Photonic Doppler Velocimetry Research Articles

Laser-plasma jet driven sub-millimeter diameter aluminum flyer and its gesture diagnosis

Laser-driven flyer has been studied for decades as it promises to possess many applications such as in measuring the equation of state (EOS) under ultrahigh pressure, investigating the material dynamic properties under high strain rate, simulating the high-speed impact for aircraft protection, and igniting explosives. However, the planarity and integrity of flyers are determined by indirect velocity lnterferometer system for any reflector (VISAR) or witness slab results due to its high speed and small dimension. For further and wide applications, it is very important to obtain direct experimental proof of the flyer gesture and configuration. Thus, the acceleration and gesture investigation of aluminum flyer driven by laser plasma are studied on Xingguang-III laser facility. The X-ray radiography is achieved by a picosecond laser irradiating the copper wire target. The shadowgraph of flyer and plasma are realized by the incidence of a bunch of infrared laser through the flyer flight path. In additon, photon Doppler velocimetry is employed to measure the flyer velocity simultaneously. The radiography, shadowgraph and velocity of typical small aluminum flyer are obtained. By optimizing the thickness of both CH ablation layer and vacuum gap, the flyer is slowly accelerated via consecutive stress wave produced by plasma colliding. The aluminum flyer has a thickness of 20 μm and diameter of about 500 μm. The whole flyer remains the integrated shape after a great angle of rotation due to uneven plasma loading. The flight distance is about 400 μm, giving an average velocity of 2.2 km/s. The planarity of the flyer is good except a little bend on the two sides due to side rarefaction of plasma. The study verifies that the laser plasma collision can generate a sub-millimeter-diameter metal flyer with integrated shape and a velocity of several kilo-meters per second, showing that it possesses the promising applications in measuring the EOS and igniting explosive .

Fabrication and Optimization Design of Multilayer Flyer Plates for Laser-Driven Loading

The laser-driven flyer plate is an important loading technology in high energy physics, shock wave physics, and explosive initiation application. How to generate a high-velocity and intact flyer plate by using the laser is a matter of concern for laser driving. In this study, the multilayer flyer plates (MFPs) of Al/Al2O3/Al and TiO2/Al/Al2O3/Al with adjustable performance were designed and fabricated by magnetron sputtering and analyzed by scanning electron microscopy (SEM), laser reflectance spectrometer, and differential thermal analysis (DTA). The effects of the structure and material on the output performance of MFPs were analyzed by photon Doppler velocimetry (PDV) and ultrahigh-speed video. The morphology results showed that the structure of MFPs had uniform and clear boundaries between side-by-side layers. The MFP velocity was controlled in the range of 4.0–6.0 km/s by adjusting the film thickness, structure, and thermite material with 43.1 J/cm2 laser ablation. Among them, the energetic flyers with the thermite ablation layer had the highest final velocity of 5.38 km/s due to the prestored energy of TiO2/Al. By appropriately increasing the thickness of Al2O3 from 0.4 μm to 0.8 μm, the complete flight of the flyer plate to 3.72 mm can be realized. In addition, TiO2/Al thermite film had characteristics of reaction heat release and lower laser reflectivity (72.13%) than the Al layer (80.55%), which explained the velocity enhancement effect of energetic flyer plates. This work provides facile strategy to enhance the output performance of MFPs, which may facilitate the practical applications of laser driving technology.

Research on time-stretched photon Doppler velocimetry

Research on time-stretched photon Doppler velocimetry

Pressure amplification and modelization in laser shock peening of Ti-6Al-4V and AA7085 with adhesive-backed opaque overlays

The effects of adhesive-backed overlays, specifically vinyl tape and aluminum tape, on the peak pressures generated by laser shock peening (LSP) of 7085 aluminum alloy and Ti-6Al-4V were investigated and the results are reported. Peak pressures were determined from photon doppler velocimetry measurements of rear surface velocity. Incorporating adhesive-backed opaque overlays in LSP provided up to 50 % pressure enhancement compared to processing without the overlays. Contrary to processing without the overlays, the temporal characteristics of LSP impulses with adhesive-backed overlays are shown to be independent of the laser pulse width in the range studied. This finding is an important impulse consideration in LSP parameter selection and has not been investigated or reported in the literature. A novel physics-based analytical model to predict shockwave pressures is proposed and validated with experimental data agreeing nominally within 15 % of the predicted values across the parameter space. The results provide insight into the improved residual stresses and enhanced material properties of published opaque overlay studies. Moreover, the results provide vital information for selecting LSP parameters for optimizing component performance and potential extension into previously unreachable pressure regimes.

Photon Doppler velocimetry measurements of impact-induced surface waves in glass and their role in fracture initiation and damage evolution

Ballistic experiments involving the impact of cylindrical Lexan® projectiles, with nominal velocities of 400 and 800 m/s, on borosilicate glass targets were conducted using a single-stage gas gun. The objective of the experiments was to develop a technique involving multiplexed photon Doppler velocimetry (PDV) to observe the distal surface motion due to the propagation of impact-induced elastic waves. Comparison of the recorded signals to finite-element simulations of the experiment allowed for the identification of the signature of fracture initiation and subsequent damage evolution in the material. By directing PDV probes through the rear surface of the target it was also found that the surface motion within the impact zone could be measured. In addition, it was discovered that two distinct velocity signals could be consistently measured beneath the impact site. One of these signals was identified as the particle velocity of the impact surface. The other signal is presently of unknown origin.

Reconstruction of Velocity Curve in Long Stroke and High Dynamic Range Laser Interferometry.

To study the law that governs the complex movements of the mechanism in the process of automatic weapon operation, the velocity tracking test technology of photon Doppler velocimetry is introduced to accurately measure velocity, displacement and acceleration, on the condition that there are long displacement and rapid velocity change. In the traditional way, out of interference signal time-frequency (TF) transformation draws TF distribution, and then by modulus maxima frequency extraction, comes to the law of velocity change. Due to the influence resulting from the change of fundamental signal as well as that of light intensity signal in the test, based on the TF distribution obtained by TF transformation, the traditional modulus maxima frequency extraction can extract frequency signals, but they show abnormal sudden changes at some moments, making the velocity discontinuous, unsmooth and unreal, which brings obvious errors to the subsequent calculation of acceleration and accurate displacement. Addressing the above-mentioned problems, this paper proposes a ridge extracting correction algorithm based on modulus maxima frequency extraction; this method, based on a large number of experiments where rodless cylinders are used to simulate the motion of a gun automatic mechanism, conducts a detailed calculation and analysis of the experimental results. A comparison of the two algorithms’ processing results, in terms of the speed, displacement and acceleration, suggests that the ridge extracting correction algorithm successfully corrects the frequency selection error, which draws a more continuous and, therefore, effective curve of the velocity change, and by so doing, the error of the displacement test (within 1.36 m displacement) is reduced from more than 3.6% to less than 0.58%, and the uncertainty dropped 97.07%. All these show that the accurate measurement of velocity, displacement and acceleration, with sudden and rapid velocity changes considered, is realized successfully.

A compact platform for the investigation of material dynamics in quasi-isentropic compression to\u2009~\u200919 GPa

This paper reports on the development of a magnetically driven high-velocity implosion experiment conducted on the CQ-3 facility, a compact pulsed power generator with a load current of 2.1 MA. The current generates a high Lorentz force between inner and outer liners made from 2024 aluminum. Equally positioned photonic Doppler velocimetry probes record the liner velocities. In experiment CQ3-Shot137, the inner liner imploded with a radial converging velocity of 6.57 km/s while the outer liner expanded at a much lower velocity. One-dimensional magneto-hydrodynamics simulation with proper material models provided curves of velocity versus time that agree well with the experimental measurements. Simulation then shows that the inner liner underwent a shock-less compression to approximately 19 GPa and reached an off-Hugoniot high-pressure state. According to the scaling law that the maximum loading pressure is proportional to the square of the load current amplitude, the results demonstrate that such a compact capacitor bank as CQ-3 has the potential to generate pressure as high as 100 GPa within the inner liner in such an implosion experiment. It is emphasized that the technique described in this paper can be easily replicated at low cost.

Energy conversion efficiency of electrical exploding foil accelerators

We evaluate the energy conversion efficiency of an electrical exploding foil accelerator that accelerates a thin dielectric foil (called the flyer) to more than 1 km/s, which is propelled by electrically exploded bridge material. The effective flyer mass ejected from the accelerator is estimated by impulse measurements obtained using a gravity pendulum as well as by time-resolving flyer velocity measurements obtained using a photonic Doppler velocimetry system. For two different bridge sizes (0.2 and 0.4 mm), the flyer velocity and impulse increase with the input energy at the bridge section. The maximum flyer velocity and impulse, that is, 4.0 km/s and 67 µN s, respectively, are obtained by supplying 0.33 J of input energy. Upon increasing the input energy, the effective flyer mass also increases and exceeds the designed-bridge mass for both bridge sizes. The energy conversion efficiency from input electrical energy to flyer kinetic energy is calculated based on the effective flyer mass, velocity, and input energy. Both bridge sizes show comparable efficiencies: 27% and 30% for 0.2 and 0.4 mm bridges, respectively. The efficiency increases with increasing specific input energy at least up to 15 MJ/kg for the 0.4 mm bridge, whereas the efficiency of the 0.2 mm bridge above 30 MJ/kg decreases. This is owing to the excessively high input energy density in the 0.2 mm bridge, which causes the effective flyer mass to increase by including surrounding materials. These results indicate that the specific input energy should be optimized for obtaining maximum efficiency.

Ethylenediamine Catalyzes Nitromethane Shock-to-Detonation in Two Distinct Ways.

Adding amines to liquid nitromethane (NM) is known to lower the threshold for the shock-to-detonation transition because amines catalyze proton transfer reactions that are the initial steps in the energy release process. We studied NM with 1 wt % ethylenediamine (NM/EDA) with 4 ns input shocks using time and space resolved diagnostics: photon Doppler velocimetry (PDV), optical pyrometry, and nanosecond video imaging. The 4 ns shocks are fast enough to time-resolve the reaction kinetics and the shock-to-detonation transition. We find that it is possible to shock ignite the NM/EDA without producing a detonation, so there is more to amine sensitization of the shock-to-detonation process than simply lowering the barrier to initial reactions. We find that although 1 wt % EDA has little effect on the ambient properties of NM, it dramatically alters the Hugoniot. The shock speed in NM/EDA is reduced, indicating that shocked NM/EDA is significantly more compressible than NM. Higher compressibility is associated with greater adiabatic heating, so EDA both lowers the barrier to proton transfer reactions and increases shock energy absorption. To explain the enhanced compressibility, we propose that shocking NM/EDA produces a reactive flow that has a much higher ionic strength than in NM. The sudden transformation from a molecular liquid to an ionic liquid with stronger intermolecular interactions is responsible for enhanced compressibility and shock heating.

Role of anisotropy in the ballistic response of rolled magnesium

Rolled magnesium alloys exhibit pronounced tension-compression asymmetry as well as anisotropy in yield and strain-hardening behavior. Although differences in the mechanical response are reasonably well-understood, it is not clear to what extent anisotropy alters the deformation and failure of plates subjected to ballistic loading conditions. In this work, the role of texture and anisotropy in the ballistic response is investigated using a combined experimental–computational approach. Sphere impact experiments are performed on rolled magnesium plates cut from orientations that exhibit differing mechanical responses. The complex failure process is characterized by in situ diagnostics, including ultra-high-speed Digital Image Correlation and Photonic Doppler Velocimetry, and compared with simulations performed on polycrystalline aggregates using a polycrystal plasticity model for hcp metals. The anisotropic deformation behavior arises from deformation mechanisms with disparate strength and strain hardening behavior, which is well captured in the model. The occurrence of low-strength extension twinning is shown to govern the initial anisotropic deformation and bulging of the plate. When compared with post-mortem 3D X-ray microscopy, regions that experience intense basal slip are shown to be sites for damage initiation that leads to eventual fracture. The combination of experiments and simulations suggest that at low to intermediate ballistic loading rates, material orientation plays a crucial role in dictating eventual fracture and failure in strongly anisotropic metals such as in rolled magnesium alloys.

Modulation based ranging for direct displacement measurements of a dynamic surface.

We developed a method for directly measuring displacement of a moving surface for use with dynamic or high explosive driven experiments. The technique, called "Modulation Based Ranging" (MBR), overcomes the errors associated with integrating a velocity history of an object undergoing non-radial flow, while also providing the exact displacement of the object with sub 100 µm resolution. A discussion of sources of phase sensitive errors is presented along with a demonstration of the applied corrections. Excellent agreement between MBR and integrated velocity from the Photonic Doppler Velocimetry (PDV) technique was observed when no non-radial flow was present. We then demonstrated the ability of MBR to accurately measure true displacement of a surface subjected to a strong non-radial component.

Free-surface velocity measurements of opaque materials in laser-driven shock-wave experiments using photonic Doppler velocimetry

We present a novel photonic Doppler velocimetry (PDV) design for laser-driven shock-wave experiments. This PDV design is intended to provide the capability of measuring the free-surface velocity of shocked opaque materials in the terapascal range. We present measurements of the free-surface velocity of gold for as long as ∼2 ns from the shock breakout, at pressures of up to ∼7 Mbar and a free-surface velocity of 7.3 km/s with an error of ∼1.5%. Such laboratory pressure conditions are achieved predominantly at high-intensity laser facilities where the only velocity diagnostic is usually line-imaging velocity interferometry for any reflector. However, that diagnostic is limited by the lower dynamic range of the streak camera (at a temporal resolution relevant to laser shock experiments) to measure the free-surface velocity of opaque materials up to pressures of only ∼1 Mbar. We expect the proposed PDV design to allow the free-surface velocity of opaque materials to be measured at much higher pressures.

Research on MEMS multi-point exploding metal foil synchronous array

Based on MEMS technology, this paper realized the chip preparation of the synchronous initiation array and obtained the ideal process parameters. The MEMS multi-point exploding metal foil synchronous array chip replaces the traditional initiation array composed of electric detonators, and has a greatly reduced volume and could be prepared in batches. By testing the electrical explosion parameters of the array chip, the peak voltage, peak current, peak time and other data of different ignition voltages are obtained. At the same time, we proposed a method to test the synchronization of the array using multi-channel photonic doppler velocimetry, and obtained the synchronization data when the firing voltage was 2500 V and 2800 V, which provided help for the subsequent optimization design.

Measurement and characterization of nanosecond laser driven shockwaves utilizing photon Doppler velocimetry

Laser driven shockwaves from nanosecond pulsed lasers were evaluated for pressure magnitude and duration using photon Doppler velocimetry (PDV). Specific investigations were performed on Ti-6Al-4V alloys and 7085-T7651 aluminum alloys with laser full-width half-max pulse lengths ranging from 6 to 20 ns and laser intensities of 2–10 GW/cm2. The rear surface velocity data obtained using a heterodyne PDV system and an automated signal processing algorithm are demonstrated and utilized to capture rear surface velocity profiles. Peak shockwave pressures measured under bare surface water confinement reached approximately 6 GPa. Shockwave loading durations were observed to correspond with laser pulse durations with impulse loading times on the order of 2×–4× the laser pulse duration.

Temperature measurements in cerium shocked from 8.4 to 23.5 GPa

Shock temperature, stress, and dynamic emissivity for cerium shocked from 8.4 to 23.5 GPa were measured. In addition, the isentropic shock release temperature as a function of release stress was determined at a window interface. Cerium samples were shock compressed by plate impact on a single-stage gun. We made time-resolved measurements of thermal radiance, reflectance, and interface velocity of samples glued to lithium fluoride windows. Reflectance was measured with an integrating sphere and velocity with photonic Doppler velocimetry. From these measurements, we determined the temperature, emissivity, and stress at the interface. For shock stresses below 10.24 GPa, the samples were shocked from the γ phase into the α phase; at higher stresses, the cerium presumably melted or entered a mixed phase upon shock. The shock Hugoniot temperature as a function of stress follows a straight line over the entire range of our measurements, disagreeing with previously published predictions that the Hugoniot would follow the melt boundary from 10.24 up to around 16–18 GPa. Between 11.9 and 16.8 GPa, all the release isentropes converged (within experimental uncertainty) to a point around 4 GPa and 900 K, near the published melt curve. For experiments shocked above ∼16 GPa, the release isentropes behave differently. This suggests that within this 12–16 GPa range, there is a phase transition taking place, probably melt, and that it is occurring somewhere along the shock and release path. We could not identify a single-valued phase boundary from our experiments. Potential reasons for this are discussed.

Time-lens photon Doppler velocimetry (TL-PDV).

We describe a time lens (TL) to expand the dynamic range of photon Doppler velocimetry (PDV) systems. The principle and preliminary design of a TL-PDV system are explained and shown to be feasible through simulations. In a PDV system, an interferometer is used for measuring frequency shifts due to the Doppler effect from the target motion. However, the sampling rate of the electronics could limit the velocity range of a PDV system. A four-wave-mixing (FWM) TL applies a quadratic temporal phase to an optical signal within a nonlinear FWM medium (such as an integrated photonic waveguide or a highly nonlinear optical fiber). By spectrally isolating the mixing product, termed the idler, and with appropriate lengths of dispersion prior to and after this FWM TL, a temporally magnified version of the input signal is generated. Therefore, the frequency shifts of PDV can be "slowed down" with the magnification factor M of the TL. M = 1 corresponds to a regular PDV system without a TL. M = 10 has been shown to be feasible for a TL-PDV system. The use of this effect for PDV can expand the velocity measurement range and allow for the use of lower bandwidth electronics. TL-PDV will open up new avenues for various dynamic material experiments.

Saver: A Peak Velocity Extraction Program for Advanced Photonic Doppler Velocimetry Analysis

Richtmyer-Meshkov instability (RMI) experiments offer a powerful, new tool for investigating materials strength at extreme strain rates, but obtaining peak velocities from the photon Doppler velocimetry (PDV) results of such tests is often subjective and time consuming. A new, semi-automated program, SAVER, was developed to address these concerns and was tested on data sets from previous RMI studies. Extreme surface velocities extracted using SAVER agreed well with human-determined values, and peak spike velocities – a key RMI metric – were reproduced to within ~ 8% in all cases. A re-analysis of materials strength based on these new SAVER values resulted in an estimated increase of ~ 20%, but no objective measure exists at this time to verify this result. While SAVER proved robust with regards to spectrogram quality, scatter in extracted velocities was seen to increase with decreasing signal quality – emphasizing the importance of PDV instrumentation and processing.

Mechatronis: The Output Energy of MEMS Explosive Train

A micron scale of microelectromechanical systems (MEMS) explosive train with applications in both space exploration and electronic control of safety airbag, the MEMS explosive train is designed to shock initiate a linear shaped charge by accelerating a thin metal plate across a small gap. This paper summarizes the experiment results of Φ0.9×0.6mm micro-charge driven different thickness flyer’s velocity by Photonic Doppler Velocimetry (PDV), compared to the calculation result of Gurney formulation, the flyer’s velocity of micro-charge driven different thickness lower than the calculation. The velocity-time variation curve and the energy variation with distance provide some information available for micro-explosive train design.

Strategy for increasing flyer launching capacity of electro-explosively actuator by coupling electric explosion and plasma discharge

In this study, a novel high-speed flyer launching strategy based on the principle of the electric explosion of metal bridge foil and plasma discharge coupling was proposed to further improve the capability of conventional electro-explosively actuators. A mini electro-explosively actuator with built-in electrodes in its barrel was drawn up and fabricated using microelectromechanical system (MEMS) and printed circuit board (PCB) technologies to verify the coupling concept. Extra energy was transferred to the plasma produced by the metal foil electric explosion through the metal electrodes embedded in the side wall of the barrel, realizing the energy coupling between electric explosion and plasma discharge and then acquiring higher flyer velocity. Photonic Doppler velocimetry (PDV) was utilized to characterize and compare the flyer velocities of this mini electro-explosively actuator under uncoupled and coupled conditions. Finally, electrical characterization and plasma spectrum tests were performed to further understand the details of plasma discharge. The results confirmed that the proposed idea was conducive to the development of the electro-explosively actuator.

Microscale Analysis of Stress Wave Propagation Through Plastic Bonded Explosives Under Micro-Sphere Shock Impact

In this article, material stress during microscale shock wave propagation through hydroxyl-terminated polybutadiene (HTPB) binder and polycrystalline cyclotetramethylene-tetranitramine (HMX) based plastic bonded explosive is analyzed. An elliptical shock wave was created by 250 µm-size zirconia sphere impactors accelerated by a laser-based particle launch setup. Particle velocity and strain rate during shock wave stress rise were measured in the target using photon Doppler velocimetry. Viscous behavior of the HTPB binder was characterized through measurement of shock viscosity at strain rates higher than 106/s. Volumetric stress was measured at the interface region close to polycrystalline HMX particle and HTPB binder using in-situ time-gated Raman spectroscopy. Results show measured shock stress rise at the HTPB-HMX interface region is close to idealized simulation values. In-situ time-gated Raman spectroscopy predicts stress due to shock wave reflection from the HMX phase at the HTPB-HMX interface region. Predicted stress is higher than stress propagated through the binder because of impedance mismatch between HTPB and HMX.