Laser-driven Flyers 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.

Enhancing laser-driven flyer velocity by optimizing of modulation period of Al/Ti reactive multilayer films

Reactive multilayer films (RMFs), a type of nanostructured energetic material, are recognized as an indispensable component for laser-driven flyer plate initiator systems. In this work, Al/Ti-RMF with three different modulation periods (600, 300, and 150 nm) were prepared and integrated into multilayer flyer plates, and energetic material with optimized performance for laser-driven flyers was obtained. Cross-sectional observations demonstrate that the modulation periods of the RMF are precisely regulated, with thickness errors falling within 3.4%. The velocity of the flyer plates was significantly higher with a modulation period of 150 nm, reaching 2174.16 m/s. Molecular dynamics simulation results show that as the modulation period decreases, the diffusion rate of atoms increases, enabling the reaction between the RMF to be completed in a shorter time span, which makes for higher velocity of the flyer. The energy coupling efficiency results indicate that the kinetic energy coupling efficiency of the RMF with a modulation period of 150 nm is 145.6% and 29.8% higher compared to those with modulation periods of 600 and 300 nm, respectively. It is proved that Al/Ti-RMF have high-energy output performance and can be a novel candidate for laser-driven flyer plates, which will play a critical role in complex electromagnetic interference environments in the future.

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.

High-Strain Rate Spall Strength Measurement for CoCrFeMnNi High-Entropy Alloy

In this study, we experimentally investigate the high stain rate and spall behavior of Cantor high-entropy alloy (HEA), CoCrFeMnNi. First, the Hugoniot equations of state (EOS) for the samples are determined using laser-driven CoCrFeMnNi flyers launched into known Lithium Fluoride (LiF) windows. Photon Doppler Velocimetry (PDV) recordings of the velocity profiles find the EOS coefficients using an impedance mismatch technique. Following this set of measurements, laser-driven aluminum flyer plates are accelerated to velocities of 0.5–1.0 km/s using a high-energy pulse laser. Upon impact with CoCrFeMnNi samples, the shock response is found through PDV measurements of the free surface velocities. From this second set of measurements, the spall strength of the alloy is found for pressures up to 5 GPa and strain rates in excess of 106 s−1. Further analysis of the failure mechanisms behind the spallation is conducted using fractography revealing the occurrence of ductile fracture at voids presumed to be caused by chromium oxide deposits created during the manufacturing process.

Investigating growth to detonation in vapor-deposited hexanitrostilbene and pentaerythritol tetranitrate films using high-throughput methods

A high-throughput experimental setup was used to characterize initiation threshold and growth to detonation in the explosives hexanitrostilbene (HNS) and pentaerythritol tetranitrate (PETN). The experiment sequentially launched an array of laser-driven flyers to shock samples arranged in a 96-well microplate geometry, with photonic Doppler velocimetry diagnostics to characterize flyer velocity and particle velocity at the explosive–substrate interface. Vapor-deposited films of HNS and PETN were used to provide numerous samples with various thicknesses, enabling characterization of the evolution of growth to detonation. One-dimensional hydrocode simulations were performed with reactions disabled to illustrate where the experimental data deviate from the predicted inert response. Prompt initiation was observed in 144 μm thick HNS films at flyer velocities near 3000 m/s and in 125 μm thick PETN films at flyer velocities near 2400 m/s. This experimental setup enables rapid quantification of the growth of reactions in explosive materials that can reach detonation at sub-millimeter length scales. These data can subsequently be used for parameterizing reactive burn models in hydrocode simulations, as discussed in Paper II [D. E. Kittell, R. Knepper, and A. S. Tappan, J. Appl. Phys. 131, 154902 (2022)].

Promoted plasma generation for laser-driven flyer by MgAl hydride films

MgAl hydride films with different hydrogen contents were fabricated in Ar and H2 mixed atmosphere using magnetron sputtering. Neutron reflectometry was performed to measure hydrogen contents, which indicated that up to 11atom% hydrogen was absorbed into MgAl. The MgAl hydride films were irradiated using pulsed lasers and the electron temperature and density of the generated plasma was measured. Results showed the electron temperature and density at delay time of 1μs were 7330 K, 7.9 × 1015 cm−3 and 8044 K, 9.0 × 1015 cm−3 for MgAl hydride films with 5atom% and 11atom% hydrogen, respectively. For comparison, the electron temperature and density for pure MgAl film were 5634 K and 6.9 × 1015 cm−3. Obviously, the electron temperature and density increases with the hydrogen content. Subsequently, the conversion ratio of the shock wave energy to the incident pulse energy were analyzed by investigating plasma induced shock wave evolution. The conversion ratio for MgAl hydride films with 5atom% and 11atom% hydrogen was 19.2% and 28.6%, which was 1.2 and 1.8 times of that for the MgAl film. Therefore, the MgAl hydride film promised to be a new class of material capable of promoting the plasma generation of the laser-driven flyers.

Launch and impact characteristics of typical multi-layered flyers driven by ns-pulsed laser

Laser-driven flyer technology not only has been widely applied in micro-scale material processing and dynamic high-pressure physics but also possesses obvious advantages in improving the instantaneity and safety of shock initiation. To investigate the launch and impact characteristics between different flyers, six types of typical laser-driven flyers were designed and prepared via magnetron sputtering. The photonic Doppler velocimetry (PDV) and a self-made polyvinylidene fluoride (PVDF) stress sensor were used to collect the velocities and impact stresses of flyers driven by ns-pulsed laser, so that the kinetic energy coupling efficiency and shock initiation property were able to be obtained. Comparisons of characteristics between different flyers were implemented. The effect of nanothermite formed between aluminum and copper oxide layers was verified. The kinetic energy coupling relationship with laser and factors that influenced impact stress of flyers were discussed as well. The results are of significance in optimizing the launch and impact characteristics of laser-driven flyers for practical application.

Laser-Driven Flyers and Nanosecond-Resolved Velocimetry for Spall Studies in Thin Metal Foils

We describe a laser-launched micro-flyer apparatus designed for spall strength measurement. The launcher uses a single pulse from a pulsed laser that is stretched in time to nominally 20 nanoseconds using an optical ring cavity, while inexpensive multi-lens arrays are used to spatially homogenize the beam. The velocimetry technique that we developed for the experiment provides the required sub-nanosecond time resolution. We demonstrate the capability of the apparatus to interrogate the spall strength of AZ31B Mg alloy thin foils, a material system with potential applications as a lightweight protection material. Numerical simulations and fractography are very useful to determine the quality of the experimental data and help to interpret our results. The simulations and fractography analyses of the experiments suggest that the short shock pulse duration in the experiment causes incipient spallation. The short pulse also likely introduces stochasticity to the measured spall strength through limited activation of failure mechanisms within the samples. The shocked AZ31B Mg alloy has spall strengths that are greater than previously reported figures for fine grained Mg alloys, likely because the laser based system achieves higher strain rates than in prior work on this material.

ABLATION AND DRIVE OF TYPICAL MULTI-LAYER FLYERS BY HIGH POWER LASER BEAM

Laser driving technology is an important technology in high energy physics, shock wave physics and explosive initiation application. In this paper, velocities and impact stresses of four types of laser-driven flyers (Al, Al/Al2O3/Al, CuO/Al2O3/Al, (CuO/Al)/Al2O3/Al) prepared via magnetron sputtering were obtained by photonic Doppler velocimetry (PDV) and a self-made polyvinylidene fluoride (PVDF) stress sensor at ns-class pulse width.The results showed that velocities and integrity of flyers were influenced by their diameters. Compared with Al single-layer flyer, other multi-layer flyers with Al2O3 insulation layer owned lower velocity but better integrity and initiation properties.CuO/Al as the ablation layer enhanced the velocities and shock initiation properties of multi-layer flyers. Velocity of a flyer improved with the increasing laser energy while shock initiation property did not perform in the same way. These results provide some references for the design of multi-layer flyers applied to laserdriven initiation technology.

Experimental progress of laser-driven flyers at the SG-III prototype laser facility

Laser-driven flyers have unique advantages of high flyer velocity, low cost, simple facility compared with the flyers driven by other conventional dynamic high-pressure loading techniques. With the fast development of laser technique, launching hypervelocity flyers with high-intensity laser pulse has become more and more prevalent. In this paper, we introduce the recent experiments of laser-driven flyers at the SG-III prototype laser facility. Three ways of launching hypervelocity flyers are developed and introduced, respectively. In the first way, multilayered aluminum flyers are gradually accelerated to a terminal velocity of 8 km/s, which is measured by optical velocimetry, without melting and vaporization. The pressure distribution within the flyer shows that the temporally ramped pulse ablation generates a compression wave, and the flyer is accelerated by this wave and its reverberation within the flyer. In the second way, a strong laser ablates the low-density reservoir foil and generates strong shock in the foil. The shock wave is strong enough, and when the shock breaks out from the free surface, the foil will unload as plasma towards the flyer with a density profile. The plasma decelerates upon colliding the flyer, and the single-layered flyer is gradually accelerated by the momentum transition. In our experiments, single-layered aluminum foil and single-layered tantalum foil are accelerated to 11.5 km/s and 6.5 km/s, respectively. According to the pressure distribution within the flyer, the flyer is also accelerated by the compression wave produced by the plasma collision, which is similar to the case of direct ablation by temporally ramped pulse. However, the way of plasma collision could better reduce X-ray and electron preheat and obtain cleaner flyers. In the last way, the flyers are launched by direct strong short-laser ablation. The multi-layered aluminum foil is accelerated to a high average velocity of 21.3 km/s by using a 3-ns quadrate laser pulse at 351 nm after spatial homogenization. A line-velocity interferometer system for any reflect (VISAR) is employed to monitor the processes of flyer launch and flight in a vacuum gap and the shock velocity associated with phase change in fused silica target after flyer impact is inferred. The reflectivity variations of the VISAR fringe pattern and the shock velocity in the fused silica suggest that the flyer owns a density gradient characteristic. Furthermore, specifically designed multi-layered flyers (polyimide/copper) are accelerated by shock impedance and reverberation techniques to a super high averaged velocity of 55 km/s, which is much faster than recently reported results. Light-emission signals of shock breakout and flyer impact on flat or stepped windows are obtained, which indicates the good planarity and integrity for the flyer. Compared with single-layer flyers, multi-layered flyers have a good planarity, and a high energy conversion efficiency from laser to flyers. In this paper, we give a comprehensive analysis and comparison of the experimental designs, technique means and data results about laser-driven flyers. This would provide a reference for further experimental study of laser-driven flyers and also verify that the SG-III prototype laser facility is a very promising facility for studying the hypervelocity flyers launching field.

Hypervelocity launching of flyers at the SG-III prototype laser facility

Experiments of laser-driven hypervelocity flyers have been conducted at the SG-III prototype laser facility. Using the continuum phase plate technique, four laser beams each with a 3-ns quadratic profile are configured to produce relatively uniform irradiated spots of diameter size either 500 μm or 2000 μm. With the former, specifically designed multi-layered flyers (polyimide/copper) were accelerated by shock impedance and reverberation techniques via direct laser ablation to a super-high averaged velocity of 55 km/s, much faster than recently reported results. Light-emission signals of shock breakout and flyer impact on flat or stepped windows were obtained that indicated good planarity and integrity for the flyer. In the latter, single-layered aluminum flyers were gradually accelerated to a terminal velocity of 11 km/s, as measured by optical velocimetry, without melting and vaporization. The results suggest that the SG-III prototype laser facility has the capability to launch high-speed flyers to create extreme conditions for investigating the science of shock compression and its equation of state.

Fiber-coupled laser-driven flyer plates system

A system for the launch of hypervelocity flyer plates has been developed and characterized. Laser-driven flyers were launched from the substrate backed aluminum-alumina-aluminum sandwiched films. A laser-induced plasma is used to drive flyers with typical thickness of 5.5 μm and diameters of less than 1 mm, to achieve velocities of a few km/s. These flyer plates have many applications, from micrometeorite simulation to laser ignition. The flyer plates considered here have up to three layers: an ablation layer, to form plasma; an insulating layer; and a final, thicker layer that forms the final flyer plates. This technique was developed aiming at improving the energy efficiency of the system. The kinetic energy of flyers launched with the additional layer was found to be enhanced by a factor of near 2 (up to 30%). The optical fiber delivery system governs the output spatial profile of the laser spot and power capacity. Moreover, a technique for coupling high-power laser pulses into an optical fiber has been developed. This fiber optic system has been successfully used to launch flyer plates, and the surface finishing quality of the fiber was found to be an important factor. Importantly, measurements of the flyer performance including the mean velocities and planarity were made by an optical time-of-arrival technique using an optical fiber array probe, demonstrating the good planarity of the flyer and the achievable average velocity of 1.7 km/s with approaching 1 mm diameter. Finally, the relationship between flyer velocities and incident laser pulses energy was also investigated.

Fiber-coupled laser-driven flyer plates experiments

A system for the launch of hypervelocity flyer plates is developed, characterized and applied. Laser-driven flyers are launched from substrate backed aluminum-alumina- aluminum sandwiched films. A laser-induced plasma is used to drive flyers with a thicknesses of a 5.5 m and a diameter of less than 1 mm, and typically, the flyer plates can achieve velocities of a few kilometers per second. These flyer plates have several applications ranging from micrometeorite simulation to laser ignition. The flyer plates considered here have three layers: an ablation layer for forming plasma; an insulating layer; and a final thicker layer for forming the final flyer plate. The optical fiber delivery system determines the spatial profile of the laser spot and power capacity. A technique and procedure for coupling high power laser pulse into optical fiber is developed. The surface finish of the fiber is found to be a critical factor. This fiber optic system is successfully used to launch flyer plates. Measurements of the flyer performance including the mean velocities and planarity are made by an optical time-of-arrival (TOA) technique with using an optical fiber array probe. The flyer shows a good planarity and achieves an average velocity of 1.7 km/s. The relationship between flyer velocity and incident laser pulse energy is investigated.

Numerical simulation of the temperature field in laser-driven flyer plates by high power nanosecond laser–material interactions

A two-dimensional numerical model was proposed for the laser-induced temperature field and the acceleration process of laser-driven flyers by a high-intensity laser pulse. This model includes absorption of laser energy, equation of state, thermal parameters and the dynamic rupture strength of the metal film. Assuming that the working vapour can be treated as a typical ideal gas, a dynamics expansion mechanism was established for the vapour generated by laser irradiation. Using the finite difference method, the temperature distributions and the magnitude of flyer velocity with a laser energy up to 10 J were simulated for aluminium films of different thicknesses. The numerical results agreed with the experimental data but systematically underestimated the flyer speeds. The present results can provide some insights into understanding the mechanism of laser-driven flyer plates as well as experimental parameters for flyer plate design.

Integrity of thin, laser-driven flyer plates

Laser-driven flyers were launched from substrate backed aluminum films. They were produced by single pulses from a Q-switched Nd:yttrium–aluminum–garnet laser, of 10 ns duration and typically up to a few hundred mJ energy, with a focused spot size of approximately 1 mm diameter. The aluminum films were between 2 and 6 μm thick and the flyers typically achieved average velocities of a few km s−1. The integrity of the flyers was studied using streak photography and microscopy of the impacted flyers. Threshold times and distances prior to plasma penetration were used as a measure of integrity and were calculated for films of different thickness, launched with various laser pulse energies.

Measurement of the ablated thickness of films in the launch of laser-driven flyer plates

Laser-driven flyers were launched from substrate-backed aluminium films. The flyers were approximately 1 mm in diameter and a few micrometres thick and were produced by single pulses from a Q-switched Nd:YAG laser, of 10 ns full-width half-maximum and typically up to a few hundred millijoules energy. The ablated thickness of the film was measured using photo-emission spectroscopy to detect the vaporized tracer material in the laser-induced plasma: tracer layers of yttrium were deposited within the aluminium films at various depths, with sub-micrometre accuracy. Using this technique, the ablation depths were measured to be between 280 and 1100 nm for laser pulse energies between 50 and 156 mJ.