Photonic Doppler Velocimetry System Research Articles

Characteristics of Step-Shaped Laser-Driven Flyers on Fiber End Face Based on a Ti/Al Ablation Layer.

Laser-driven flyers (LDF), which can launch the flyer on the interaction of a laser pulse with a thin film of metal, have been widely used in many fields, such as ignition, space scrap metal science, and dynamic high-pressure physics. However, at present, further development of LDF is being hindered by the high reflectivity of the ablation layer and low energy utilization efficiency of LDF on the fiber end face. Herein, improved LDFs were designed and fabricated by mask plate and magnetron sputtering. Improved LDF incorporates a Ti/Al composite film as the ablation layer, while the flyer layer features a smaller diameter round platform design. Reflectivity of samples under static and dynamic conditions and driving characteristics of samples were tested using an optical isolator and photonic doppler velocimetry system. The velocity of the improved LDFs reaches 1.7 km/s with a peak acceleration of 8.7 × 1010 m/s2. LDF with a Ti/Al composite film as the ablation layer demonstrates a static absorption rate of 59%, which gradually increases to 65% under laser irradiation. This absorption rate is notably higher compared with the static absorption rate (20%) and the peak absorption rate under laser irradiation (40%) of an Al layer. Consequently, there is a substantial improvement of about 35% in the flyer velocity. In contrast to the plane-shaped LDF, the velocity profile of the flyer and impact crater morphology suggest that the step-shaped LDF offers a 15% improvement in velocity and a 50% increase in acceleration, with better flyer integrity observed.

Modeling and validation of forming by vaporizing foil actuators

Sheet metal forming by means of vaporizing actuators attempts to utilize the explosion-like expansion of a cheap and simple aluminum foil, which occurs when a high capacitor discharge current heats it up to the gas phase within a very short time. Until now, mainly basic experimental investigations of this flexible, relatively young manufacturing process exist. In this work, the modeling of the complete forming process, i.e. including the expanding actuator, is conducted for the first time. A crucial parameter is the energy deposition until the so-called burst point, eventually representing an initial equation of state condition for the actuator in the forming simulation. The actuator is modeled using Smoothed Particle Hydrodynamics (SPH), while deformable solid bodies interacting with it are modeled by the Finite Element Method (FEM). The dynamic character of the process especially necessitates the incorporation of the strain rate-dependent constitutive behavior of the solids, which are a steel sheet and a protective polyurethane interlayer. Free forming histories of circular DC01 blanks are recorded with a Photon Doppler Velocimetry (PDV) system. The resulting curves validate the adopted modeling approach for three different, experimentally measured electrical energy depositions. Further aspects of the established numerical model are addressed: It is found, i.a., that the impulse of the bursting actuator is transferred to the blank through the elastomeric interlayer principally without changing the amplitude and effective area. The maximum free kinetic particle energy, however, restricts the possible energy available for deforming solids, and therefore expresses an upper bound for the process efficiency.

Experimental study on the pressure characteristics of laser-induced shock waves under different energy distribution

Laser shock peening has been widely used in anti-fatigue strengthening of metals. The different types of lasers (Flattop laser and Gaussian laser) used have influence on the pressure characteristics of shock wave and the strengthening effect. In this work, using PDV (photonic doppler velocimetry) system, the difference of peak pressure of Flattop laser-induced shock wave and Gaussian laser-induced shock wave is studied experimentally and theoretically. Research shows that the peak pressure of shock wave induced by Gaussian laser is higher than that of Flattop laser, because of the infinitesimal power density in the center of Gaussian laser spot is higher than that of Flattop laser. Furthermore, at the same power density, Gaussian laser can implant a larger and deeper residual stress field in the material.

Study on the cylinder test of aluminized explosives with different content of Al powder

To investigate the effect of the content of Al powder on the acceleration ability of the aluminized explosives, a series of 25mm cylinder experiments was conducted, and the content of Al/LiF powder is 0%, 15%, 30%. The radial expansion velocities of the cylinder wall were measured through the Photonic Doppler Velocimetry (PDV) system. The cylinder experiments verify that Al powder could react with detonation products to release energy, which enhance the accelerating ability of explosives. By comparing the different content of Al powder of aluminized explosives, it is found that the accelerating ability of aluminized explosives is related to the content of Al powder, and the optimal content of Al powder is 15% in this experiment. By comparing the same content of Al/LiF powder, it is indicated that the reaction delay time of Al powder in RDX80/Al15 explosive does not exceed 2.8µs, and as RDX65/Al30, the reaction delay time is less than 3µs. Finally, through normalization method, the specific reaction delay time of Al powder are 0.147 and 0.203, with the content of Al powder 15% and 30%.

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.

High performance laser-driven flyers based on a refractory metamaterial perfect absorber.

Laser-driven flyers (LDFs), which can drive metal particles to ultra-high speeds by feeding high-power laser, have been widely used in many fields, such as ignition, space debris simulation, and dynamic high-pressure physics. However, the low energy-utilization efficiency of the ablating layer hinders the development of LDF devices towards low power consumption and miniaturization. Herein, we design and experimentally demonstrate a high-performance LDF based on the refractory metamaterial perfect absorber (RMPA). The RMPA consists by a layer of TiN nano-triangular array, a dielectric layer and a layer of TiN thin film, and is realized by combing the vacuum electron beam deposition and colloid-sphere self-assembled techniques. RMPA can greatly improve the absorptivity of the ablating layer to about 95%, which is comparable to the metal absorbers, but obviously larger than that of the normal Al foil (∼10%). This high-performance RMPA brings a maximum electron temperature of ∼7500 K at ∼0.5 µs and a maximum electron density of ∼1.04 × 1016 cm-3 at ∼1 µs, which are higher than that the LDFs based on normal Al foil and metal absorbers due to the robust structure of RMPA under high-temperature. The final speed of the RMPA-improved LDFs reaches to about 1920 m/s measured by the photonic Doppler velocimetry system, which is about 1.32 times larger than the Ag and Au absorber-improved LDFs, and about 1.74times larger than the normal Al foil LDFs under the same condition. This highest speed unambiguously brings a deepest hole on the Teflon slab surface during the impact experiments. The electromagnetic properties of RMPA, transient speed and accelerated speed, transient electron temperature and density have been systematically investigated in this work.

Multi-wavelength crosstalk-free velocimetry demonstration and uncertainties.

Multiplexed photonic Doppler velocimetry is required to measure 2D surface velocities of novel materials in shock physics experiments. Very close measurements lead to crosstalk issues, which can be fixed by wavelength multiplexing. The benefits of our 16 wavelengths photonic Doppler velocimetry system over conventional systems are demonstrated here. First, a dedicated crosstalk comparative experiment has been carried out using a high-pulsed-power generator at about 80m/s. Second, velocity uncertainties are discussed with 16 wavelengths measuring the same point in an experiment at 130m/s.

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.

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.

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.

Multi-wavelength crosstalk-free photonic Doppler velocimetry.

Multiplexed photonic Doppler velocimetry systems are developed to measure velocities with high density in physics experiments such as shock physics experiments on novel materials. Decreasing the mesh size can lead to crosstalk issues, which can be overcome by wavelength multiplexing. Crosstalk has been characterized on a line of eight collimators with a pitch of 1 mm. A crosstalk-free photonic Doppler velocimetry system with 16 telecom wavelengths was built. Wavelength multiplexing provides as well a reduction in the total number of fiber components. The system was designed for velocities up to 1000 m/s and with a bandwidth of 2 GHz. In the frequency domain, the channel spacing is 100 GHz, which is more than enough to prevent any crosstalk. A ramp compression experiment was carried out by a high-pulsed-power generator to demonstrate the dynamic performances of the crosstalk free system at about 80 m/s.

A non-isentropic model of aluminized explosives involved with the reaction degree of aluminum powder for post-detonation burning behavior

The post-detonation burning effect of aluminum (Al) powder plays an important role during the expansion of detonation products (DPs) of aluminized explosives (AEs). Lithium fluoride (LiF) is an inert substitute for Al, and hence, a comparison of the performance of composite explosives based on cyclotrimethylenetrinitramine (RDX), such as RDX/Al and RDX/LiF, clearly illustrates its contribution to accelerating ability due to Al oxidation. A series of metal plate tests is conducted to measure the velocity history of a metal plate driven by RDX/Al and RDX/LiF through a photonic Doppler velocimetry system with 5%, 15%, and 25% Al or LiF contents. The detonation and expansion process of the AEs is generally divided into two stages: the detonation zone (DZ) and the post-detonation zone (PDZ). In the DZ, the Al powder remains inert, while it absorbs the detonation energy from pure explosives. Therefore, the equivalent inert dilution model is established and the equivalent inert dilution coefficient of the Al powder is introduced. In the PDZ, the Al powder reacts with DPs, and the Al oxidation reaction results with a change in entropy related to the reaction degree of the Al powder. Based on the local isentropic assumption, as well as the function of the reaction degree of the Al powder, a non-isentropic model is established. The method of the non-linear characteristic line is applied to theoretically calculate the metal plate velocity based on the non-isentropic model. In addition, the theoretical results show good agreement with the metal plate test results with an acceptable error (less than 10%), indicating that the non-isentropic model can be effectively applied to analyze the accelerating ability of the AEs.

Shock wave induced cavitation of silicone oils

The cavitation threshold of polydimethylsiloxane (silicone) oils was studied using the planar impact of flyer plates to generate large transient negative pressures within the liquids. The plate-impact experiments used a 64-mm-bore gas-gun to launch thin sabot-supported flyer plates onto liquid capsule targets in which a thin Mylar diaphragm formed a free surface at the back of the sample. The shock wave driven into the target capsule by the flyer impact placed the silicone oil in tension upon reflection from the rear free surface, eventually causing the sample to cavitate. The spall strength, or critical tension which cavitates the liquid, was determined by monitoring the free-surface velocity using a photonic Doppler velocimetry system. This study explored the effect of viscosity and loading strain rate on a system of three silicone oils having vastly different viscosities (4.8×10−2Pas to 2.9×101Pas), but otherwise similar properties. The spall strength was found to remain constant over the ranges of strain rate and viscosities probed in this work. A comparison of the experimental results to models for the cavitation threshold of liquids suggested that homogeneous nucleation of bubbles was the dominant mechanism for tension relief at the onset of cavitation.

A non-isentropic model and the method of characteristic line for the post-detonation expansion of aluminized explosives

The entropy of the flow field could not keep constant due to a strong exothermic reaction of aluminum powders during the post-detonation expansion of the aluminized explosives. Therefore, a non-isentropic model incorporating the aluminum oxidation in the detonation products was established. To solve the non-isentropic expansion process analytically, it was assumed that the whole expansion process was divided into several time ranges and the flow field was isentropic in each time range. Besides, the method of characteristic line was applied to theoretically calculate the velocity of the metal plate driven by aluminized explosives. Moreover, the effects on the pressure, density, sound speed, and temperature of detonation products due to the change of the entropy were analyzed. Finally, the metal plate-pushing tests were conducted to measure the velocity of metal plate driven by aluminized explosives through the Photonic Doppler Velocimetry system, and the degree of aluminum reaction was calculated indirectly from the test results. By comparing the results based on the isentropic model and novel non-isentropic model, it was proved that the non-isentropic model could more correctly describe the driving process of detonation products of aluminized explosives.

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.

Optimisation of modulation period of TiO2/Al reactive multilayer films for laser-driven flyer plates

Reactive multilayer films (RMFs) are a type of energetic nano-composite material and can be integrated into ablative layer structures using micro-electro-mechanical systems technology. They could potentially be applied to a laser-driven flyer detonator. In this study, TiO2/Al-based RMFs with two different modulation periods were integrated into multilayer flyer plates using magnetron sputtering technology, which produced nano-energetic multilayer films with optimised performance for laser-driven flyer plate systems. The effects of the TiO2/Al RMFs on the laser-driven properties of the RMF flyer were then systematically investigated in terms of the electron temperature and density using laser-induced breakdown spectroscopy. Measurements of the RMF flyer velocity were performed using a photonic Doppler velocimetry system. Electron temperatures of 6331.2–10563.0 K, electron densities of 4.03–12.69 × 1015 cm−3, and flyer velocities of 3854.7–6417.7 m/s were produced in the RMFs merely by changing the modulation period of the TiO2/Al RMF. This behaviour was also consistent with that found using a thermal conduction model in combination with a mass-diffusion reaction model. The RMF flyer exhibited a high level of integrity and promoted laser-driven energy efficiency, which are significant for improving the performance of a laser-driven flyer detonator for military and civilian applications.

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.

In-situ investigation of dynamic deformation in NiTi shape memory alloys under laser induced shock

The free surface particle velocity of NiTi target shocked with a pulsed laser beam was measured with a photonic Doppler velocimetry (PDV) system to study the dynamic deformation behavior of NiTi alloys at ultra-high strain rate of 106∼107/s. Through the analysis of the particle velocity profiles, the shock wave intensity was found to have the influence on the process of austenite-martensite transformation. Theoretical analysis of shock wave propagation showed that the first plateau in the velocity profile was caused by martensitic transformation and the second plateau if existing is caused by the subsequent plastic yielding of shock induced martensite. Residual martensite of the NiTi, which exhibited as needlelike structures, was observed in the laser shocked region. Based on the present results, and the studies by Nemat-Nasser et al. (2005), Liao et al. (2012), and Wang et al. (2013), we concluded that laser induced shock can cause the martensitic transformation as long as the laser induced shock pressure reaches a critical value. The dynamic transition stress and the dynamic tensile strength of NiTi alloys were also determined from the experimentally measured surface velocity profile.

A novel method for the measurement of the von Neumann spike in detonating high explosives

We present detonation wave profiles measured in T2 (97 wt. % TATB) and TX1 (52 wt. % TATB and 45 wt. % HMX) high explosives. The experiments consisted in initiating a detonation wave in a 15 mm diameter cylinder of explosive using an explosive wire detonator and an explosive booster. Free surface velocity wave profiles were measured at the explosive/air interface using a Photon Doppler Velocimetry system. We demonstrate that a comparison of these free surface wave profiles with those measured at explosive/window interfaces in similar conditions allows to bracket the von Neumann spike in a narrow range. For T2, our measurements show that the spike pressure lies between 35.9 and 40.1 GPa, whereas for TX1, it lies between 42.3 and 47.0 GPa. The numerical simulations performed in support to these measurements show that they can be used to calibrate reactive burn models and also to check the accuracy of the detonation products equation of state at low pressure.

Analysis of Fiber Optic Sensor to Measure Velocities During High Deformation Rate Material Forming Processes

Previous methods of measuring high velocities, e.g., during electromagnetic forming (EMF) and magnetic pulse welding processes where the workpiece is deforming, include photon Doppler velocimetry (PDV), laser micrometers, and high speed photography. In this paper, an alternative method is presented, implementing a fiber optic, reflectance dependent sensor. The sensor is shown to be an attractive, low purchase cost solution to measure high velocities. Data are shown with respect to sensor characterization including various surface reflectivity values, curvatures, and misalignments; implementation in two EMF/welding processes; and verification with high velocity PDV measurements. The sensor system is one twentieth the purchase cost of a PDV system, and yet measures velocities accurately (using PDV measurements as the reference) to at least 150 m/s provided that local curvature is not extreme and the displacement is less than approximately 27 mm. Sensor performance is also enhanced by the use of retroreflective tape, which is shown to increase the displacement range by 9×, decrease sensitivity to misalignment, and increase repeatability and ease of implementation.