Velocity Interferometer Research Articles

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.

Simulation of magnetically driven flyer plate experiments with an improved magnetic field boundary formula

To consistently simulate magnetically driven flyer plate experiments with a magnetohydrodynamics code by taking the measured current as the input, a relation is needed that connects the measured current and the magnetic field on the loading surface. Lemke’s magnetic field formula [Lemke:2011, Davis:2014] gives deep insights into this relation, but it also leads to noticeable discrepancy from experiments if the formula is directly used for flyer plate simulation. In this paper, we find that, by adding to Lemke’s formula a new term that reveals the ablation effect of the loading surface, the simulation can be much improved. With this modified version of Lemke’s formula, we simulated one-sided and two-sided flyer plate experiments performed on the Primary Test Stand facility in China, which show satisfactory agreement with the velocity interferometer system for any reflector measurements.

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.

격벽착화기 화약의 충격파와 민감도 분석

격벽착화기의 최적 설계를 위하여 여폭약에 의해 생성되는 충격파의 감쇄 특성과 수폭약의 민감도에 관하여 연구하였다. 충격파의 감쇄 특성은 레이저 광간섭계인 VISAR를 이용하여 측정한 격벽의 자유 표면 속도로부터 유도하였고 수폭약의 민감도는 SSGT (Small Scale Gap Test) 결과로부터 구하였다. 격벽착화기의 기폭시험을 통하여 SSGT로부터 구한 화약의 민감도가 격벽착화기와 같이 소량의 화약을 사용하는 시스템에는 충격파 지속 시간이 상이하여 적합하지 않다는 것을 밝혀내었다. We studied attenuation characteristics of shock waves induced by a donor charge and the sensitivity of an acceptor for optimal design of a TBI (Through-bulkhead initiator). The attenuation behavior of shock waves was studied by measuring free surface velocity using a VISAR (Velocity Interferometer System for Any Reflector), and the sensitivity of the acceptor explosives was analyzed via SSGT (Small Scale Gap Test). It was found that the acceptor sensitivity obtained by the SSGT may be inappropriate for the design of the small-scale explosive devices such as TBI due to the different shock duration time.

Performance characterization of a miniaturized exploding foil initiator via modified VISAR interferometer and shock wave analysis

A pyrotechnic device that consists of a donor/acceptor pair separated by a gap or a bulkhead relies on the shock attenuation characteristics of the gap material and the shock sensitivity of the donor and acceptor explosives. In this study, a miniaturized exploding foil initiator, based on high pulsed electrical power generator, was designed to launch a micro Kapton® flyer for impact initiation of a high explosive in order to understand its performance characteristics. Here, the explosive substance was replaced with a witness plate because the flyer poses various flight motions of rotation, bend, and fragmentation due to its extreme thinness. By using a Velocity Interferometer System for Any Reflector and ANSYS Explicit Dynamics, the averaged velocity of a flyer is measured, which then allows for the calculation of the shock pressure and the duration imparted to the explosive for an initiation. Subsequently, the relationship between the flyer velocity, the amplitude, and the width of impact loading can be used to assess the performance of the designed exploding foil initiator of a micro pyro-mechanical device.

Shock compression behavior of low density powder materials and the double-shock Hugoniot

Particle velocity measurements of window/sample interface by velocity interferometer have been performed on a low density powder material, Ce:YAG, under various shock loading conditions generated by plate impact with a maximum impact velocity of 1.564km/s (SUS304). The observed interface velocities and the reshock pressures are very high, indicating that the shock impedance of shock-compressed powder in the first-shock state is much higher than that of the initial powder because the density and the shock velocity for the first-shock state are largely increased. Measurements with three different window materials with almost the same impact velocities have been carried out to determine the reshock (double-shock) Hugoniot which is much stiffer than the principle Hugoniot, leading to a characteristic shock reverberation behavior of low density powder materials.

Plastic behavior of steel and iron in high strain rate regime

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

Velocity interferometer signal de-noising using modified Wiener filter

The accuracy and precision of the non-contact velocity interferometer system for any reflector (VISAR) depends not only on the good optical design and linear optical-to- electrical conversion system, but also on accurate and robust post-processing techniques. The performance of these techniques, such as the phase unwrapping algorithm, depends on the signal-to-noise ratio (SNR) of the recorded signal. In the present work, a novel method of improving the SNR of the recorded VISAR signal, based on the knowledge of the noise characteristic of the signal conversion and recording system, is presented. The proposed method uses a modified Wiener filter, for which the signal power spectrum estimation is obtained using a spectral subtraction method (SSM), and the noise power spectrum estimation is obtained by taking the average of the recorded signal during the period when no target movement is expected. Since the noise power spectrum estimate is dynamic in nature, and obtained for each experimental record individually, the improved signal quality is high. The proposed method is applied to the simulated standard signals, and is not only found to be better than the SSM, but is also less sensitive to the selection of the noise floor during signal power spectrum estimation. Finally, the proposed method is applied to the recorded experimental signal and an improvement in the SNR is reported.

Ramp-Wave Compression Experiment with Direct Laser Illumination on ShenGuang-III Prototype Laser Facility**Supported by the National Natural Science Foundation of China under Grant No 11305160.

Ramp-wave compression experiment to balance the high compression pressure generation in aluminum and x-ray blanking effect in transparent window is demonstrated with an imaging velocity interferometer system for any reflector (VISAR) on ShenGuang-III prototype laser facility. The highest pressure is about 500 GPa after using the multilayer target design Al/Au/Al/LiF and 1013 W/cm2 laser pulse illuminated on the planar Al target, which generates the spatial uniformity to over 500 μm on the ablation layer. A 2-μm-thick Au layer is used to prevent the x-ray from preheating the planar ablation Al layer and window material LiF. The imaging VISAR system can be used to record the abrupt loss of the probe beam ( nm) caused by absorption and reflection of 20-μm, 30-μm and 40-μm-thick Al, i.e., the blanking effect. Although there are slight shocks in the target, the peak pressure 500 GPa, which is the highest data up to now, is obtained with ramp-wave compression.

Response of aramid honeycomb sandwich panels subjected to intense impulse loading by Mylar flyer

The deformation/failure modes and impact resistance of composite sandwich panels with titanium alloy plates and aramid honeycomb cores subjected to intense impulse loading were investigated experimentally using electric gun technique. A velocity measuring instrument, known as the velocity interferometer system for any reflector (VISAR), measured the velocity history at the mid-point of the back plates. Typical deformation/failure modes were analyzed and classified systematically, and then a comparison of structural deformation resistance between sandwich panels and plates of equivalent mass was studied. It was found that both the deformation/failure modes and the dynamic response of the plates were sensitive to the impact velocity. The impact resistance of sandwich panels could be improved by core crushing and deformation of the front plate. Finally, effects of impact velocity and boundary conditions on the dynamic response of sandwich panels were identified. It was worth noting that the fully clamped boundary reduced the damage extent of the front plate, but increased the local plastic deformation of the back plate.

Absolute equation of state measurement of aluminum using laser quasi-isentropic-driven flyer plate

AbstractIn this paper, we perform an absolute equation of state (EOS) measurement on the principal Hugoniot of aluminum using a near-symmetric impact method. The flyer plates are accelerated to high velocities using the laser-ramp-driven method. An aluminum flyer plate of ~25 µm is accelerated to the velocity range from 4 to 12 km/s. Then the aluminum flyer plate propagates across a vacuum gap and impacts with an aluminum step target. A line-imaging optical recording velocity interferometer for any reflector (ORVIS) is used to measure the aluminum flyer plate and the shock velocity simultaneously. Aluminum EOS data were measured with pressures range from 50 to 200 GPa. This absolute EOS measurement method may be used for studying a variety of materials.

Valence electron structure model of spall strength in middle carbon steel

A phenomenological theory to model spall strength in middle carbon steel was proposed by using empirical electron theory of solids and molecules (EET). Under plate impact load (strain rate, 105–106 s−1), valence electron structure (VES) of α-Fe, α-Fe-C, and ε-Fe3C phases have obvious change than quasi-static load (strain rate, 10−3–10−2 s−1). Theoretical spall strength of ASTM1045 were calculated at plate impact pressure range of 5.37–8.17 GPa. Experimental spall strength were experimented by using 57 mm-diameter, 12.5 m-long single-stage gas gun and velocity interferometer system for any reflector (VISAR). The relative error of theoretical and experimental spall strength is less than 7.6%.

基于神光Ⅲ原型的双轴速度干涉仪

基于神光Ⅲ原型的双轴速度干涉仪

复合式多点测量速度干涉仪光学系统设计

复合式多点测量速度干涉仪光学系统设计

Line-imaging VISAR for laser-driven equations of state experiments

The paper presents the diagnostic system for velocity measurements in laser- driven equations of state experiments. Two Mach-Zehnder line-imaging VISAR-type (velocity interferometer system for any reflector) interferometers form a vernier measuring system and can measure velocity in the interval of 5 to 50 km/s. Also, the system includes a passive channel that records target luminescence in the shock wave front. Spatial resolution of the optical layout is about 5 μm.