Laser-driven Shock Wave Experiments Research Articles

Free-surface velocity measurements of opaque materials in laser-driven shock-wave experiments using photonic Doppler velocimetry

We present a novel photonic Doppler velocimetry (PDV) design for laser-driven shock-wave experiments. This PDV design is intended to provide the capability of measuring the free-surface velocity of shocked opaque materials in the terapascal range. We present measurements of the free-surface velocity of gold for as long as ∼2 ns from the shock breakout, at pressures of up to ∼7 Mbar and a free-surface velocity of 7.3 km/s with an error of ∼1.5%. Such laboratory pressure conditions are achieved predominantly at high-intensity laser facilities where the only velocity diagnostic is usually line-imaging velocity interferometry for any reflector. However, that diagnostic is limited by the lower dynamic range of the streak camera (at a temporal resolution relevant to laser shock experiments) to measure the free-surface velocity of opaque materials up to pressures of only ∼1 Mbar. We expect the proposed PDV design to allow the free-surface velocity of opaque materials to be measured at much higher pressures.

Time-dependent effects in melting and phase change for laser-shocked iron

Using the LCLS facility at the SLAC National Accelerator Laboratory, we have observed X-ray scattering from iron compressed with laser driven shocks to Earth-core like pressures above 400GPa. The data shows shots where melting is incomplete and we observe hexagonal close packed (hcp) crystal structure at shock compressed densities up to 14.0 gcm-3 but no evidence of a double-hexagonal close packed (dhcp) crystal. The observation of a crystalline structure at these densities, where shock heating is expected to be in excess of the equilibrium melt temperature, may indicate superheating of the solid. These results are important for equation of state modelling at high strain rates relevant for impact scenarios and laser-driven shock wave experiments.

Fluorescence temperature sensing based on thermally activated singlet-triplet intersystem crossing in crystalline anthracene

The temperature dependence of a steady-state fluorescence spectrum of anthracene crystals ranging from 300 K to 500 K had been investigated, which was in the temperature range of most tabletop laser driven shock wave experiments. An interesting finding is that the fluorescence intensity of the 2-0 transition increases more rapidly than other transitions as the temperature increases. In particular, the logarithm of intensity ratios γn shows a linear correlation with inverse temperature, which can be used for fluorescence temperature sensing. The analysis of sensitivity η and random uncertainty ΔT has demonstrated that the intensity ratio γ1 is the best comprehensive performance physical quantity for temperature sensing. Theoretical analysis and experimental results demonstrated that the unusual increase in the intensity of 2-0 transition originated from a second excited triplet state T2, which was thermally coupled with the first excited singlet state S1. In a word, we established a new fluorescence temperature sensing method based on the intensity ratio and clarified that the mechanism of this method was the thermally activated singlet-triplet intersystem crossing.

Measurement of the sound speed in dense fluid deuterium along the cryogenic liquid Hugoniot

Many experimental studies, spanning several decades of research and using various dynamic compression schemes, have been conducted to investigate cryogenic liquid deuterium under strong shock compression. The consensus emerging from these studies is that of a progressive dissociation of the D2 molecules into an electrically conducting, atomic plasma, when subjected to shock pressures exceeding ∼50 GPa. While state-of-the-art numerical simulations based on density-functional-theory or quantum Monte-Carlo techniques capture this behavior quite well, subtle differences subsist between these simulations and the available experimental data regarding the pressure-density compressibility. Here, leveraging a recently developed analysis method for high-resolution Doppler interferometric velocity data, we present Eulerian sound speed measurements in compressed deuterium to shock pressures between 50 and 200 GPa. These results, extracted from laser-driven shockwave experiments, are found to agree with several of the most accurate equation of state models for deuterium at those conditions up to ∼150 GPa. However, the data indicate that these models fail to reproduce the experimentally observed sound speed at higher pressures, approaching 200 GPa. In particular, we unveil a discrepancy between the experimental results and the equation of state model that is most commonly used in inertial confinement fusion at the National Ignition Facility.

Thermodynamic properties of MgSiO3 at super-Earth mantle conditions

Recent discoveries of terrestrial exoplanets distant from our solar system motivate laboratory experiments that provide insight into their formation and thermal evolution. Using laser-driven shock wave experiments, we constrain high-temperature and high-pressure adiabats and the equation of state of ${\mathrm{MgSiO}}_{3}$, a dominant mantle constituent of terrestrial exoplanets. Critical to the development of a habitable exoplanet is the early thermal history, specifically the formation and freezing of the magma ocean and its role in enabling convection in the mantle and core. We measure the adiabatic sound speed and constrain the melt transition along the Hugoniot and find that the adiabats and melt boundary of silicate magmas are shallower than predicted. This suggests that small changes in the temperature of a super-Earth mantle would result in rapid melting and solidification of nearly the entire mantle.

Plastic behavior of steel and iron in high strain rate regime

The shock response of anti-hydrogen steel (HR-2) and iron was studied in a series of laser-driven shock wave experiments. A line-imaging optical recording velocity interferometer system for any reflector was used to record the free surface velocity histories of shock loaded samples, 100–300 \(\upmu \hbox {m}\) thick and with an initial temperature ranging from 296 to 1073 K. Based on the recorded free surface velocity profiles, the elastic precursors, dynamic yield and tensile (spall) strengths of HR-2 and iron were calculated. The dependence of the measured HEL stresses on the propagation distance for HR-2 and polycrystalline iron is approximated by a power law relationship.But, that for the single crystal iron with orientation of (110) seems to be constant. Spall strengths \((\upsigma _{\mathrm{sp}})\) of HR-2 estimated from the magnitude of the pull-back signal show that the spall strength dependence on the strain rate \((\dot{\upvarepsilon })\) is approximated by a power law relationship \(\upsigma _{\mathrm{sp}} =0.24\left( \dot{\upvarepsilon } \right) ^{0.24}\,\left( {\hbox {GPa}} \right) \). The spall strength of HR-2 and single crystal iron at the initial temperatures of 296–1073 K decreases slightly with increasing temperature and that of poly crystal iron abnormally increases at a temperature of 873 K. The X-ray diffraction results on the recovered poly crystal samples indicate significant changes in the relative peak intensity and the change in the crystal orientation may be the reason for the abnormal increasing at 873 K. The spall fracture surfaces of HR-2 were observed using a 3D laser scanning confocal microscope. The spall surface contains many dimples, suggesting that the fracture mode is that of ductile fracture. At ambient temperatures, the dimples and crowns were evenly distributed at the fracture surface. At high temperatures, many large crowns appeared and were unevenly distributed at the fracture surface.

A joint diagnostic system for laser-driven shock wave experiments

A joint diagnostic system was established for the diagnosis of laser-driven shock wave experiments. The system has high temporal resolution (time resolution ~12 ps) and high spatial resolution (spatial resolution ~7 μ m) and fits for diagnostics of the experiment with small sample size and short time physical process. The joint diagnostic system was applied for shock wave measurement on the Shenguang-II laser facility. The passive shock breakout signal and active diagnostic signal were simultaneously obtained. The temporal measurement reliability of the system was verified using a multi-layered target. The experimental results show that the two measurement results were consistent.

Lithographically fabricated gratings for the interferometric measurement of material shear moduli under extreme conditions

Electron beam lithography and photolithography were used to fabricate diffraction gratings on targets for laser-driven shock-wave experiments. This target design was used with an optical interferometric system to measure transverse wave motion of the target during dynamic (shock-wave) compression. A wedged-shaped diamond substrate and reflective grating on the sample's surface allowed detection of transverse motion. Proof of principle tests on single-crystal 〈100〉 Si samples gave a transverse wave speed of 5.9 km/s at 5 GPa and a shear modulus of 81 GPa. This experimental design has tremendous potential, including the possibility of measuring the shear properties of pure iron at Earth core conditions.

A modified illumination system for a line-imaging optically recording velocity interferometer and its application in equation of state measurement

A line-imaging optically recording velocity interferometer system (L-ORVIS) fitting the high-strain rate motion of solids as in shock wave experiments requires a high power single-mode laser. We have put forward a new illumination method which can increase threefold the luminosity of such a diagnostic. A modified illumination system is applied to the L-ORVIS which was implemented at ‘Shenguang-II' laser facility. The modified L-ORVIS is applied to laser-driven shock wave experiments and platinum Hugoniots were obtained at both high pressure and low pressure.

High-precision measurements of the diamond Hugoniot in and above the melt region

High-precision measurements of the diamond principal Hugoniot have been made at pressures between 6 and 19 Mbar. Shock velocities were determined with 0.3%--1.1% precision using a velocity interferometer. Impedance-matching analysis, incorporating systematic uncertainties in the equation of state of the quartz standard, was used to determine the Hugoniot with 1.2%--2.7% precision in density. The results are in good agreement with published ab initio calculations, which predict a small negative melt slope along the Hugoniot, but disagree with previous laser-driven shock wave experiments, which had observed a large density increase in the melt region. In the extensive solid-liquid coexistence regime between 6 and 10 Mbar, the present measurements indicate that the mixed phase is a few percent more dense than what would be expected from a simple interpolation between liquid and solid Hugoniots.

Streaked optical pyrometer system for laser-driven shock-wave experiments on OMEGA

The temperature of laser-driven shock waves is of interest to inertial confinement fusion and high-energy-density physics. We report on a streaked optical pyrometer that measures the self-emission of laser-driven shocks simultaneously with a velocity interferometer system for any reflector (VISAR). Together these diagnostics are used to obtain the temporally and spatially resolved temperatures of approximately megabar shocks driven by the OMEGA laser. We provide a brief description of the diagnostic and how it is used with VISAR. Key spectral calibration results are discussed and important characteristics of the recording system are presented.

Laser induced shock pressure multiplication in multi layer thin foil targets

The impedance mismatch technique has been used for shock pressure amplification in two- and three-layer thin planar foil targets. Numerical simulation results using one-dimensional radiation hydrocode MULTI in two-layer targets consisting of Al–Au and Al–Cu and three-layer target consisting of plastic–Al–Au and foam–Al–Au, respectively, are presented. These results show a pressure enhancement up to 25 and 29 Mbar for plastic–Al–Au and foam–Al–Au targets, respectively, from an initial pressure of 7 Mbar in the reference material using laser intensity of 5 × 1013 W cm−2 at 1.064 µm. This enhancement is more as compared with 18 and 22 Mbar found in plastic–Au and foam–Au two-layer targets, respectively. Results of laser driven shock wave experiments for equation of state (EOS) studies of Au and Cu in two-layer target are also presented. A Nd : YAG laser chain (2 J, 1.06 µm wavelength, 200 ps pulse FWHM) is used for generating shocks in the planar Al foils and Al–Au (or Al–Cu) layered targets. The EOS of Au and Cu in the pressure range of 9–14 Mbar obtained shows remarkable agreement with the simulation results and with experimental data of other laboratories and SESAME data.

Mean-field potential calculations of shock-compressed porous carbon

The mean-field potential approach with first-principles total energy data as input is used to investigate shock-compressed carbon with initial densities of 3.51, 2.2, 1.85, and $1.6\phantom{\rule{0.3em}{0ex}}\mathrm{g}∕{\mathrm{cm}}^{3}$ up to $1400\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. We have calculated the shock Hugoniot, and the temperature and electronic contribution to the heat capacity along the Hugoniot. While excellent agreement with underground nuclear explosion data is obtained, our results do not show the high compressibility of carbon at megabar pressures indicated by recent laser-driven shock wave experiments.

Laser driven shock wave experiments for equation of state studies at megabarpressures

We present the results from laser driven shock wave experiments for equation ofstate (EOS) studies of gold metal. An Nd:YAG laser chain (2 J, 1.06 μmwavelength, 200 ps pulse FWHM) is used to generate shocks in planar Al foils andAl + Au layered targets. The EOS of gold in the pressure range of 9–13 Mbar isobtained using the impedance matching technique. The numerical simulationsperformed using the one-dimensional radiation hydrodynamic code support theexperimental results. The present experimental data show remarkableagreement with the existing standard EOS models and with other experimentaldata obtained independently using laser driven shock wave experiments.

Computer simulations for shock-compressed liquid deuterium: Failure of density-functional theory and molecular dynamics

We show that the disagreement of recent laser driven shock wave experiments for liquid deuterium with Hugoniots calculated with the generalized gradient approximation molecular dynamics (GGA-MD) method stems from the underestimate of the metalization density associated with density functional theory (DFT). This leads to an enhanced degree of thermally induced molecular dissociation with the result that computer simulations fail to achieve the compression observed experimentally. Scaling of the GGA-MD Hugoniot to hypothetical GW Hugoniot results, using the ratio of volumes at which the GW and DFT band gaps close, leads to a prediction in good agreement with experiment.

Integrated two-dimensional simulations of hohlraum targets

We report on integrated simulations of hohlraum targets based on the new code MULT12D for two-dimensional radiation hydrodynamics. Targets for heavy ion beam fusion with two-sided irradiation are discussed; l = 4 asymmetry is controlled by low-albedo rings in the hohlraum wall. Targets designed for laser-driven shock wave experiments are also presented. MULT12D in its present version is still restricted to the use of simplified (power-law-type) equations of state and opacities, so the results are of semi-quantitative nature. We focus on hohlraum dynamics and aspects of radiation field symmetrization.

Experimental measurement of the conditions for the planarity of laser-driven shock waves

Laser-driven shock wave experiments have been conducted in the regime of 1011 W/cm2 in which incipient spall in a 300-μm-thick aluminum foil was used as an indicator of a constant shock state. Laser input energy and deposition spot size were varied to obtain a condition of incipient spall. The results agree with previous models in that a constant laser irradiance produces a constant shock state. The effective diameter of the laser is indicated to be about 0.44 mm larger than the laser spot diameter due to lateral effects during the shock generation or propagation phases.

Laser-driven shock wave experiments at the University of British Columbia

A review of recent laser-driven shock wave experiments at the University of British Columbia is presented. These include emissivity and reflectivity measurements on target rear surfaces when the shock wave emerges as well as measurements of the trajectories of shock propagation in initially transparent targets irradiated by temporally tailored laser pulses. The rear surface measurements allowed us to study the equation of state and electron conductivity of dense plasmas while coalescence of shock waves was evident in the trajectory of shock waves driven by a shaped pulse.

Density measurements for thick-film laser targets

We measured the absolute densities of 5 μm-thick films of gold, silver, copper, and aluminum that had been vapor deposited on thicker substrates intended for use in laser-driven shock wave experiments. Proton backscattering measurements show that the film densities are equal to the densities of bulk materials to within the experimental accuracy of about ± 1%. The films were uniform to ± 1% over spatial dimensions > 10 μm as determined by the Los Alamos Nuclear Microprobe.