Velocity Interferometer System Research Articles

Shock compression of crystalline TeO2 to the high-pressure fluid regime: Insights from ab initio molecular dynamics simulations

The shock response of fully-dense and porous crystalline tellurium dioxide (TeO2) to the high-pressure and high-temperature fluid regime was investigated within the framework of density functional theory with Mermin’s generalization to finite temperatures. The principal and porous shock Hugoniot curves were predicted from canonical ab initio molecular dynamics (AIMD) simulations, with the phase space sampled along isotherms up to 80 000 K, for densities ranging from ρ=3 to 17 g/cm3. The polymorphs investigated are α-TeO2 paratellurite (P41212), TeO2 cotunnite (Pnma), and TeO2 post-cotunnite (P21/m). Based on the discontinuity found in the calculated Us−up slope of TeO2 post-cotunnite at a shock velocity of Us≃8.35 km/s and a particle velocity of up≃3.64 km/s, the shock melting temperature and pressure are predicted to be ≃6500 K and ≃170 GPa. Results from the AIMD simulations are in line with the static compression data of TeO2 paratellurite and cotunnite, and with the recent shock Hugoniot data for single-crystal α-TeO2 for pressures up to 85 GPa, obtained using the inclined-mirror method and the velocity interferometer system for any reflector combined with powder gun and two-stage light-gas gun.

Dynamic failures at the metal-glass interface under impact loading

To investigate dynamic fracture behavior in the metal, three metal spheres (e.g., steel sphere, high purity tungsten sphere, and high purity lead sphere) are accelerated by the gas gun devices to impact glass spheres under the critical speed range (i.e., from 70 m/s to 210 m/s). The velocity interferometer system for any reflector (VISAR) devices are employed to measure the particle velocities at the back surface of glass sphere, and high-speed photographs are utilized to capture the failure process at the metal-glass interface. Due to the asynchronous evolutions of stress fields and strain fields in the violent failure process, the results illustrate quite different failure mechanisms from those by the Split Hopkinson Pressure Bar (SHPB) impacting. Fragmentations of the glass sphere are caused mainly by the radial cracks and the lateral cracks around the metal-glass interface and the edges of the sphere with increasing impact velocity. Dynamic failures in the three metal impactors exhibit different modes, e.g., tensile fracture in the steel impactor, shear fracture in the tungsten impactor, and compressed yielding in the lead impactor. The transferring of strain energy releasing is introduced to describe the failure behavior at the metal-glass interface, and a relaxation-diffusion equation of strain energy releasing is then established based on the experimental results and the numeric results by the discrete element method (DEM). The evolutions of failures at the metal-glass interface are discussed. Further investigation is conducted to describe the dynamic fractures in tungsten impactors and steel impactors based on the dimensional analyses, and the quantitative expressions of these strain rate dependent fracture strains and crack width in the metal impactors are obtained. The results are helpful for the profound understanding of the dynamic fracture in the metal structures and the dynamic fragmentations in the brittle material when subjected to impact loading.

Enhanced fractional-order total variation regularization-based velocity field reconstruction for CUP-VISAR diagnostic system.

The fusion of a velocity interferometer system for any reflector with compressed ultrafast photography systems in recent literature can achieve two-dimensional spatiotemporal diagnosis of shock wave velocities. Addressing the limitations posed by 7 × 7 coded aperture sampling, this study introduces an enhanced three-dimensional reconstruction algorithm grounded in fractional-order total variation regularization (E-3DFOTV). Simulated reconstructions and analysis were conducted on 80 frames of 350 × 800 fringes. The results show that compared with TWIST, ADMM, and E-3DTV, the average PSNR of the E-3DFOTV algorithm is increased by 16.81 dB, 14.46 dB, and 2.98 dB, respectively, and the average SSIM of the E-3DFOTV algorithm is increased by 53.20%, 27%, and 3.19%, respectively. Moreover, the reconstruction time consumption of E-3DFOTV is reduced by 33.48% compared with the E-3DTV algorithm and 2.94% compared with the ADMM algorithm. The two-dimensional distribution of shock wave velocity fields reconstructed using E-3DFOTV exhibits minimal errors, with percentages within 1.67%, 1.00%, and 2.14% at different slices, respectively. Moreover, the experiment was conducted on the ShenGuang-III prototype laser facility and VISAR data has been reconstructed in 1.25 ns range. Reconstruction results from experimental data demonstrate that the percentage errors at maximum velocity location for ADMM, E-3DTV, and E-3DFOTV are 12.08%, 19.27%, and 3.59%, and the maximum percentage error for E-3DFOTV is 6.65%, underscoring the feasibility of the algorithm.

Simultaneous measurement of surface velocity and plasma density with interferometric velocimetry.

The apparent velocity measured by an interferometric surface velocimeter is a function of both the surface velocity and the time derivative of the refractive index along the measurement path. We employed this dual sensitivity to simultaneously measure km/s surface velocities and 1018cm-3 average plasma densities with combined VISAR (velocity interferometer system for any reflector) and PDV (photonic Doppler velocimetry) measurements in experiments performed on the Z Pulsed Power Facility. We detail the governing equations, associated assumptions, and analysis specifics and show that the surface velocity can be extracted without knowledge of the specific plasma density profile.

Optimization and application of shock wave measurement technology for shock-timing experiments on small-scale capsules

In laser fusion research, the precision of shock-timing technology is pivotal for attaining optimal adiabatic tuning during the compression phase of fusion capsules, which is crucial for ensuring the high-performance implosion. The current main technological approach for shock-timing experiments is to use keyhole targets and VISAR (velocity interferometer system for any reflector) diagnostics to measure the shock velocity history. Nonetheless, this approach encounters limitations when scaling down to smaller capsules, primarily due to the reduced effective reflection area available for VISAR diagnostics. In this work, a novel high-precision shock-timing experimental methodology is used to realize a double-step radiation-driven implosion of a 0.375 mm radius capsule on a 100 kJ laser facility. By calculating the intensity of VISAR images with spherical reflective surfaces, a new experimental technical route is proposed, i.e. using the keyhole cone reflection effect to enhance the VISAR diagnostic spatial area, which can effectively increase the effective data collection region by nearly threefold for small-scale capsules. The technique has been adeptly used to measure shock waves in cryogenic liquid-deuterium-filled capsules under shaped implosion experimental conditions, thus obtaining high-precision shock-timing experimental data. The experimental data reveal that the application of this technology can markedly enhance both the image quality and the precision of data analysis for shock wave velocity measurements in small-scale capsules. Furthermore, it is discovered that under similar laser conditions, there exist considerable variations in the shock velocity profiles. Simulation analysis shows that the difference in chasing behavior of the “<i>N</i>+1” reflected shock wave caused by small changes in laser intensity is the main reason for the significant difference in merging speed. It is demonstrated that small changes in laser parameters can significantly affect the transmission behavior of the shock wave. This experiment highlights the complex sensitivity of shock wave transmission in high-performance forming implosion physics process on a current small capsule scale, making it essential to conduct shock-timing experiments to accurately adjust actual shock wave behavior. This research not only lays a robust technical foundation for promoting adiabatic tuning experiments ofour 100 kJ laser facility but also has profound significance for the ultra-high pressure physics research based on the spherical convergence effect.

Measurements of improved stability to achieve higher fuel compression in ICF

While nuclear fusion ignition has been achieved at the National Ignition Facility in inertial confinement fusion (ICF) experiments, obtaining higher gain and more efficient burn is still desired. In that regard, increasing the compression of the fuel is an important factor. In recent indirect-drive capsule implosions, the SQ-n campaign is testing the hypothesis that reducing the hydrodynamic growth of perturbations is key to achieving higher compression of high-density carbon based-ablators for ICF. SQ-n uses a design at lower adiabat with a ramped foot laser pulse shape to minimize early-time hydrodynamic instability growth, predicted to be reduced by a factor of 10, and an optimized ablator dopant distribution. Subsets of experiments were conducted within the SQ-n campaign to study the implosion symmetry, laser backscatter, stability, and compression. Only the latter two will be reviewed here. Shock timing experiments using the velocity interferometer system for any reflector diagnostic enabled the development of a gently accelerating shock velocity. The ice–ablator interface acceleration, important for managing the Richtmyer–Meshkov phase growth, was observed with refraction enhanced radiography and the ablation front growth was measured using radiography of pre-imposed modulations. Finally, layered tritium-hydrogen-deuterium (∼75% H, ∼25% T, ∼2–10% D) and deuterium–tritium implosions demonstrate that between 15% ± 3% and 30% ± 6% improved compression has been achieved.

L1 norm based sparse reconstruction approach to compressed ultrafast photography of velocity interferometer system for any reflector.

The one-dimensional velocity interferometer system for any reflector (VISAR) is widely used in inertial confinement fusion (ICF). Compressed ultrafast photography (CUP) is recently combined with VISAR to enable two-dimensional imaging, in which, the time-varied two-dimensional fringe images reconstruction from one image is a challenging problem. In this Letter, a new l1 norm based sparse reconstruction approach is presented, to the best of our knowledge, in which: (1) fringe images are sparsely represented with a few coefficients over the discrete cosine transformation (DCT) basis; (2) the image reconstruction is formulated as an l1 norm based sparse coefficients optimization problem; and (3) the two-step iterative shrinkage/threshold algorithm combined with a soft-thresholding operator is proposed to efficiently solve such a problem. Finally, the results show that, compared with the usual method, reconstructed fringe images with clear boundaries and good continuity are obtained. Additionally, the maximum relative error of the velocity is reduced from 14% to 8%, which is a reduction of approximately half.

Experimental and numerical investigation of the laser shock-based paint stripping process on CFRP substrates

Various laser shock wave applications have been proposed for metals; the most popular is the laser shock peening. In composites, the use of the laser shock wave technology is still at an early stage. Laser shock paint stripping (LSPS) is a method where a laser-induced shock wave is used to remove a paint from a substrate. In this study, a combined experimental–numerical study was performed on the LSPS on Carbon Fiber Reinforced Polymers (CFRP) substrates. Two types of specimens were investigated: specimens with an epoxy structural primer covered by a polyurethane base and a clear coat and specimens with the above layers plus an epoxy exterior primer between the epoxy structural primer and the polyurethane layer. The specimens were shot using various laser intensities aiming to obtain the stripping threshold and the stripping percentage as functions of the laser intensity. The diameter of the stripped area was measured by a profilometer. An electron microscope was used to evaluate the stripping efficiency. In parallel, a numerical model was developed using the LS-DYNA explicit Finite Element (FE) software. For the modeling of the CFRP, a progressive damage model with strain rate parameters was used while for the epoxy an elastic–plastic-hydrodynamic material model combined with the Gruneisen equation of state. The interface between these two materials was modeled using cohesive zone elements of zero thickness, which follow a bi-linear traction-separation law. Calibration of the modeling of the shock wave propagation inside the composite material was achieved through the comparison of the back-face velocity profiles, measured by a velocity interferometer system for any reflector system. The results from the multiple tests conducted revealed the repeatability of the process and the significant effect of the laser intensity. On the other hand, the model managed to predict well the back face velocity–time profile but in most cases, it failed to predict the stripping pattern. This is mainly due to lack of crucial material properties and the assumptions made in the modeling of the applied pressure profile.

Absolute calibration of a streaked optical pyrometer at nanosecond time scale with a luminescent concentrator

The Streaked Optical Pyrometer (SOP) is a visible diagnostic widely used to study the warm dense matter regime at high energy laser facilities, gas guns, or ion accelerators. It is usually coupled with a Velocity Interferometer System for Any Reflector (VISAR) diagnostic for simultaneous shock wave velocity, reflectivity, and temperature measurements to study the Equation of State (EOS) of materials. While VISAR is a well-mastered technology that provides velocity measurements with low relative uncertainties (close to percent), SOP diagnostics still suffer from high imprecision. In this article, we present a new calibration method in order to obtain absolute temperature measurements with reduced uncertainties. This approach is based on a novel light source: a Ce:YAG luminescent concentrator pumped by LEDs. This device produces enough optical power for calibration at the nanosecond sweep duration of the streak camera. As a demonstration, it has first been installed at the LULI facility and tested on quartz samples shocked at temperatures above 4000 K.

Dynamical modeling of bi-layer Aluminium adhesive tape for laser shock applications

The presented work covers the response of Aluminium tape (Al tape) under high strain rate of deformation (order of 106s−1) using laser shock. High power laser (J) with a short pulse duration (ns) is used to create laser shock within the water confinement regime on two Al tape configurations in order to apply low and high pressure (order of MPa and GPa). Al tape has been modeled using Johnson-Cook (J-C) material model for the Al layer, and Steinberg–Cochran–Guinan (SCG) material model (elastic with pressure dependence) for the adhesive layer, both material models are coupled with Grüneisen equation of state. The Al tape model has been validated by comparing the simulated Back Face Velocity (BFV) of the target with the measured one by the Velocity Interferometer System for Any Reflector (VISAR). In addition, the validated material model is used to conduct the sensitivity studies about the transmitted pressure depending on the acoustical impedance of the target and adhesive thickness. Moreover, location and magnitude of maximum tensile stress within the target are calculated in function of the adhesive thickness of the Al tape. Finally, it has been proved that using one laser beam configuration, maximum tensile zone could appears close to the front face by increasing the adhesive thickness.

High-pressure shock compression of olivine: Dynamic pulverization and frictional melting

In order to study seismo-mechanochemical processes, two kinds of shock compression experiments – the recovery and the in-situ VISAR (velocity interferometer system for any reflector) experiments – were performed, using a single crystal of forsteritic olivine and a single-stage powder propellant gun. The generated peak shock pressures were 31.1 GPa for both the recovery and the VISAR experiments. Many shear planes were generated in the olivine crystal. The VISAR experiment revealed detailed time profiles for the particle and shock wave velocities during the compression and yielded the Hugoniot elastic limit (HEL) of the olivine to be 7.44 GPa. We observed, in one recovered sample, a clear local melting occurring along a shear plane. The microstructural observations of the shear plane and its walls using a field-emission scanning electron microscopy (SEM) and an analytical transmission electron microscopy (ATEM) revealed that plastically-deformed olivine was pulverized into a few hundred-nanometer size particle assemblages within a few-micrometers narrow zone along the shear plane and, locally have partially melted. Moreover, injection veins filled with olivine melt were found in the wall of the same shear plane. It was shown from the estimate of the critical resolved shear stress (CRSS) along the shear planes and the inferred slip velocity using the Hugoniot elastic limit (HEL) data that the power generated by the friction was large enough for melting to occur in the olivine. The whole process occurred in a time scale <0.6 microsecond. Pulverization (comminution) before melting is considered to be a common phenomenon in the formation of pseudotachylytes and in dynamically expanding seismic faults upon earthquakes.

Extended x-ray absorption fine structure measurement of ramp compressed Ti using laser-irradiated metallic foil as x-ray source on SGIII prototype laser facility

Laser-irradiated metallic foils were considered as x-ray sources for extended x-ray absorption fine structure (EXAFS) measurements and confirmed by experiments on the SGIII prototype facility. The Au foils were irradiated by laser beams with a total energy of 2.77 kJ and full width at half maximum (FWHM) of 1 ns to create an x-ray source. The x-ray emission was spectrally smooth in the energy range of Ti EXAFS, the FWHM of Au foil x-ray radiation pulse in the energy range of 0.1–4000 eV was 0.99 ns, and the FWHM of x-ray pulse in the energy range of 5000–6000 eV was deduced to be 0.55 ns according to simulation results. A shaped laser pulse was designed to achieve the Ti sample’s laser-direct-driven ramp compression process. By creating a quasi-stable state lasting longer than 1 ns as the probing window during the compression process, the demand for temporal resolution was reduced. EXAFS spectra of compressed Ti in α and ω-phase were obtained and compared, and structural phase transition was verified by EXAFS pattern changes. The velocity of the back interface of the Ti sample was measured by the velocity interferometer system for any reflector, and the maximum of the deduced pressure in the middle of the Ti sample was 8.2 GPa, which is consistent with the α-ω phase transition.

Experimental evidence of shock wave measurements with low-velocity (

We present a series of shock-wave measurements on aluminum based on the use of a simultaneous Photon Doppler Velocimetry (PDV) and triature velocity interferometer system for any reflector. Our dual setup can accurately measure shock velocities, especially in the low-speed range (<100ms-1) and fast dynamics (<10ns) where measurements are critical in terms of resolution and unfolding techniques. Especially, the direct comparison of both techniques at the same measurement point helps the physicist in determining coherent settings for the short time Fourier transform analysis of the PDV, providing increased reliability of the velocity measurement with a global resolution of few m s-1 in velocity and few ns FWHM in time. The advantages of such coupled velocimetry measurements are discussed, as well as new opportunities in dynamic materials science and applications.

Imaging velocity interferometer system for any reflector (VISAR) diagnostics for high energy density sciences.

Two variants of optical imaging velocimetry, specifically the one-dimensional streaked line-imaging and the two-dimensional time-resolved area-imaging versions of the Velocity Interferometer System for Any Reflector (VISAR), have become important diagnostics in high energy density sciences, including inertial confinement fusion and dynamic compression of condensed matter. Here, we give a brief review of the historical development of these techniques, then describe the current implementations at major high energy density (HED) facilities worldwide, including the OMEGA Laser Facility and the National Ignition Facility. We illustrate the versatility and power of these techniques by reviewing diverse applications of imaging VISARs for gas-gun and laser-driven dynamic compression experiments for materials science, shock physics, condensed matter physics, chemical physics, plasma physics, planetary science and astronomy, as well as a broad range of HED experiments and laser-driven inertial confinement fusion research.

Extensive characterization of Marshak waves observed at the LIL laser facility

We detail results of an experiment performed at the Ligne d'Intégration Laser facility aimed at studying supersonic and diffusive radiation front propagation in low-density SiO2 aerogel (20 and 40 mg/cm3) enclosed in a gold tube, driven by thermal emission from a laser-heated spherical gold cavity. The evolution of the front is studied continuously by measuring its self-emission with a 1D (one-dimensional) time-resolved soft x-ray imager. Measurement is performed along (through a 200-μm-wide observation slit) and at the exit of the tube giving access to the dynamics and the curvature of the front. Experimental results are then compared successfully to results from the 3D (three-dimensional) radiation hydrodynamics code TROLL, which shows that if continuous tracking of the front position is accessible with this experimental scheme, measurement of its maximum radiation temperature is on the contrary affected by radiation closure of the observation slit. 3D simulations also indicate that this effect can even be worsened if one includes pointing errors of the x-ray imager. Radiation temperature along the tube was then inferred by combining results from the imager to a wall shock breakout time measurement using a velocity interferometer system for any reflector and results from a broadband x-ray spectrometer used to determine the temperature at the exit of the tube. A decrease in the radiation temperature along the tube is observed, the decrease being more important for the higher SiO2 aerogel density.

Application of dislocation-based constitutive model into research of dynamic mechanical behavior under shock loading

To understand the plastic deformation mechanism of an FCC metal (pure aluminum) under shock loading and describe its dynamic mechanical behavior accurately, a multiscale constitutive model based on the dislocation substructure is developed, which comprehensively considers the controlling mechanisms of dislocation motion and dislocation evolution. Then, the model is extended to the loading of strong shock waves by incorporating the homogeneous nucleated dislocation within the constitutive framework. The model parameters are successfully determined by the normal plate impact experiments with different thicknesses of specimens. Additionally, shock front perturbation decay experiments are performed using a line velocity interferometer system for any reflector, where the modulated surface of the specimen is subjected to a laser-driven loading. Then, the model is applied to reproduce the perturbation decay of shock fronts in experiments. During the post-process of simulated results, the method based on the pressure gradient is used to determine the amplitude and the location of distributed shock fronts. The extended model shows promise as an effective method to figure out the role of strength (shear response) on the evolution of perturbation amplitude.

Interaction of multiple shocks in planar targets with a ramp-pulse ablation

Interaction of multiple shocks plays a critical role in setting up an adiabatic compression of megabar pressure in nanosecond timescale in inertial confinement fusion. In this paper, we present observations of dynamic behavior and interaction of multiple shocks in polystyrene (CH) planar targets driven by a single-ramp pulse of 2.5 ns at the SG-II laser facility with a specially designed velocity interferometer system for any reflector (VISAR). A maximum pressure of 7.65 Mbar and a mass density of 3.46 g/cm3 are measured, respectively. Radiation-hydrodynamic simulations reveal the interaction process of the multiple shocks and are in good agreement with the measurements. A theoretical model is proposed to invert the space-time history of the shock generation with the VISAR data. Moreover, an optimized double-slope ramp pulse is proposed for further compression experiments. The improved multiple-shock coalescence is expected to effectively enhance both density and velocity for an initial compression of the CH target.

A new woven composite constitutive model validated by shock wave experiments

In this paper, we present results of plate impact simulations of shock compressed woven glass fiber-reinforced plastic (GRP) performed using the Arbitrary Lagrangian–Eulerian three-dimensional finite element code. A hyperelastic large-strain-based empirical Continuum Damage Mechanics (CDM) formulation is employed to describe damage initiation and growth in the shock-compressed GRP. The model parameters calibration scheme utilizes the Velocity Interferometer System for Any Reflector normal particle velocity measurements at the free surface of the GRP target plates. The impact velocity in the experiments ranged from 8.5 to 418 m/s. The finite element model considered planar 0°/90° bidirectional plies with an individual ply thickness of 0.68 mm, stacked to reach a total laminate thickness of 6.8 mm. The anisotropic elastic strains were estimated from the experimentally determined tetragonal symmetry stiffness matrix for the GRP. The strain-based damage model captures several salient features observed in the measured free surface particle wave profiles, including the shock rise time, onset of Elastic—Elastic Cracking, and the shape of the nonlinear portion of the experimental particle velocity profiles. The CDM model predicts the dominant damage mode to be matrix microcracking due to shear and the associated bulk expansion (bulking) under the global compressive loading in the plate impact configuration.

Study on the length of diagnostic time window of CUP-VISAR

One-dimensional line velocity interferometer system for any reflectors (VISARs) acts as a significant diagnostic device in the process of the inertial confinement fusion, which can be used to measure the free surface velocity and the history of shockwave velocity in transparent media over time. But the lack of spatial information makes it impossible to measure the flatness of wavefront. The compressed ultrafast photography (CUP) system developed in recent years can realize two-dimensional ultrafast photography. We have designed a new two-dimensional VISAR diagnosis system by combining the CUP with line-VISAR, called the CUP-VISAR, which will be of great significance in two-dimensional shockwave front measurement. In this paper, a data simulation method of CUP-VISAR is proposed to study the length of diagnostic time window of the device. A series of original VIASR images are firstly generated and then the compressed images are simulated after image encoding and compression. Finally, two-step iterative shrinkage thresholding algorithm is used to reconstruct the compressed images, which results in corresponding reconstructed images. In order to verify the restoration effect of the algorithm, we calculate the velocity error and image correlation coefficient value. Finally the relationship between the image reconstruction quality and the number of images is preliminarily obtained, which provides reference for the actual experiment.

Novel Confinement Possibility for Laser Shock: Use of Flexible Polymer Confinement at 1064 nm Wavelength

Through the years, laser shock peening became a treatment of choice in the aerospace industry to prolong the life of certain critical pieces. Water flow is commonly used as a confinement to improve the process capability but some applications cannot allow for water presence in the area of interest. In a previous article, an alternative to the water confinement was presented, a flexible polymer confinement was used and demonstrated the production of pressures equivalent to the water configuration treatment. However, laser parameters have been restricted to a wavelength in the visible range at 532 nm. In this paper, the study is extended to 1064 nm which is commonly used in LSP applications and with two different pulse durations. A 1064 nm near infra-red laser is used to do pressure characterization of shots with polymer confinement through Velocity Interferometer System for Any Reflector (VISAR) measurements coupled with Finite Element Modelling on Abaqus software. The results show that the pressures produced by the confinement is slightly lower with the 1064 nm wavelength, similar to what is observed with the classic water confined regime when switching from 532 nm to a near infra-red wavelength. Nevertheless, the high level of pressure produced by laser shock under the polymer confinement configuration allows for the treatment of common types of metal alloys used in the aerospace industry. Although the use of such a confinement has yet to be applicable to peening setups, it has already uses in some single shot configurations such as LasAT where it allows the avoidance of the water flow optimization.