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Abstract
The thin flyer is a small-scale flying object, which is well known as the core functional element of the initiator. Understanding how flyers perform has been a long-standing issue in detonator science. However, it remains a significant challenge to explore how the flyer is formed and functions in the barrel of the initiator via tabletop devices. In this study, we present dynamic and unprecedented images of flyer in barrel via high intensity short-pulse laser. Advanced radiography, coupled with a high-intensity picosecond laser X-ray source, has enabled the provision of state-of-the-art radiographs in a single-shot experiment for observing micron-scale flyer formation in a hollow cylinder in nanoseconds. The flyer was clearly visible in the barrel and was accelerated and restricted differently from that without the barrel. This first implementation of a tabletop X-ray source provided a new approach for capturing dynamic photographs of small-scale flying objects, which were previously reported to be accessible only via an X-ray phase-contrast imaging system at the advanced photon source. These efforts have led to a significant improvement of radiographic capability and a greater understanding of the mechanisms of “burst” of exploding foil initiators for this application.
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1. Introduction
Exploding foil initiators offer multiple safety and timing benefits, establishing themselves as highly secure and reliable pyrotechnic devices extensively employed in detonation systems [1–3]. Their primary components include a reflector plate, bridge foil, flyer, barrel, and an inert explosive. When a strong electric current traverses the bridge foil, it causes the foil to explode, swiftly producing plasma that propels the flyer to adhere to the bridge foil. Subsequently, the flyer accelerates within the barrel and exits through the port at velocities reaching several kilometers per second. Upon exiting, it impacts and ignites the explosive column, leading to energy release [4–7] . The thin plastic flyer is the core functional element of the initiator. However, comprehending its performance has remained elusive over time. While current detonator modeling captures performance metrics, it doesn't provide a granular understanding of the flyer's initiation mechanics. This gap hinders the evaluation of safety margins and the formulation of new or enhanced designs [4,8]. Hence, there is an immediate demand to directly observe the flyer in action. Achieving this necessitates advanced X-ray phase-contrast imaging (PCI) with high temporal and spatial resolution [9–15]. Dynamic flyer imaging was conducted at the advanced photon source (APS), leveraging cutting-edge X-ray PCI [2,16]. This approach enabled direct observation of slapper flight dynamics and plasma instabilities with impressive temporal and spatial resolutions of 100 ps and 5 µm, respectively. However, capturing dynamic flyer imagery via tabletop devices remains a formidable challenge. There are limited studies on this subject, with most of the significant studies being conducted at large-scale facilities like APS [1,9,17–19].
In the initiator, the flyer is accelerated to a higher speed by a barrel, resulting in an enhanced conversion efficiency of electrical energy compared to when no barrel is used. The flyer can be shaped into a specific form using a hollow cylinder and then refined using confined plasma [20]. To directly observe the flyer within the barrel, dynamic X-ray radiography with high 2D resolution is crucial for discerning intricate details [17]. Moreover, to penetrate a cylindrical barrel, the X-rays employed must possess an energy of at least 20 keV. High-contrast radiography is imperative for this application, given that the absorption of the lightweight plastic flyer diminishes quickly as photon energy rises. Consequently, imaging the dynamic flyer within the barrel is more challenging than imaging it outside of the barrel, presenting a novel challenge for advanced X-ray radiography.
The remainder of this paper is organized as follows. In Sec. 2, we present an experimental setup for imaging a dynamic flyer in a barrel using a high-intensity short-pulse laser. A Monte Carlo simulation is conducted to enhance the contrast of the flyer in the barrel. In Sec. 3, the high-level X-ray radiograph is achieved via high-intensity short-pulse laser in single-shot experiments. The dynamic flyer is clearly visible within the barrel, and its characteristics are estimated from the images. The acceleration and restrictions of the flyer in the barrel are discussed. A summary of the study is provided in Sec. 4.
2. Experimental design and simulation method
2.1 Experimental design
The experiments were performed at the Xingguang III laser facility of the China Academy of Engineering Physics [21–25]. We achieved dynamic radiography of the flyer through the point projection from an X-ray source. This source was produced by the interplay between a high-intensity ps-pulse laser and metallic microwire target [24–29]. Figure 1 visually details the experimental setup. The picosecond pulse laser was situated at the equator and aimed at the microwire target. It imparted an energy of 100 J @ 1064 nm to the target within a 10 ps timeframe at a focal point. This focal point typically spanned 35 × 50 µm2, resulting in an intensity approximating 1018 W/cm2. The microwire, constructed on a freestanding CH substrate, has dimensions of 10 ± 2 µm in diameter and 500 µm in length. Under these stipulated conditions, the microwire exhibited a pronounced X-ray emission.
Fig. 1. a) Schematic view of the flyer inside the barrel imaged using a ps-laser, where the high-brightness X-ray emissions result from the interaction between the picosecond laser and microwire; b) energy spectra of the typical microwire using metals such as Cu, Mo, and Au; c) Kα emission measurements associated with the Mo microwire; d) schematic view of exploding foil initiator, in which the flyer is in the initial position.
The microwire's axis is precisely aligned with the objective, ensuring that the X-ray source is reduced to a singular point. This alignment facilitates high 2D resolution radiography through point-projection geometry. The X-rays pass through the flyer and Cu filter, and they are subsequently captured by a specialized high-energy X-ray instrument (HXI). [24] The HXI is uniquely designed to capture X-ray radiography, ensuring that scattered X-rays and hot electrons do not interfere with the radiographic image. This X-ray data is recorded on an imaging plate within the HXI and later scanned for interpretation. The objective's distance from the source is 30 mm, while the distance between the source and the detector is 450 mm. This results in a radiographic magnification factor of 15. The radiography's field of view is approximately 8.0 × 6.0 mm2, derived from a rectangular collection area measuring 120 × 90 mm2. Each pixel in the image represents 25 µm, corresponding to a real-world size of 1.67 µm in the field of view.
The spatial resolution of the high-energy X-ray radiography primarily hinges on the micro-size of the X-ray source and is determined to be 10 ± 2 µm, as referenced. The temporal resolution closely mirrors the duration of the picosecond pulse, ensuring motion blur is effectively minimized during the experiment. The diagnostic apparatus comprises a pinhole X-ray camera (PHC) and a filter stack spectrometer (FSS). The PHC captures the X-ray spot induced by the ps-laser, providing insight into the interaction between the picosecond pulse laser and the microwire target. Meanwhile, the FSS is instrumental in diagnosing the spectrometry of Bremsstrahlung X-ray emissions, ranging from 10 keV to 1.5 MeV.
2.2 Simulation method
A Monte Carlo code was utilized for the simulation of flyer radiography. This simulation model incorporated an X-ray source, a synthetic flyer, and a detector [11,12,30]. A diameter of 10 µm was assigned to the X-ray source with a divergence angle set at 30 mrad. The energy spectrum was derived from experimental measurements using a filter-stack spectrometer. The synthetic flyer, made of polyimide, contained an aluminum barrel. A distance of 30 mm from the source was established for this flyer. The thickness and diameter of the artificial flyer was 100 µm and 580 µm, respectively. The barrel had an internal diameter of 600 µm with a wall thickness of 200 µm. The detector is positioned 50 cm behind the flyer, as depicted in Fig. 1, and is configured as an SR imaging plate with a pixel size of 25 µm. High-flux X-ray photons were emitted in each simulation in accordance with the experimental X-ray spectrum. Energy deposited in each detector pixel was documented as a radiographic image. The signal-to-noise ratio (SNR) of the radiograph was calculated using the formula: SNR = average value/standard deviation. An increase in the brightness of the source resulted in an observed increase in the SNR.
Additionally, the physical processes, including X-ray absorption and scattering in the simulation, were considered by using the “standard EM” process. Moreover, we introduced an X-ray boundary process from the gate to simulate X-ray refraction at the geometry boundaries. This enables the phase-contrast image effect to be observed in the simulation results.
3. Results and discussion
High-energy X-ray and high 2D resolution radiography were performed for flyer-in-barrel imaging. The MC simulation showed that a high-SNR radiograph is essential for this purpose, and the phase contrast is strengthened for illustration. Dynamic imaging was performed using point-projection radiography at a high-intensity laser facility. The flyer behaved differently from one without a barrel. The acceleration and restriction mechanisms were analyzed.
3.1 High SNR radiograph via MC simulation and experimental improvements
An MC simulation, involving an X-ray source, an artificial flyer, and a detector, was utilized to optimize the contrast of the artificial flyer in the radiograph. The typical radiograph in Fig. 2 clearly shows the artificial flyer when the signal-to-noise ratio is increased to 20 with an estimated contrast of 0.5, which is sufficient to resolve the details. The absorption coefficient of the artificial flyer in the barrel of the Al alloy was estimated to be 3%. The phase contrast of the flyer was strengthened and compared with the results obtained using X-ray micro-CT. It should be noted that the diameter of the source is crucial for the phase contrast. Specifically, a diameter less than 5 µm results in a 10% increase in phase contrast for the flyer. It may be achieved via laser-plasma based X-ray source [31]. These simulation results provide insight into dynamic imaging experiments.
Fig. 2. Imaging of the artificial flyer in the barrel using various methods: Monte Carlo simulation, scalar diffraction, and hybrid simulation. The Monte Carlo simulation results lack phase contrast, whereas it is accentuated in the scalar diffraction results. Phase contrast, based on MC simulation and the embedded diffraction process, is achieved through the hybrid simulation. Results from micro X-ray CT and laser X-ray radiography align more closely with the hybrid simulation.
The signal-to-noise ratio is primarily determined by the number of X-ray photons, and a signal-to-noise ratio of 20 indicates that each pixel receives approximately 400 photons. To increase the SNR of the dynamic images in the experiments, it was essential to increase the brightness of the source. First, the microwire material was optimized, and the bremstraughters emitted from Mo, Cu, and Au were investigated. The three metals emit bremstraughters with similar spectra, but Mo is preferred because it emits higher radiation than the other two metals. In addition to the spectrum, a peak is observed near 20 keV due to Kα emission. The conversion efficiency from laser to X-ray demonstrates an increase in photon yield within the intensity range of 1–10 × 1018W/cm2. High-energy protons, that is generated at this intensity range, severely affect the quality of X-ray imaging, but they cannot be deflected or shielded by magnets or filters. This is a challenging problem that needs to be solved for high signal-to-noise ratio imaging. We employ proton deflection technology and utilize the characteristics of high-energy proton target back emission. The end face of the wire target is cut at a specific angle, which is offset from the imaging angle. In this way, emitted protons deviate from the imaging direction, avoiding interference with the imaging process.
In the radiograph, an artificial flyer within the barrel was visualized using a high-intensity short-pulse laser. The estimated signal-to-noise ratio (SNR) approached 15, aligning closely with an ideal radiograph's metrics. With the flyer's absorption coefficient at 2.5% of the background level, the radiograph mirrored an ideal case.
3.2 Dynamic flyer in barrel imaging
The flyer is driven via an exploding foil initiator and then accelerated in plastic and Al barrels for hundreds of nanoseconds. The dynamic image of the plastic flyer is shown in Fig. 3(a) and Fig. 3(b), where the inner diameter of barrel is 800 µm. At a certain moment post-burst, it has flown a distance of 188 µm from its initial position, and the center of the flyer is flat during flight. The width and thickness of the flyer are measured to be 543 µm and 37 µm, respectively. The diameter of the flyer was close to the inner diameter of the barrel, with the two sides bent to a certain radius and connected to the barrel wall. The density at the interface of the flyer and plasma was slightly higher than that at other positions.
Fig. 3. Imaging of the flyer inside (a) plastic barrel, (b) plastic barrel and (c) Al alloy captured at various intervals. The flyer appears distinct and predominantly undamaged, exhibiting intricate details. Over an extended delay, its traversed distance expands.
At a subsequent moment post-burst, with the flyer having traveled a distance of 377 µm, its center remains flat and its width is gauged at 558 µm. The sides curve to an even broader radius. However, it stays attached to the barrel wall, and the flyer's diameter stays proximate to the barrel's inner diameter. The thickness registers at 37 µm. During its trajectory, the plasma persists in accelerating, while the barrel shapes the flyer. Nonetheless, the plastic barrel might deform upon exposure to the high-temperature plasma bursts.
The behavior of the flyer within the Al alloy barrel was analyzed. Notably, even when exposed to the high-temperature plasma resulting from the exploding foil, the barrel maintained its form. The flyer's behavior in the Al alloy barrel deviated slightly from its actions in the plastic barrel. After an extended amount of time post-burst, as depicted in Fig. 3(c), the flyer moves 477 µm from the substrate, with its center remaining unchanged. Distinctly, its sides were not affixed to the barrel wall. The challenges posed by the low absorption coefficient necessitate refinements in X-ray radiography techniques for effective imaging.
3.3 Acceleration and restriction mechanisms of flyer in barrel
To gain a deeper insight into the barrel's acceleration and restriction mechanisms, the simulation model is compared with experimental outcomes under comparable conditions, as illustrated in Fig. 4. Utilizing ANSYS software, a secondary development was initiated, wherein custom user subroutines were crafted using the Fortran programming language. The derived effective energy was incorporated into the dynamic equation for explosive products, leading to the creation of a finite-element model for the shock plate transducer. Adhering to the material properties of each component, the reflector is alumina ceramics, which adopted the Gruneisen equation of state and J-C strength model [32]. Conversely, the exploding foil is copper, which integrated the Gruneisen equation of state with the segmented J-C strength model. The barrel is stainless steel, which employed the Gruneisen equation of state in tandem with the J-C strength model [33]. The acceleration of the polyimide flyer, given the barrel's constraint, was ascertained using this simulation framework [34]. Observations highlighted that the simulated flyer contour resonated closely with experimental data, especially when the barrel's inner diameter was consistently restricted to 600 µm. This revealed a rapid central acceleration compared with a more gradual edge acceleration during its flight. With the imposition of pulse-current loading, the exploding foil spawned a plasma environment characterized by elevated temperature and pressure. The propagation of this plasma was modulated by lateral rarefaction waves, resulting in an edge pressure on the flyer that was inferior to its central counterpart. As a consequence, the flyer's central segment experienced an initial boost in acceleration, culminating in a central velocity surpassing that at the edges. Concurrently, the inner section of the flyer endured tensile stresses. Upon surpassing the flyer's shear strength, this stress culminated in the complete detachment of the flyer's edge from the reflector.
Fig. 4. Comparison of the flyer's acceleration process derived from dynamic model calculations with experimental results for barrels of identical diameter. In the top image, captured after a brief delay, the flyer moves a minimal distance. Conversely, in the bottom image, taken after an extended delay, the flyer covers a considerably larger span.
Utilizing the devised model, variations in the reflector materials, mechanical attributes of the flyer, barrel material, and design parameters of the barrel-flyer-exploding foil combination were analyzed to determine their impact on the velocity and morphology of the airborne flyer. Influential factors on the sensitivity of explosive ignition were discerned from the short-pulse initiation criteria of heterogeneous explosives. This offers data-backed insights to enhance the reliability of the physical parameters in impact detonators.
4. Conclusion and future work
In this study, dynamic barrel imaging of a flyer is introduced, utilizing a high-intensity short-pulse laser. Through the point projection of the X-ray source derived from the picosecond laser, high-penetration and 2D resolution radiography are attained. A typical SNR of 15 is observed for the radiograph, a value notably high for single-shot radiography. The flyer's propulsion is initiated by an exploding foil, followed by its acceleration within the barrel over hundreds of nanoseconds. A vivid dynamic image of the flyer within the barrel is discernible. Notably, the flyer's shape inside the barrel differs from its shape outside of it. The characteristic parameters of this shape are subsequently estimated and scrutinized. The introduction of tabletop X-ray sources offers an innovative method for capturing dynamic images of minute flying objects. Future endeavors will aim to amplify the contrast of the flyer when situated inside the Al alloy barrel. Enhancements can be achieved by intensifying the X-ray source's brightness, which would elevate the SNR. Furthermore, by diminishing the source's diameter via laser-plasma based X-ray source, phase contrast can be improved.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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