Velocity Interferometer System For Any Reflector Research Articles

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.

First experimental observation of a photoabsorption-edge induced shock by its coalescence onto a regular ablation-shock

Implosion experiments of an inertial confinement fusion (ICF) target on the laser megajoule (LMJ) and the National Ignition Facility require, for certain designs, a precise timing coalescence of four shocks at a specific point of the capsule, which strongly depends on the ablator equation of state. In experiments at the Ligne d'Intégration laser facility, a prototype for the LMJ, coalescence of two shocks was studied in a planar polystyrene (CH) sample in an indirect drive configuration. Shocks were driven by x-ray emission generated in a spherical hohlraum radiatively heated using a 12 ns duration laser pulse temporally shaped to produce two steps in the radiation temperature history that launches these two successive ablation-shock waves. Shock velocity was inferred from a Velocity Interferometer System for Any Reflector (VISAR). Shot performed with 10 ns long truncated laser pulses reduces VISAR blanking, which allows us, for the first time to our knowledge, to observe a photoabsorption-edge induced shock, edge-shock for short, which is a third shock out of the two-step radiation temperature history, coalescing with the second ablation shock. The accurate measurement of this shock with well-controlled x-ray drive should potentially help to constrain the equation of state and opacity of carbon in coronal plasma conditions since the behavior of this shock is very sensitive to both. Moreover, since they can drastically alter the speed of coalesced shocks (in keyhole experiments or ignition designs for ICF), measurements of these edge-shocks may also contribute to improving our ICF design capabilities.

Experimental investigation of shock wave pressure induced by a ns laser pulse under varying confined regimes

The article presents a study of shock waves induced by a nanosecond laser pulse in samples in the form of steel plates. Its contents include a description of the measurement system, methods of calculations and some characteristics of the measuring instruments and materials used. The quoted formulas enabled the processing of recorded measurement signals. The influence of material used as a confining layer (glass, PMMA - Plexiglass, water) and substrate (PMMA, aluminium, steel) on the amplitude and shape of recorded pressure waves was studied. Pressure behind the shock wave measurements were conducted using piezoelectric polymer PVDF (polivinylidene fluoride) sensors. Verifications of PVDF results were conducted by the measurements of velocity of back sample surface by VISAR (Velocity Interferometer System for Any Reflector). A qualitative compliance between the PVDF's pressure and VISAR's velocity rescaled to pressure was achieved. The strains (about 0.3%) and strain rates (about 3 × 105 1/sec) were evaluated. The obtained results will allow for a better selection of test conditions for studying material properties by using a shock wave induced by the laser pulse.

Simulation of laser-driven cratering experiments on aluminum

After a brief description of the physical principles involved in the cratering process, the authors present a specific methodology to simulate laser-driven cratering experiments performed with a long pulse duration (100 ns) and a small focal spot diameter (220μm). This methodology can be divided into two steps. First, the 2D-axisymmetrical pressure field generated by the laser on the target is determined from laser parameters. Second, this pressure is applied on the surface of the target in a Eulerian simulation. In order to validate this methodology, the authors simulate a laser shot on a thin aluminum target whose rear surface velocity is recorded by a VISAR (Velocity Interferometer System for Any Reflector). Once validated, they use the methodology to simulate laser-driven cratering experiments on semi-infinite aluminum targets. Numerical results are compared to experimental measurements of the craters. Although slight differences are pointed out and discussed, the proposed methodology is well adapted to simulate craterization laser shots.

Study on equation of state and spall strength of sintered Nd-Fe-B magnets

The experiments were carried out in a gas gun system to examine the equation of state (EOS) below or near the Hugoniot elastic limit (HEL) for the sintered Nd-Fe-B magnets. A laser velocity interferometer system for any reflector (VISAR) was used to measure the shock and particle velocity of the specimens. The obtained EOS data indicated that the EOS at high pressure could not be extrapolated to the lower pressure below or near the HEL for the Nd-Fe-B magnets. The spall strength of the Nd-Fe-B magnets was determined in the gas gun plane spall tests using the analysis of the free surface velocity profile of the specimens measured with the VISAR. It is found that the spall strength of the Nd-Fe-B magnets is dependent on the strain rates and larger than the tension strength of the magnets in the dynamic Brazilian tests.

Laser Shock Peening: Toward the Use of Pliable Solid Polymers for Confinement

This paper presents the first extensive study of the performances of solid polymers used as confinement materials for laser shock applications such as laser shock peening (LSP) as opposed to the exclusively used water-confined regime up to now. The use of this new confinement approach allows the treatment of metal pieces needing fatigue behavior enhancement but located in areas which are sensitive to water. Accurate pressure determination in the polymer confinement regime was performed by coupling finite element simulation and experimental measurements of rear free-surface velocity using the velocity interferometer system for any reflector (VISAR). Pressure could reach 7.6 and 4.6 GPa for acrylate-based polymer and cross-linked polydimethylsiloxane (PDMS), respectively. At 7 and 4.7 GW/cm 2 , respectively, detrimental laser breakdown limited pressure for acrylate and PDMS. These results show that the pressures produced were also as high as in water confinement, attaining values allowing the treatment of all types of metals with LSP and laying the groundwork for future determination of the fatigue behavior exhibited by this type of treated materials.

A high temporal resolution numerical algorithm for shock wave velocity diagnosis

We propose a high temporal resolution numerical algorithm for shock wave velocity diagnosis. By analysing variations in the optical path and phasor of a light field, we determine a high temporal resolution shock wave velocity equation for a velocity interferometer system for any reflector (VISAR). The equation can be transformed into matrix form for numerical solution. To solve noise problems, a ‘filtering velocity spectrum’ method is proposed. Analysis of a VISAR data example shows that the resolution precision of shock wave velocity obtained from the numerical algorithm is the same as the temporal resolution of a streak camera. Moreover, it can observe the shock wave in greater detail. This algorithm can be used to observe detailed images and determine the mechanism and evolution of extreme shock waves, as well as provide data for research into hydrodynamic behaviour in inertial confinement fusion.

Simulated refraction-enhanced X-ray radiography of laser-driven shocks

Refraction-enhanced x-ray radiography (REXR) is used to infer shock-wave positions of more than one shock wave, launched by a multiple-picket pulse in a planar plastic foil. This includes locating shock waves before the shocks merge, during the early time and the main drive of the laser pulse that is not possible with the velocity interferometer system for any reflector. Simulations presented in this paper of REXR show that it is necessary to incorporate the refraction and attenuation of x rays along with the appropriate opacity and refractive-index tables to interpret experimental images. Simulated REXR shows good agreement with an experiment done on the OMEGA laser facility to image a shock wave. REXR can be applied to design multiple-picket pulses with a better understanding of the shock locations. This will be beneficial to obtain the required adiabats for inertial confinement fusion implosions.

Hugoniot Measurements Utilizing In Situ Synchrotron X-ray Radiation

Pressure–density relationships derived from the experimentally obtained shock and particle velocities are critical to define a material’s equation of state (EOS). Typically, impact experiments coupled with velocimetry are used to map a material’s Hugoniot. Limitations such as sample geometry and varying indices of refraction may prevent proper characterization using traditional techniques such as photon doppler velocimetry (PDV) or velocity interferometer system for any reflector (VISAR). Here, traditional Hugoniot measurements using PDV are compared to dynamic x-ray imaging encompassing two different sample geometries on the gas gun platform. Through each of these methods an experimentally derived Hugoniot is determined for a previously uncharacterized polymeric material, Somos Watershed XC11122, that is used 3D printed stereolithography parts. A Us–up relationship was determined to be Us = 2.93 us + 1.73 mm/μs through traditional PDV. Slope and sound speed values determined from x-ray imaging methods varied 11% from PDV measurements. Each method yielded a Hugoniot with densities similar to poly(methyl methacrylate) (PMMA). The similarity shows the viability of such analyses for dynamic properties and Hugoniot data. The performance and analysis of both PDV and dynamic x-ray measurements are laid out in this work. Comparing PDV and x-ray imaging highlights distinct advantages and disadvantages among each method. PDV provides less uncertainty for velocity measurements, however x-ray imaging is more spatially resolved allowing for shock steadiness observations of value when studying heterogeneous materials. Additionally, x-ray imaging provides greater insight into the shape and heterogeneity of the shock front as well as uniaxial strain state (1D zone) assumptions.

Dynamic Anisotropic Response of β‐HMX and α‐RDX Single Crystals Using Plate Impact Experiments at ∼1 GPa

AbstractDynamic responses of several orientations of cyclotetramethylene tetranitramine (HMX) and cyclotrimethylene trinitramine (RDX) single crystals were investigated by using plate impact experiments. The orientations studied included (011), (010), (100), (−111), (01−1), and (11−1) planes of HMX single crystals and (210), (100), (11−1), (2−10), and (111) planes of RDX single crystals. Crystal/window interface particle velocity profiles, which were measured by using velocity interferometer system for any reflector, showed distinct elastic‐plastic double‐wave structures. Dynamic mechanical properties were obtained according to impedance matching method. Elastic‐plastic responses of HMX and RDX exhibited strong anisotropy, nonlinear elasticity, and pressure/strain‐rate dependency. Anisotropy was explained by analyzing the relationship between the impact plane and deformation systems. Results revealed insights into ignition mechanisms and impact sensitivity and provided multiple data for calibration of physical‐based constitutive models.

Optical diagnostic systems based on Shenguang III

Optical diagnostic system plays an important role in inertial confinement fusion (ICF) experiment. Some key physical quantities and parameters in fusion can be diagnosed intuitively. In this review, three typical diagnostic systems on Shengang III facility are introduced, including imaging velocity interferometer system for any reflector (VISAR), fusion-reaction diagnostic system and backscatter light diagnostic system. VISAR can be used to measure velocity of free surface or shock wave traveling history in transparent media, which is an important diagnostic technique for shock wave velocity regulation, material compression characteristics and radiation temperature measurement. The fusion reaction-history diagnostic system can be used to accurately measure the fusion rate of ICF experiment. It’s a key diagnostic technique to study implosion dynamics and understand the mixed effect. Backscatter light diagnostic system is capable to measuring the energetic quotient stimulated by laser and plasma interaction (LPI), which produce important supports for calculating the coupling efficiency of driven laser and target.

Research on a wide-angle diagnostic method for shock wave velocity at SG-III prototype facility

A wide-angle diagnostic method for shock wave velocity is proposed in this paper. An ellipsoidal equation deduced for wide-angle diagnosis by using geometrical optical laws is presented. The combination of an ellipsoidal mirror and a traditional velocity interferometer system for any reflector (VISAR) forms a wide-angle VISAR. Several fundamental problems such as assembly, imaging, and interference in the wide-angle VISAR are discussed. The method is proven to be feasible based on the results of an eight-beam laser direct drive experiment at SG-III prototype facility. The diagnostic method is expected to be usable in a wide variety of experiments, especially those related to implosion compression, including high-pressure equation of state experiments, materials characterization experiments, shock characterization for Rayleigh–Taylor experiments, and shock timing experiments for inertial confinement fusion research.

Design and testing of a magnetically driven implosion peak current diagnostic

A critical component of the magnetically driven implosion experiments at Sandia National Laboratories is the delivery of high-current, 10s of MA, from the Z pulsed power facility to a target. In order to assess the performance of the experiment, it is necessary to measure the current delivered to the target. Recent Magnetized Liner Inertial Fusion (MagLIF) experiments have included velocimetry diagnostics, such as PDV (Photonic Doppler Velocimetry) or Velocity Interferometer System for Any Reflector, in the final power feed section in order to infer the load current as a function of time. However, due to the nonlinear volumetrically distributed magnetic force within a velocimetry flyer, a complete time-dependent load current unfold is typically a time-intensive process and the uncertainties in the unfold can be difficult to assess. In this paper, we discuss how a PDV diagnostic can be simplified to obtain a peak current by sufficiently increasing the thickness of the flyer. This effectively keeps the magnetic force localized to the flyer surface, resulting in fast and highly accurate measurements of the peak load current. In addition, we show the results of experimental peak load current measurements from the PDV diagnostic in recent MagLIF experiments.

Characterization of a signal recording system for accurate velocity estimation using a VISAR

The linearity of a signal recording system (SRS) in time as well as in amplitude are important for the accurate estimation of the free surface velocity history of a moving target during shock loading and unloading when measured using optical interferometers such as a velocity interferometer system for any reflector (VISAR). Signal recording being the first step in a long sequence of signal processes, the incorporation of errors due to nonlinearity, and low signal-to-noise ratio (SNR) affects the overall accuracy and precision of the estimation of velocity history. In shock experiments the small duration (a few µs) of loading/unloading, the reflectivity of moving target surface, and the properties of optical components, control the amount of input of light to the SRS of a VISAR and this in turn affects the linearity and SNR of the overall measurement. These factors make it essential to develop in situ procedures for (i) minimizing the effect of signal induced noise and (ii) determine the linear region of operation for the SRS. Here we report on a procedure for the optimization of SRS parameters such as photodetector gain, optical power, aperture etc, so as to achieve a linear region of operation with a high SNR. The linear region of operation so determined has been utilized successfully to estimate the temporal history of the free surface velocity of the moving target in shock experiments.

Failure mechanism of resistance-spot-welded specimens impacted on base material by bullets

The tests of bullet impact on the base material (BM) of a simple specimen with a single resistance-spot-welded (RSW) nugget of TRIP800 steel are performed to investigate the response of the RSW specimen to the ballistic debris impact on the RSW specimen. A one-stage gas gun is used to fire the bullets while a laser velocity interferometer system for any reflector (VISAR) is used to measure the velocity histories of the free surfaces of the RSW specimen. The recovered RSW specimens are examined with the three-dimensional super depth digital microscope (SDDM) and the scanning electro microscope (SEM). For the tests of small multiple-bullet impact, it is revealed that the wave train of the VISAR measured results and the detachment of the base material interfaces in the recovered RSW specimens are directly related to the reflection and refraction of the curved stress waves incoming to the interfaces and the free surfaces in the RSW specimens. The detachment of BM interfaces can lead to the impact failure of the RSW joints for the larger multiple-bullet impact at higher velocity, the mechanism of which is different from the case for normal incidence (spalling). For the tests of single large bullet impact, it is brought to light experimentally that the plastic strain concentration at the “notch tip” spurs either the crack near the RSW joint or the split of the nugget. The numerical simulation shows up the process of splitting the nugget: a crack initiates at the “notch tip”, propagates across the nugget interface and splits the nugget into two parts. It is indicated that the interaction between the stress waves and many interfaces/free surfaces in the RSW specimen under ballistic impact causes variable local stress triaxialities and stress Lode angles, which affects the deformation and fracture mechanism of the RSW specimen including stretching and shearing failure. It is shown that the impact failure of the RSW joints is a mixture of brittle fracture and ductile fracture while the fracture or perforation of the BM is ductile.

Progress of laser-driven quasi-isentropic compression study performed on SHENGUANG III prototype laser facility

The equation of state for solid at extreme pressure and relatively low temperature is an important topic in the study of astrophysics and fundamental physics of condensed matter. Direct laser-driven quasi-isentropic compression is a powerful method to achieve such extreme states which have been developed in recent years. A lot of researches have been done in Research Center of Laser Fusion in China since 2012, which are introduced in this article. The researches include an analytical isentropic compression model, a developed characteristic method, techniques for target manufacture, and experiments performed on SHENGUANG Ⅲ prototype laser facility. The analytical isentropic compression model for condensed matter is obtained based on hydrodynamic equations and a Murnaghan-form state equation. Using the analytical model, important parameters, such as maximum shockless region width, material properties, pressure pulse profile, and pressure pulse duration can be properly allocated or chosen, which is convenient for experimental estimation and design. The characteristic method is developed based on a Murnaghan-form isentropic equation and characteristics, which can be used for experimental design, simulation, and experimental data processing. Based on the above researches, several rounds of experiments have been performed to obtain better isentropic effect by upgrading the target configurations. Five kinds of target configurations have been used up to now, which are three-step aluminum target, CH-coated planar aluminum target, CH-coated three-step aluminum target, planar aluminum target with Au blocking layer, and three-step aluminum target with Au blocking layer. The rear surface of three-step aluminum target is found to be destroyed when the loading pressure rises up to 194 GPa, and weak shock appears in CH-coated planar aluminum target and CH-coated three-step aluminum target. Besides, velocity interferometer system for any reflector (VISAR) fingers are found to decrease when the pressure rises up to about 400 GPa and disappears at 645 GPa. By reducing laser intensity, the whole interface velocities on three steps are obtained in the CH-coated three-step aluminum target and a stress-density curve is calculated. In order to eliminate the weak shock, the target configurations are upgraded by changing the ablation layer and putting a gold blocking layer after it. The experimental results show that the weak shock is eliminated and much clearer VISAR fingers are obtained when pressure rises to as high as 570 GPa.

Dynamic measurements in non-uniform flows

The response of gauges registering the flow velocity and pressure in highly non-uniform media (for example, a powder under shock compression or powdered low-density explosive) is simulated. The modeling employs an acoustic approach. Against the average level of the signal, the fluctuations generated by the heterogeneity of the medium are observed which may distort the results completely. For reliable measurements, gauges larger than the characteristic scale of the medium non-uniformity are required. Under this condition, electromagnetic flow measurements and the velocity interferometer system for any reflector (VISAR) produce quite similar flow velocity profiles with small level of noise.

VISAR signal analysis with an emphasis on ellipse fitting.

A method based on ellipse fitting for analysis of VISAR (Velocity Interferometer System for Any Reflector) signals has been described. The errors introduced on the free surface velocity history due to common measurement imperfections in the amplitude and phase angle of the VISAR signal has been investigated and a remedy to mitigate this using a proper ellipse fitting technique has been presented. Performance comparison among various ellipse fitting techniques based on empirical evidence is carried out, and the geometric fitting technique of the Gauss Newton algorithm with initial conditions from the Bookstein algebraic fitting method is proposed. The new method provides better result in terms of accuracy than existing techniques. The method is especially useful when a fractional fringe signal is present which is quite common in VISAR based measurements. The applicability of the ellipse fitting technique is first theoretically justified, and then numerical simulation of ellipse fitting is carried out. Finally, the proposed method is validated by applying it for the analysis of VISAR signals recorded in the shock wave experiment conducted on the Al-2024T4 target material. The results of present analysis display a good agreement with the data available from other sources.

Shock drive capabilities of a 30-Joule laser at the matter in extreme conditions hutch of the Linac Coherent Light Source.

We measure the shock drive capabilities of a 30 J, nanosecond, 527 nm laser system at the matter in extreme conditions hutch of the Linac Coherent Light Source. Using a velocity interferometer system for any reflector, we ascertain the maximum instantaneous ablation pressure and characterize its dependence on a drive laser spot size, spatial profile, and temporal profile. We also examine the effects of these parameters on shock spatial and temporal uniformity. Our analysis shows the drive laser capable of generating instantaneous ablation pressures exceeding 160 GPa while maintaining a 1D shock profile. We find that slope pulses provide higher instantaneous ablation pressures than plateau pulses. Our results show instantaneous ablation pressures comparable to those measured at the Omega Laser Facility in Rochester, NY under similar optical drive parameters. Finally, we analyze how optical laser ablation pressures are compare with known scaling relations, accounting for variable laser wavelengths.