Shock Velocity Measurements Research Articles

Simultaneous measurement of laser-induced shock wave and released particle velocities at Mbar pressure

We show the feasibility of simultaneous measurement of shock velocity and released particle velocity after shock at Mbar pressure. The shock wave is driven by a laser pulse of 1.2 ps duration (full width at half maximum), with the intensity of ∼1014 W/cm2 at 785 nm, irradiating a 500-nm-thick aluminum foil. A chirped laser pulse split from the main pulse is applied to detect the shock breakout process at the rear surface of the target based on frequency domain interferometry. The mean shock velocity determination benefits from the precise synchronization (<100 fs resolution) of the shock pump and probe laser pulse, which is calculated from the time the shock takes to travel the 500-nm-thick aluminum. The released particle velocity determination takes advantage of the chirped pulse frequency domain interferometry. The two measured parameters are self-consistent.

Impedance match equation of state experiments using indirectly laser-driven multimegabar shocks

Measurements of equation of state (EOS) points on the principal Hugoniots of Cu, Au, Pb and the plastics Parylene-C and brominated CH at multimegabar pressures have been made using the 1 TW HELEN laser at AWE. The aim was 1% accuracy in shock velocity measurement (3%–4% in pressure) in order to compare with data from gas-gun and nuclear underground test experiments and the theoretical EOS’s based on, or supported by, these data. Experiments comprised a hohlraum heated by two 500 J, 0.53 μm wavelength, 1 ns Gaussian laser pulses generating an x-ray flux which drove a shock into a target consisting of a base, with steps of a known EOS material and of the material of unknown EOS. Shock breakout from base and steps was detected by monitoring light emission from the target with optical streak cameras and shock velocities were derived from the transit times across the known-height steps.

Refractive indices of sapphire under elastic, uniaxial strain compression along the a axis

Sapphire crystals were shocked to 190 kbar along the a axis to characterize their use as optical windows, for velocity interferometry measurements, up to their Hugoniot elastic limit. When partially polarized light is incident on the samples, birefringence in the material is manifested as a beat frequency in the probe light that is returned from the specimens. Proper procedures for interpreting the velocity interferometry data for various polarization conditions were developed. The refractive indices at 514.5 nm wavelength decreased linearly with the density. The data were analyzed to yield three photoelastic coefficients: p12, p31, and p41. Calibration is developed for any polarization state of the probe light. Particle velocity wave forms are consistent with elastic behavior up to 170 kbar shock stress, and evidence of deviation from elastic behavior is present at 190 kbar impact stress. High precision shock velocity measurements are reported to 170 kbar stress along the a axis.

Shock induced mechanical response of a γ-TiAl alloy

Plate impact experiments have been performed on samples of the γ-titanium alloy, Ti-48Al-2Cr-2Nb-1B to determine its mechanical response during shock loading. This alloy displays pronounced precursor decay. Comparison with the findings of a previous investigation on a similar material shows that the variation in the Hugoniot Elastic Limit can be explained in terms of the microstructure. The Hugoniot has also been determined, and stress, shock, and particle velocity have been measured as independent variables. The difference of these data from those of a similar intermetallic alloy have been explained in terms of the microstructure. It has been observed that the measured Hugoniot (in stress-particle velocity space) lies above that calculated from the measured shock velocities. We believe that this is due to the high degree of work hardening that this type of intermetallic alloy displays.

Hohlraum temperature inference via measurement of aluminum shock velocity and time- and spatially resolved x-ray re-emission

The radiation field in a hohlraum (4 mm diameter×4 mm length) with a ∼10 TW x-ray input has been simultaneously measured by two independent techniques. An active shock breakout measurement of radiation-driven shock velocity in aluminum indicates a peak radiation field with incident flux equivalent to a Planckian distribution with a temperature of 147±3 eV. In the same experiment, a time- and spatially resolved measurement of x-ray re-emission from the gold hohlraum wall indicates re-emission flux equivalent to a Planckian brightness temperature of 145±5 eV.

Refractive index and elastic properties of z-cut quartz shocked to 60 kbar

Shock wave experiments were performed to characterize z-cut, α-quartz as an optical window for velocity interferometry measurements. Refractive index changes and shock velocities were determined to 60 kbar peak stress. Results indicate that the window correction to velocity measurements is a constant fraction of the actual particle velocity, Δu/u=0.081 07. Shock velocity measurements provide the Hugoniot curve to 60 kbar, and yield the fourth-order elastic constant, C3333=174 810 kbar. By combining the velocity correction factor and the shock velocity measurements, the refractive index is determined to be a linear function of density. This finding is used to show that the velocity correction appropriate for a step jump may be applied to monotonically time-dependent wave forms with negligible error. The present work has demonstrated the use of z-cut quartz as an optical window to 60 kbar.

Shock-induced transformation of liquid deuterium into a metallic fluid

Simultaneous measurements of shock velocity and optical reflectance at 1064, 808, and 404 nm of a high pressure shock front propagating through liquid deuterium show a continuous increase in reflectance from below 10% and saturating at approximately (40-60)% in the range of shock velocities from 12 to 20 &mgr;m/ns (pressure range 17-50 GPa). The high optical reflectance is evidence that the shocked deuterium reaches a conducting state characteristic of a metallic fluid. Above 20 &mgr;m/ns shock velocity (50 GPa pressure) reflectance is constant indicating that the transformation is substantially complete.

Molecular dynamics simulations of shocked benzene

The behavior of benzene at high temperatures and pressures is studied using nonequilibrium molecular dynamics. The interatomic forces were generated using linear-scaling tight-binding electronic structure theory on systems containing 128 and 576 molecules. The shock Hugoniot, calculated directly from the simulations without predetermining the equation of state, is compared with experiment. Piston velocities of 4 km/s or greater result in a pressure-induced polymerization. This transition is consistent with the bend in the experimental measurements of shock versus piston velocity.

Use of Z-pinch sources for high-pressure equation-of-state studies

In this paper, we describe a new technique for using a pulsed power source (Z pinch) to produce planar shock waves for high-pressure equation of state (EOS) studies. Initial EOS experiments indicate that these sources are effective for shock wave studies in samples with diameters of a few millimeters and thicknesses of a fraction of one millimeter, and thus provide the possibility for achieving accuracy in shock and particle velocity measurements of a few percent. We have used the Z pinch source to produce the first in-situ time-resolve particle velocity profiles obtained with pulsed radiation sources in the Mbar regime. Particle velocity profiles obtained with a VISAR interferometer are compared with 1-D numerical simulations performed with a radiation-hydrodynamics code, ALEGRA. Good agreement with experimental results was achieved in the simulations and suggests that the Z pinch source should be a valuable tool for high-pressure EOS studies in thermodynamic regimes important to hypervelocity impact.

Fibre Optic Shock Velocity Sensor for Solids

This paper reports a fibre optic sensing technique for the measurement of shock velocity in solid materials. The shock-induced changes in the light transmission properties of an optical fibre are employed as the principal transduction mechanism. A polycarbonate flyer plate generated shock waves by impacting a perspex target. The shock velocity was determined from the difference in arrival times of the shock front at the spatially separated optical fibres embedded in the target. The main advantage of this sensor system lies in its simplicity and immunity to optical and radio frequency (RF) noise. Consideration is also given to the effect of release waves on the uniform shock pressure region generated by the ˚yer impact which can degrade the accuracy of the velocity measurement.

Color temperature measurement in laser-driven shock waves

A simultaneous measurement of color temperature and shock velocity in laser-driven shocks is presented. The color temperature was measured from the target rear side emissivity, and the shock velocity by using stepped targets. A very good planarity of the shock was ensured by the phase zone plate smoothing technique. A simple model of the shock luminosity has been developed in order to estimate the shock temperature from the experimental rear side emissivity. Results have been compared to temperatures obtained from the shock velocity for a material of a known equation of state.

Shock compression of 6H polytype SiC to 160 GPa

The Hugoniot compression curve for 6H polytype SiC has been determined to pressures of 160 GPa by a two-stage light-gas gun and an optical method which provides direct measurements of shock velocity and particle velocity of the shock compressed SiC. A phase transition has been detected at 105\ifmmode\pm\else\textpm\fi{}4 GPa, with a volume reduction of 15\ifmmode\pm\else\textpm\fi{}3 %. This volume change suggests that the high-pressure phase has sixfold coordination and most probably rocksalt structure. The transition of 6H SiC and the high-pressure behaviors of diamond structure Si and the other polytypes of SiC imply that the post-diamond phase of C may possess a sixfold-coordinated simple-cubic structure as predicted previously by a certain computer calculation.

Time resolved imaging of the surface ablation of soft tissue with IR picosecond laser pulses

Time resolved video photographs have been taken in order to investigate plasma induced surface ablation of soft tissue by infrared picosecond laser pulses with energies of about 1 mJ. The emission and propagation of shock waves in the irradiated tissue as well as in the surrounding air environment was studied. The pressure amplitudes of the shock transients were determined from the measured shock velocities. A decay of the pressure amplitude below 100 MPa was observed within a distance of about 200 \(\mu\)m from the center of laser induced optical breakdown. The dynamics of the ablation crater and the ejection of the ablation fragments was studied on a larger timescale. The maximum expansion of the ablation crater was measured to be about 200 \(\mu\)m at temporal delays of 4–5 \(\mu \)s referring to the laser pulse. Furthermore, generation and propagation of a surface deformation wave was observed. Thus, we present a detailed and consistent description of all phenomena occuring during plasma induced surface ablation of soft tissue.

Hugoniot EOS measurements at Mbar pressures

The AWE HELEN laser is being used to measure high-pressure Hugoniot data by the impedance match method. Indirect drive is used to generate pressures of up to 10 Mbar in the aluminum reference material. We are aiming to measure shock velocities in our targets to ± 1 %, leading to errors in pressure and particle velocity of the order of ±2%. This requires improvements in target fabrication and characterization, streak camera calibration and data analysis, coupled with investigations into shock planarity and attenuation. Our latest results have accuracies of 2–4% in shock velocity but have identified work that should allow us to achieve our intended level of accuracy. Experiments have been done with copper and chlorinated plastic; our results are compared with theory, and, for copper, with U.S. and Russian gas-gun, explosively or nuclear-driven experiments.

Optical smoothing techniques for shock wave generation in laser-produced plasmas.

Shock wave experiments have been performed at the Laboratoire pour l'Utilisation des Lasers Intenses in a regime of interest for inertial confinement fusion and equation of state measurements. Laser beams have been optically smoothed either with random phase plates or with phased zone plates. This last technique allowed the production of a flat-top intensity distribution in the focal spot and hence of flat shock fronts. High quality shocks have been produced and used with stepped targets to allow direct measurements of shock velocity by time-resolved imaging of shock breakthrough at the rear of irradiated targets.

Demonstration of 0.75 Gbar planar shocks in x-ray driven colliding foils.

We have directly observed 0.75\ifmmode\pm\else\textpm\fi{}0.2 Gbar planar shocks in gold target foils impacted by ablatively driven gold flyer foils. The x-ray drive for the flyer foils was produced in a gold Hohlraum by ten beams of the Nova laser. The dynamic pressure inferred by shock velocity measurements is more than a factor of 7 greater than previous dynamic shocks produced in the laboratory and over a surface area significantly larger than most previous ultrahigh pressure experiments. The technique allows the laboratory investigation of equations of state in the gigabar regime.

Non-invasive determination of shock wave pressure generated by optical breakdown

Shock waves generated by a laser-induced plasma were investigated using a pump-and-probe technique. Both 7-ns and 40-ps laser pulses at 1.06 μm were employed to initiate breakdown in water. Two He-Ne laser beams were used as a velocity probe, allowing the accurate measurement of the shock velocity around the plasma. The maximum shock pressure was determined from the measured shock velocities, the jump condition and the equation of state for water. The conservation of the total momentum of the shock front was used to derive expressions for the shock velocity, particle velocity and shock pressure vs. the distance (r) from the center of the plasma. For a shock wave of spherical symmetry, the shock pressure is proportional to 1/r2. Our work shows that the expanding plasma initially induces a shock wave; the shock wave dissipates rapidly becoming an acoustic wave within 300–500 μm.

Time-resolved measurements of laser-induced shock waves in deuterated polystyrene porous targets by x-ray backlighting

Experimental studies are presented of laser-driven shock waves in deuterated polystyrene porous targets observed with x-ray streak shadowgraphy. Using two different target-mass densities of 0.1 g/cm3 and 0.2 g/cm3, the density dependence of the shock velocity was obtained as vs∝ρ−0.45±0.02 and agreed well with a simple scaling obtained from the Hugoniot relation. X-ray shadowgraphy has shown a shock formation in porous targets and analysis suggests that there are two parts to compression waves: a shock front and a following pileup zone of multiple shock waves. Temporal histories of the pressure, the temperature, and the compression were estimated from the measured shock velocities and the opacities. It was found that the pressure peak due to the shock front was retarded to that by the pileup zone. The pileup zone had a higher compression and followed a thermodynamic condition closer to an adiabat than the shock front.

Shock-induced solid-state chemical reactivity studies using time-resolved radiation pyrometry

Time-resolved radiation pyrometry has been used to study materials which undergo solid-state chemical reactions due to shock loading. Shock-induced chemical reactivity in solids is fundamentally different than that in high explosives and other energetic materials because, if no volatiles are present, the reaction products end up in the condensed, rather than the vapor, state. Bulk property changes accompanying the solid-state reactions may therefore be too small to be observable with wave profile or shock-velocity measurements. However, some solid-state reactions, such as that between metallic nickel and aluminum, are exothermic enough to give rise to a measurable increase in temperature, so pyrometry can be used to detect the reactions. Unfortunately, these measurements are complicated by the large temperature increases generated by other sources. Possible mechanisms for generation of these high temperatures, and their effect on the chemical reaction, are suggested

Shock Velocity Measurement in Water by Sulfur Probes

AbstractSulfur‐coated pin oscillographic technique (SPOT) has been developed as a new technique for measurement of shock velocity in water. Unlike other techniques, generally employing piezoelectric transducer or high speed photography, the application of this technique is not restricted to the measurement of shock wave pressures over a limited range. To illustrate the application and accuracy of the technique the detonation pressures of a few typical explosives have been measured with this technique and compared with those determined elsewhere by using other techniques. The new technique has yielded measurement of shock velocity in water within an accuracy of 2%.