D. E. Fratanduono

Multiple spherically converging shock waves in liquid deuterium

The fuel entropy and required drive energy for an inertial confinement fusion implosion are set by a sequence of shocks that must be precisely timed to achieve ignition. This Letter reports measurements of multiple spherical shock waves in liquid deuterium that facilitate timing inertial confinement fusion shocks to the required precision. These experiments produced the highest shock velocity observed in liquid deuterium (U(s) = 135  km/s at ∼2500  GPa) and also the first observation of convergence effects on the shock velocity. Simulations model the shock-timing results well when a nonlocal transport model is used in the coronal plasma.

Analysis of laser shock experiments on precompressed samples using a quartz reference and application to warm dense hydrogen and helium

Megabar (1 Mbar = 100 GPa) laser shocks on precompressed samples allow reaching unprecedented high densities and moderately high ∼103–104 K temperatures. We describe here a complete analysis framework for the velocimetry (VISAR) and pyrometry (SOP) data produced in these experiments. Since the precompression increases the initial density of both the sample of interest and the quartz reference for pressure-density, reflectivity, and temperature measurements, we describe analytical corrections based on available experimental data on warm dense silica and density-functional-theory based molecular dynamics computer simulations. Using our improved analysis framework, we report a re-analysis of previously published data on warm dense hydrogen and helium, compare the newly inferred pressure, density, and temperature data with most advanced equation of state models and provide updated reflectivity values.

The direct measurement of ablation pressure driven by 351-nm laser radiation

The instantaneous scaling of ablation pressure to laser intensity is directly inferred for ramp compression of diamond targets irradiated by 351-nm light. Continuously increasing pressure profiles from 100 to 970 GPa are produced by direct-drive laser ablation at intensities up to 7 × 1013 W/cm2. The free-surface velocity on the rear of the target is used to directly infer the instantaneous ablation-pressure profile at the front of the target. The laser intensity on target is determined by laser power measurements and fully characterized laser spots. The ablation pressure is found to depend on the laser intensity as P(GPa)=42(±3)[I(TW/cm2)]0.71(±0.01).

Precision equation-of-state measurements on National Ignition Facility ablator materials from 1 to 12 Mbar using laser-driven shock waves

A large uncertainty in the design of ignition capsules for use in the National Ignition Campaign (NIC) is the ablator equation of state. In this article, we report equation-of-state measurements for two candidate NIC ablator materials, glow-discharge polymer (GDP), and germanium-doped GDP. These materials were driven to pressures of 1 to 12 Mbar using laser-driven shock waves. Hugoniot measurements were obtained using the impedance matching technique with an α-quartz standard. This article presents the first kinematic measurements in the high-pressure fluid regime for these materials, which show to be in close agreement with Livermore equation-of-state model predictions.

Examining the radiation drive asymmetries present in the high foot series of implosion experiments at the National Ignition Facility

This paper details and examines the origins of radiation drive asymmetries present during the initial High Foot implosion experiments. Such asymmetries are expected to reduce the stagnation pressure and the resulting yield of these experiments by several times. Analysis of reemission and dual axis shock timing experiments indicates that a flux asymmetry, with a P2/P0 amplitude that varies from −10% to −5%, is present during the first shock of the implosion. This first shock asymmetry can be corrected through adjustments to the laser cone fraction. A thin shell model and more detailed radiation hydrodynamic calculations indicate that an additional negative P2/P0 asymmetry during the second or portions of the third shock is required to reach the observed amount of asymmetry in the shape of the ablator at peak implosion velocity. In conjunction with symmetry data from the x-ray self emission produced at stagnation, these models also indicate that after the initially negative P2/P0 flux asymmetry, the capsule...

A novel approach to Hugoniot measurements utilizing transparent crystals

A new absolute equation of state measurement technique is described and demonstrated measuring the shock state and the refractive index of MgO up to 226 GPa. This technique utilizes steady shock waves and the high-pressure transparency of MgO under dynamic shock compression and release. Hugoniot measurements performed using this technique are consistent with the previous measurements. A linear dependence of the shocked refractive index and density is observed up to 226 GPa, over a magnitude greater in pressure that previous studies. The transparency of MgO along the principal Hugoniot is higher than any other material reported to date. We observe a significant change in the refractive index of MgO as the Hugoniot elastic limit is exceeded due to the transition from uniaxial to hydrostatic strain. Measurements of the elastic-plastic two-wave structure in MgO indicate a nucleation time for plastic deformation.

Hugoniot experiments with unsteady waves

Recent development of transparent shock wave standard materials, such as quartz, enables continuous tracking of shock waves using optical velocimetry, providing information on shock wave steadiness and pressure perturbations in the target. From a first order perturbation analysis, we develop a set of analytical formulas that connect the pressure perturbations at the drive surface to the shock velocity perturbations observed in measurements. With targets that incorporate a calibrated transparent witness material, such as quartz, and with the analytical formulas describing the perturbation response, it is possible to determine the sound speed and Gruneisen coefficient of an unknown sample by using evolution of the non-steady perturbations as a probe. These formulas can also be used to improve the accuracy of traditional shock wave impedance match Hugoniot experiments of opaque samples driven with non-steady waves.

Shock-wave equation-of-state measurements in fused silica up to 1600 GPa

The properties of silica are important to geophysical and high-pressure equation-of-state research. Its most-prevalent crystalline form, α-quartz, has been extensively studied to TPa pressures. This article presents Hugoniot measurements on amorphous silica, commonly referred to as fused silica, over a range from 200 to 1600 GPa using laser-driven shocks and an α-quartz standard. These results extend the measured Hugoniot of fused silica to higher pressures. In the 200- to 600-GPa range, the data are in very good agreement with those obtained by Qi et al. [Phys. Plasmas 22, 062706 (2015)] using magnetically driven aluminum impactors and aluminum as a standard material. A new shock velocity/particle velocity relation is derived to fit the experimental data.

Differential heating: A versatile method for thermal conductivity measurements in high-energy-density matter

We propose a method for thermal conductivity measurements of high energy density matter based on differential heating. A temperature gradient is created either by surface heating of one material or at an interface between two materials by different energy deposition. The subsequent heat conduction across the temperature gradient is observed by various time-resolved probing techniques. Conceptual designs of such measurements using laser heating, proton heating, and x-ray heating are presented. The sensitivity of the measurements to thermal conductivity is confirmed by simulations.

X-ray area backlighter development at the National Ignition Facility (invited).

1D spectral imaging was used to characterize the K-shell emission of Z ≈ 30-35 and Z ≈ 40-42 laser-irradiated foils at the National Ignition Facility. Foils were driven with up to 60 kJ of 3ω light, reaching laser irradiances on target between 0.5 and 20 × 10(15) W/cm(2). Laser-to-X-ray conversion efficiency (CE) into the Heα line (plus satellite emission) of 1.0%-1.5% and 0.15%-0.2% was measured for Z ≈ 30-32 and Z ≈ 40-42, respectively. Measured CE into Heα (plus satellite emission) of Br (Z = 35) compound foils (either KBr or RbBr) ranged between 0.16% and 0.29%. Measured spectra are compared with 1D non-local thermodynamic equilibrium atomic kinetic and radiation transport simulations, providing a fast and accurate predictive capability.