onset of melting at higher pressures than those accessible by a single shock. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

Atom-in-jellium equations of state for cryogenic liquids

Modeling the electronic structure of an atom as a homogeneous electron gas or jellium yields a computationally efficient method for calculating the equations of state (EOS) that has been shown to be consistent with the more rigorous methods employing path-integral Monte Carlo and quantum molecular dynamics (QMD) simulations of metals in a warm dense matter regime. Here we apply the atom-in-jellium model to predict wide-ranging EOS for the cryogenic liquid elements nitrogen, oxygen, and fluorine. The principal Hugoniots for these substances were surprisingly consistent with the available shock data and Thomas-Fermi (TF) EOS for very high pressures and exhibited systematic variations from TF associated with shell ionization effects. The new EOS extend much higher in pressure than previous widely-used models for nitrogen and oxygen in particular, and should allow much more accurate EOS to be predicted for oxides and nitrides in the liquid, vapor, and plasma regime, where these have previously been constructed as mixtures containing the older EOS.   

Temperature-induced changes in hP4-Sodium Electride: An Ab Initio Study

Crystalline alkali metals generally exhibit near free-electron behavior at ambient conditions. For sodium at high pressures, a deviation from such a metallic behavior was found in the hP4 phase. Diamond anvil cell experiments and associated cold curve calculations have concluded that this change is caused by the formation of hP4 electrides with interstitially localized electrons, which renders this phase transparent. However, higher temperature solid-states accessible by ramp-compression have showed results to the contrary. In this work, we investigate the subtleties of the electronic and optical property changes in such a system with increasing temperatures, using density functional theory with the Kubo-Greenwood formulation. The properties looked into include, among others, the electron localization function, frequency-dependent conductivity, density of electronic states, and electronic band structure for a quantitative explanation of the causes for the emergence of reflectivity in hP4-sodium under high pressure--temperature conditions.   

Modifying the Random Phase Approximation for the Average Atom

The dielectric function corresponding to the Random Phase Approximation (RPA) has been applied in many studies to determine the response properties of plasmas like dynamic structure factors and stopping powers. However, the RPA fails to take into account short range electron-electron and electron-ion interactions. ~These interactions can be calculated by average-atom models and used to modify the RPA dielectric function to obtain self-consistent response properties. We show how the modified dielectric function changes both dynamic structure factors and stopping powers for charged particles traveling through a plasma and compare with results in the literature.   

Sarkas: An object-oriented molecular dynamics code for plasmas in Python

Plasma science enjoys a wide array of simulation methods, including hydrodynamics, smoothed-particle hydrodynamics, and particle-in-cell. This list increasingly includes molecular dynamics (MD), a powerful method to investigate many-body systems at the atomistic scale that yields closure properties, such as equations of state and transport coefficients, required by macroscale methods. In spite of these merits, MD has been limited to extremely small length and time scales. However, due to advances in architectures (e.g., GPUs), MD can now be applied to a wider array of scales, and can be included in multiscale approaches alongside other methods. Today, many MD codes are available; however, there are no plasma-specific codes and none that employ high-performance Python. We are developing such a code, called Sarkas, which is an object-oriented MPI-parallel code. Sarkas is an open-source code, and takes advantage of the user-friendliness of Python, suitable for wide use in various research contexts, as well as execution speeds comparable to compiled languages through Numpy and Numba. Sarkas employs a highly efficient P$^{3}$M algorithm for long-range electric fields. Sarkas will be introduced and several examples of its use for a range of plasma physics problems will be illustrated.   

Sensitivity of stellar physics to the equation of state

The formation and evolution of stars depends on various physical aspects of stellar matter, including the equation of state (EOS) and transport properties. Although often dismissed as `ideal gas-like' and therefore simple, states occurring in stellar matter are dense plasmas, and the EOS has not been established precisely. EOS constructed using multi-physics approaches found necessary for laboratory studies of warm dense matter give significant variations in stellar regimes, and vary from the EOS commonly used in simulations of the formation and evolution of stars. We have investigated the sensitivity of such simulations to variations in the EOS, for sun-like and low-mass stars. We find a high sensitivity of the lifetime of the Sun and of the lower luminosity limit for red dwarfs, and a significant sensitivity in the lower mass limit for red dwarfs. Simulations of this type are also used for other purposes in astrophysics, including the interpretation of absolute magnitude as mass, the conversion of inferred mass distribution to the initial mass function using predicted lifetimes, simulations of star formation from nebulae, simulations of galactic evolution, and the baryon census used to bound the exotic contribution to dark matter.