Dissociation along the principal Hugoniot of the Laser Mégajoule ablator material

Glow discharge polymer hydrocarbon (GDP-CH) is used as the ablator material in inertial confinement fusion (ICF) capsules for the Laser Mégajoule and National Ignition Facility. Due to its fabrication process, GDP-CH chemical composition and structure differ from commercially available plastics and detailed knowledge of its properties in the warm dense matter regime is needed to achieve accurate design of ICF capsules. First-principles ab initio simulations of the GDP-CH principal Hugoniot up to 8 Mbar were performed using the quantum molecular dynamics (QMD) code abinit and showed that atomic bond dissociation has an effect on the compressibility. Results from these simulations are used to parametrize a quantum semiempirical model in order to generate a tabulated equation of state that includes dissociation. Hugoniot measurements obtained from an experiment conducted at the LULI2000 laser facility confirm QMD simulations as well as EOS modeling. We conclude by showing the EOS model influence on shock timing in a hydrodynamic simulation.

Figure 1

GDP-CH isochores calculated using QMD simulations. The inset shows a close-up of isochores in the energy-temperature plane. Dotted lines are fits to guide the eye.

Figure 2

C-C pair correlation function for GDP-CH along simulated Hugoniot states.

Figure 3

GDP-CH Grüneisen parameter deduced from cold curve and Hugoniot QMD calculations (blue filled squares) is fitted using a Hill function (continuous blue line) to produce an effective $\mathrm{\Gamma }$, and compared with QEOS ion EOS (dashed pink line) and Hamel [24] ${\mathrm{CH}}_{1.36}$ EOS (orange line).

Figure 4

GDP-CH Hugoniot in (a) pressure-compression plane and (b) temperature-compression plane. The QSEM (blue line) fitted to QMD calculations (blue filled squares) is compared to QEOS (pink dashed line) and LEOS5350 (gray line, taken from Ref. [24]). Other plastic materials are also shown: QMD calculations on polyethylene [5] (purple open circles) and polystyrene [9] (cyan line and filled squares), and Mie-Grüneisen EOS fitted on QMD calculations for ${\mathrm{CH}}_{1.36}$ mixtures [24] (orange line). Insets show high pressure and temperature region of the Hugoniot, with the same units as in the main graphs. Green filled circles in top graph are high-temperature orbital free molecular dynamics calculations from Ref. [11]. Also shown in blue dashed line in panel (a) is the QMD cold curve from Ref. [31].

Figure 5

Experimental configuration and corresponding VISAR and SOP images, with identical time and space scales.

Figure 6

GDP-CH Hugoniot data (red filled circles) in (a) pressure-compression plane and (b) temperature-compression plane, compared to the QSEM (blue line), QMD calculations (blue filled squares), and QEOS (pink dashed line). Also included are pressure-compression data for General Atomics GDP [23] (gray filled circles) and LEOS5350 (gray line, taken from Ref. [24]). Gray filled circles in temperature-compression plane (bottom graph) are polystyrene data from Ref. [15]. Data from Ref. [15] and Ref. [23] were reanalyzed using quartz standard from Ref. [45].

Figure 7

CHIVAS simulations of time history of pressure in a monodimensional $500\mu \mathrm{m}$ thick GDP-CH slab irradiated by the radiative drive for a typical LMJ ignition design. Pressure profiles showing first and second shocks were taken at $150\mu \mathrm{m}$ and shock merge occur at $\sim 250$–$300\mu \mathrm{m}$ depending on the EOS model. Color code is pink for QEOS and blue for QSEM. Continuous lines are profiles taken at $150\mu \mathrm{m}$ depth and dotted lines are for profiles taken at $\sim 250$–$300\mu \mathrm{m}$.