Session GO07: HED: Materials and Atomic Physics

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The Rayleigh-Taylor instability can be used to measure flow strength in solids, where the growth of perturbations is mitigated by the material's resistance to plastic flow.$^{\mathrm{1}}$ This phenomenon has been exploited to investigate the strength of various materials at high strain rates, including Cu, Fe, Ta, and Pb.$^{\mathrm{2-4}}$ Here, a Be plasma, formed by three lasers of Omega EP, drives a ramped compression pulse into a rippled Sn target to an average pressure of 1 Mbar. The growth of a rippled Sn surface against less dense CH plastic is measured using face-on radiography. We find that the growth is significantly less than predicted by a simple Steinberg-Guinan strength model. 1.Park, H.-S. \textit{et al.} Grain-Size-Independent Plastic Flow at Ultrahigh Pressures and Strain Rates. \textit{Phys. Rev. Lett.} \textbf{114}, (2015). 2.Huntington, C. M. \textit{et al.} Investigating iron material strength up to 1 Mbar using Rayleigh-Taylor growth measurements. \textit{AIP Conf. Proc.} \textbf{1793}, 110007 (2017). 3.McNaney, J. M. \textit{et al.} Measurements of Rayleigh-Taylor growth in solid and liquid copper in the Mbar regime. in \textit{Bull. of the Am. Phys. Soc.} (2019). 4.Krygier, A. \textit{et al.} Extreme Hardening of Pb at High Pressure and Strain Rate. \textit{Phys. Rev. Lett.} \textbf{123}, (2019).   

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Experiments conducted at the National Ignition Facility and the Omega laser have used plasma-driven ramp compression to induce strength-dependent Rayleigh-Taylor instability allowing us infer strength in solid body-centered cubic (bcc) lead and lead alloy at high pressure [1]. Experiments are underway for bcc tin. Here we model the strength (flow stress) of lead and tin at high pressure. Previous models were built from data in the ambient-pressure phase. Plasticity in bcc can be very different. We have developed Improved Steinberg-Guinan models for bcc lead strength [2] and tin strength using ab initio calculations of the shear modulus at pressure. The lead model agrees well with those experiments. The lead alloying, which increases strength 4x at ambient conditions, has no measurable effect at high-pressure. [1] A. Krygier et al., Phys. Rev. Lett., 123, 205701 (2019). [2] R.E. Rudd et al., AIP Conf. Proc. 1979, 070027 (2018).   

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We present the findings of a double-shock experiment in polystyrene, where the reflectance of the second shock behind the transparent first shock was observed in addition to the resulting coalesced shock. We deduce mechanical, thermal, and transport, properties of the double-shocked material to pressures of up to 8 Mbar with reflectivity, shock velocity, and temperature data obtained from the VISAR (velocity interferometer system for any reflector) and the SOP (streaked optical pyrometer) diagnostics. From these data we explore the off-Hugoniot behavior of polystyrene and conclude with a comparison of our findings to previous experiments. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856, the University of Rochester, and the New York State Energy Research and Development Authority.   

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Kinetics can play an important role in the transformation of materials to different high-pressure phases on the short time scales associated with dynamic-compression experiments. The study of phase-transition kinetics has motivated many theoretical and experimental works on the rapid freezing of liquid water into the ice-VII phase. We present measurements of the over-pressurization of the water-ice VII phase transition at 10x higher compression rates than previously studied. Water was ramp compressed to peak pressures of $\sim$15 GPa over $\sim$10 ns at the Omega laser facility. The pressure at which water froze into the ice VII phase is deduced from wave-profile measurements and compared to predictions using a phase-transition-kinetics model recently developed at Lawrence Livermore National Laboratory.   

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X-ray Free Electron Lasers deliver sufficiently high intensities to drive samples directly to high energy density states. This occurs faster than heating by charged particle beams, and with much greater uniformity than optical irradiation. However, since the energy deposition occurs by exciting high-energy photo- and Auger electrons, it is complex to model, and tends to heat a larger region of the sample than the irradiated spot. This talk will present results from the Japanese XFEL SACLA (SPring-8 Angstrom Compact free electron LAser). Using its two-color split and delay mode, we irradiated samples with incident X-ray intensities on the order of 10\textasciicircum 19 W/cm2, and probed changes in the atomic structure by diffraction and scattering at delays of up to 300 fs. In silicon, nonthermal melting occurs within 100 fs, with the appearance of the liquid state discerned from the rise in incoherent scattering. In graphite, anisotropic disordering proceeds on a similar timescale, with weak interplanar bonds breaking, but in-plane Bragg peaks becoming stronger as the intact planes deform.   

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Shock release is the fundamental process that takes place when a material rapidly decompresses from a high-pressure state. The conventional treatment of shock release assumes that the process is isentropic. To test this assertion, we performed an experiment at the Matter in Extreme Conditions instrument, wherein tantalum foils were compressed to a megabar shock state via laser-plasma ablation, and subsequently probed as they unloaded with an x-ray free-electron laser. From the resulting diffraction patterns, the post-release temperatures of the foils were deduced from their thermal expansion, and were found to exceed significantly the isentropic release temperature. These results are corroborated by large-scale molecular dynamics simulations of tantalum crystals in shock and release, which demonstrate that heating occurs due to the colossal plastic work that must be expended to overcome the crystal's extreme strength during rapid release.   

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Magnesium oxide is an important window material in dynamic shock experiments, a pressure standard in diamond-anvil cell experiments, and a fundamental planet-forming mineral. Elucidating the mechanism of its phase transition under high pressure is important for high-energy-density sciences, planetary sciences, as well as for materials sciences. Significant progress in theoretical and experimental research has been made to determine the equilibrium phase boundary of this transition. We are determining/predicting transient structures and their compression-rate dependence by combining geometrical analysis, empirical models, and quantum computations. These findings are important for deciphering kinetic and thermodynamic origins of solid-state phase transitions at extreme conditions. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

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We present a thermodynamically consistent description of plasma under collisional radiative equilibrium (CRE). Many plasmas composed of mid- to high-$Z$ materials are much more accurately described by CRE than local thermodynamic equilibrium (LTE), particularly when plasma properties, such as specific heat and sound speed require accurate accounting of the ionization component of the internal energy. CRE opacity/emissivity tables have been in use for many years in radiation-hydrodynamic simulations. We seek self-consistency by implementing equation-of-state tables based on the same CRE atomic models. CRE extends LTE by accounting for the steady-state atomic-kinetic effects of radiation escaping freely from the point of emission. As under LTE, plasma under CRE is described completely in terms of local thermodynamic variables. Consequently, radiative and thermodynamic properties can be tabulated in terms of local thermodynamic variables, and thermodynamically consistent descriptions of both limits are possible. Conceptual problems arise from CRE not being a true equilibrium, which requires a description in terms of nonequilibrium thermodynamics.   

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Recently, we have emphasized the renewed interest to study hard x-ray non-thermal inner-shell emission from Z-pinch plasmas of high-atomic-number materials on Sandia's Z and NRL Gamble II generators and have presented the time history of relative intensities of cold L lines (alpha, beta, and gamma) from W wire arrays (in a spectral range between 1 and 1.7 {\AA}) produced on the UNR Zebra generator. Another important signature of non-thermal high-atomic-number multiply ionized plasmas is polarization of hard x-ray line emission. Here we present the theoretical study of polarization of hard x-ray spectra of Na-like W produced by dielectronic recombination in the same spectral range. In particular, the polarization of the intense dielectronic satellite (DS) lines in the vicinity of the most intense Ne-like W resonance lines at 1.19 {\AA} and 1.36 {\AA} is calculated and analyzed in detail. The DS spectral features mostly affected by polarization are identified and highlighted. Future work and applications to High-Energy-Density Science are discussed. This research was supported by NNSA under the DOE grant DE-NA0003877 and in part through the Krell Institute Laboratory Residency Graduate Fellowship under DE-NA0003864.   

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The ability of radiation magneto-hydrodynamics codes to accurately predict the behaviour of high energy density physics experiments depends on the provision of accurate data for the constituent properties of the materials involved. The predicted properties vary depending on the microphysical model used to represent them. Furthermore, the interplay between the hydrodynamics and radiation transport leads to a source of error when using inconsistent models for material properties. This work presents the development of the DCA atomic code SpK [1] to enable the calculation of the equation of state, based on the solution to the Saha equation and following the work of Faussurier et al. [2]. SpK was initially developed to produce opacity data from the excited state level populations and distribution of charge states calculated. The extension of SpK to provide a description of the EoS enables tabulated data valid for dense plasmas to be produced at low computational cost compared to first-principles methods. References [1] J. P. Chittenden, et al., Physics of Plasmas 23, 052708 (2016) [2] G. Faussurier, et al., HEDP 4, 114 (2008)   

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Laser-driven experiments at the University of Rochester's Laboratory for Laser Energetics were performed to probe the crystal structure of shock and shock-ramped platinum with the powder x-ray diffraction platform\footnote{ J. R. Rygg \textit{et al.}, Rev. Sci. Instrum. \textbf{83}, 113904 (2012).} on OMEGA EP. Platinum is of interest because it is often used as a calibration standard in high-pressure experiments. The samples remained face-centered cubic when shock-ramped up to \textasciitilde 350 GPa and liquid diffraction was observed upon further compression. These experiments serve as additional measurements of shock-ramped, shocked, and shock-released platinum completed at Sandia National Laboratories and the National Ignition Facility. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.   

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Inertial confinement fusion (ICF) implosions can create extreme high-energy-density (HED) states of matter such as super-high densities and temperatures. For example, a stagnating ICF shell accesses pressures up to hundreds of Gbar (\textasciitilde 10$^{\mathrm{16}}$ Pa) due to spherical convergence, even for an non-igniting target. How atomic physics might change in these extreme conditions is an intriguing and fundamental question that remains to be understood by the HED physics community. A better understanding of such extreme atomic physics can, in turn, help diagnose the stagnated-shell conditions. For these purposes, we have initiated a combined experimental and theoretical campaign on OMEGA to examine detailed atomic physics at super-high pressures above \textasciitilde 5 Gbar using stable Cu-doped CH-shell implosions. Time-resolved, high-resolution spectroscopy has been used to simultaneously measure K$_{\alpha }$ and K$_{\beta }$ emission/absorption of Cu in the stagnating shell during the hot-spot formation. These observations have been interpreted by radiation-hydrodynamic simulations coupled with either commonly used collisional-radiative (NLTE) models or self-consistent density-functional-theory--based multi-band kinetic modeling. We will present our understanding of extreme atomic physics and how we can apply this time-resolved spherical spectroscopy to tracking the return shock and heat-wave propagation in a stagnating ICF shell. This work is supported by the DOE/NNSA under Award Number DE-NA0003856 and US NSF Grant No. PHY-1802964.