Velocity Interferometer System For Any Reflector Research Articles

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.

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.

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.

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.

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.

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.

Periodic error while processing data from a velocity interferometer system for any reflector

When processing experimental data with systems such as velocity interferometer system for any reflector (VISAR), periodic measurement uncertainty arises. The appearance of its uncertainty is investigated and computer modeling of signal processing with random noise is carried out. Analytical dependences of the error value on the signal-to-noise ratio and signal phase are obtained. Methods of reducing this error are considered.

A hybrid interferometric system for velocity measurements in shock-wave experiments.

This paper presents a hybrid interferometric system designed to measure the surface velocity of tested specimens in shock-wave experiments. The system integrates the All-Fiber Velocity Interferometer System for Any Reflector (AFVISAR) and the Photonic Doppler Velocimeter (PDV) interferometric channels using a single probing system to measure the velocity of one surface point of specimens under study. This design allows the same optical signal containing the Doppler frequency shift to be processed by the AFVISAR and PDV independent interferometric devices. The interferometric system has been tested in dynamic experiments and provides the velocity measurement accuracy of at least 1.5 m/s with a nanosecond time resolution.

Experimental study on deformation and fracture of resistance-spot-welded (RSW) overlapping plates impacted at RSW joint by round-nosed bullet

The ballistic tests of Q&P980 resistance-spot-welded (RSW) overlapping plates impacted at RSW joint by round-nosed bullet were performed using a single-stage gas gun system with a laser velocity interferometer system for any reflector (VISAR). The deformation and fracture mechanism of the RSW specimens were investigated by using the VISAR measured results, the metallographic cross-section observation and the scanning electron microscope (SEM) analysis of the recovered RSW specimens. The dynamic response of the RSW specimens was related to the reflection and refraction of the impact stress waves on the material interfaces and the free surfaces in the RSW specimens. The bulging, dishing, shearing near the notch tip and partial opening of the RSW specimens were discussed. With the range of loading speed and the joint configuration (overlap structure, nugget size, etc.) studied in this work, the plugging fracture should be a prevailing mode. The crack propagation of the corona bond and pre-crack of the RSW specimens under impact loading was found to affect the dynamic fracture process of the RSW specimens under bullet impact.

Spall Characterization in Epoxy Via Laser Spallation

Background: The strength of materials under extreme dynamic loading conditions, such as in the case of shock wave loading, is assessed from their spallation characteristics. Under laboratory conditions, flyer plate impact, or sometimes laser-induced stress waves, is employed to instigate spall in a material. These methods are often combined with velocity interferometer system for any reflector (VISAR) technique for performing transient measurements. Although the VISAR can record the velocity of extremely fast-moving surfaces, it requires a complex optical setup and a specialized data reduction technique. Objective: In this study, a simpler approach is adopted by extending laser spallation method to determine the spall strength of epoxy, while performing in situ interferometric measurements, directly on top of thick epoxy films. Methods: The glass/epoxy test samples are prepared by transferring an aluminum coating on top of epoxy layers with different thicknesses. Laser-induced stress waves transmit across the substrate/film interface and induce subsurface failure in the epoxy at sufficiently high incident laser energy. The nature and magnitude of the waves are deciphered from the out-of-plane displacement histories of the top reflective sample surfaces, which are recorded by using a Michelson interferometer. Results: The interferometric data reveal the development of two (temporally) well-separated stress waves: an ablation-induced high-amplitude short-duration longitudinal pulse, which is referred to as the primary wave, and a secondary wave, which travels at a comparatively slower speed. The complex constructive interaction of the two waves develops a high-magnitude tensile stress region in the epoxy layer. The spall strength is quantified by superimposing the two stress wave histories associated with the critical energy fluence. Conclusions: The spall depths predicted from spatiotemporal wave travel analyses are in excellent agreement with the experimental observations. The newly adopted methodology estimates the spall strength of epoxy as 260 ± 20 MPa.

Combination of the velocity interferometer system for any reflector and the photon Doppler velocimetry methods to study transparent and absorbing condensed matter

The paper proposes a method for combining photon Doppler velocimetry and a system of speed interferometers for any reflective systems, which allows one to study the behavior of transparent media under the action of powerful shock waves without the use of additional reflective surfaces. The paper presents the results of an experiment on the propagation of a shock wave in water. In this experiment, water was an absorber; however, the proposed method would also work provided that the test material is transparent and provides reflection.

Detonation properties of tetranitromethane–methanol mixture

An experimental study of the reaction zone structure and stability of detonation waves was performed for tetranitromethane–methanol (TNM–M) mixture using a velocity interferometer system for any reflector (VISAR). At the near stoichiometric concentration of methanol, it was observed that the amplitude of Von Neumann spike decreases significantly, whereas the detonation parameters increase. The instability of detonation waves in TNM–M mixture was shown, both with respect to one-dimensional longitudinal perturbations and to the curvature of the front leading to the formation of the cellular structure. The dependence of detonation velocity of TNM–M on the diluent concentration and limits of detonation propagation were found.

Physical Experiments on the HEAVEN-I KrF Laser Facility

The HEAVEN-I KrF excimer laser facility is housed at the China Institute of Atomic Energy, Beijing, China. There are six laser beams at this facility, whose combined energy is up to 100 J with a pulsewidth of 28 ns at a wavelength of 248 nm. The uniformity distribution of laser intensity is less than 2%. The diameter of the focal spots was about 500 μm, producing a laser intensity of about 1012 W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . HEAVEN-I's capability to provide extremely uniform focal profiles and its deep ultraviolet (UV) wavelength facilitate the conducting of experiments with laser-produced shocks with negligible effects from laser imperfections and laser-plasma instability. Thus, the laser facility was used in the research of high-energy-density physics and other related field physical experiments, such as the shock dynamics and the equation of state at high pressure, simulation of space debris, and direct laser-driven quasi-isentropic compression. Sidelight shadow photography and imaging velocity interferometer system for any reflector (VISAR) were developed for the purpose of active diagnostics for physical experiments.

Forward modeling of Doppler velocity interferometer system for improved shockwave measurements.

When near-instantaneous shocks are recorded by using a Doppler velocity interferometer velocity interferometer system for any reflector (VISAR), they typically exceed the detector's ability to react and "skipped fringes" result, where its visibility briefly reduces. Traditionally, replacing skipped fringes required guesswork in analysis, which increased arrival time errors. Second, the use of long but velocity-sensitive interferometer delays with fast detectors which can resolve the delay has traditionally been avoided because of the fear of confusing the arrival time signal. However, shorter delays produce smaller fringe phase shifts per velocity and, thus, can decrease velocity precision. We realize that while some loss of fringe information occurs at shock events, this is often just a partial loss and the residual fringe information can still hold valuable information. We describe a forward model (FM) of the interferometer action and detector blurring that assists with VISAR fringe analysis at skipping events: (1) more precise shock arrival times, (2) arrival time precision not limited by long delays, and (3) improved ghost subtraction, which improves accuracy over a broad time region. We demonstrate the utility of a FM on National Ignition Facility and Laboratory for Laser Energetics Omega shots and discuss data process methods to increase the high time resolution of VISAR systems despite their limitations and complexities.

The design of a line velocity interferometer for any reflector for inertial confinement experiments on the Z-machine

A line VISAR (Velocity Interferometer System for Any Reflector) has been designed and commissioned at the Sandia National Laboratory's Z-machine. The instrument consists of an F/2 collection system, beam transport, and an interferometer table that contains two Mach-Zehnder type interferometers and an eight channel Gated Optical Imaging (GOI) system. The VISAR probe laser operates at the 532 nm wavelength, and the GOI bandpass is 540-600 nm. The output of each interferometer is passed to an optical streak camera with four selectable sweep speeds. The system is designed with three interchangeable optics modules to select a full field of view of 1 mm, 2 mm, or 4 mm. The optical beam transport system connects the target image plane to the interferometers and the gated optical imagers. The target is integrated into a sacrificial final optics assembly that is integral to the transport beamline.