First-principles equation of state and shock compression predictions of warm dense hydrocarbons

Abstract

We use path integral Monte Carlo and density functional molecular dynamics to construct a coherent set of equation of state for a series of hydrocarbon materials with various C:H ratios (2:1, 1:1, 2:3, 1:2, and 1:4) over the range of 0.07 − 22.4 g cm −3 and 6.7 × 10 3 − 1.29 × 10 8 K. The shock Hugoniot curve derived for each material displays a single compression maximum corresponding to K-shell ionization. For C:H=1:1, the compression maximum occurs at 4.7-fold of the initial density and we show radiation effects significantly increase the shock compression ratio above 2 Gbar, surpassing relativistic effects. The single-peaked structure of the Hugoniot curves contrasts with previous work on higher-Z plasmas, which exhibit a two-peak structure corresponding to both K-and L-shell ionization. Analysis of the electronic density of states reveals that the change in Hugoniot structure is due to merging of the L-shell eigenstates in carbon, while they remain distinct for higher-Z elements. Finally, we show that the isobaric-isothermal linear mixing rule for carbon and hydrogen EOSs is a reasonable approximation with errors better than 1% for stellar-core conditions.Introduction. Hydrocarbon ablator materials are of primary importance for laser-driven shock experiments, such as those central to the study of inertial confinement fusion (ICF) [1][2][3] and the measurement of high energy density states relevant to giant planets [4] and stellar objects [5]. Accurate knowledge of the equation of state (EOS) of the hydrocarbon ablator is essential for optimizing experimental designs to achieve desired density and temperature states in a target. Consequently, a number of planar-driven shock wave experiments have been performed on hydrocarbon materials, including polystyrene (CH) [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23], glow-discharge polymer (GDP) [24][25][26][27][28], and foams [29][30][31][32], to measure the EOS. The highest pressure achieved among these experiments is 40 Mbar [14,15], which is yet not high enough to probe the effects of Kshell ionization on the shock Hugoniot curve. Since the first X-ray scattering results on CH at above 0.1 Gbar (1 Gbar=100 TPa) [33], ongoing, spherically-converging shock experiments using the Gbar platform at the National Ignition Facility (NIF) [34][35][36][37][38] and the OMEGA laser [39] will extend measurements of the shock Hugoniot curve of polystyrene to pressures above 0.35 Gbar and into the K-shell ionization regime [40].

Path integral Monte Carlo simulations of warm dense aluminum

“…Rigorous discussions of the PIMC [76][77][78] and DFT molecular dynamics (DFT-MD) [79][80][81] methods have been provided in previous works, and the details of our simulations have been presented in some of our previous publications [74,75,[82][83][84][85][86][87][88][89][90][91][92][93]. Here, we summarize the methods and provide the simulation parameters specific to simulations of aluminum plasma.…”

High Pressure Hydrocarbons Revisited: From van der Waals Compounds to Diamond

“…Dynamic compression experiments on hydrocarbons are used to probe their properties at combined high-pressure and -temperature conditions [33][34][35][36][37][38][39][40], both to investigate their potential decomposition, but also to better understand a critical ablator material used in inertial confinement fusion experiments. The thermal equation of state of hydrocarbons has therefore been a subject of several ab initio molecular dynamics studies [41][42][43][44][45][46]. Recent shock experiments on polystyrene ([C 8 H 8 ] n ) [37,38] reported its decomposition into diamond, however only above 140 GPa and 4000 K in an apparent disagreement with diamond anvil cell experiments [23,24].…”

Absolute Equation-of-State Measurement for Polystyrene from 25 to 60 Mbar Using a Spherically Converging Shock Wave

“…Here we present the first radiographic EOS measurements of a spherically converging shock wave in the laboratory. A polystyrene sample was used because a large body of experimental and theoretical work existed [9,13,[20][21][22][23][24], high quality spherical targets were readily available, and experimental requirements could be met by existing NIF target platforms. Our experiments significantly advance the accuracy of high-pressure Hugoniot measurements and extend the data set up to 60 Mbar.…”