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Underwater pulsed spark discharge has been widely used in industrial fields as a source of shock waves or acoustic waves, and numerical modeling of the discharge and pressure wave characteristics is necessary to improve the application performance. In this paper, a numerical model is proposed that couples the circuit equation, the mass and energy conservation equations, and a momentum conservation equation based on the Rankine–Hugoniot conditions. A tabulated wide range equation of state and conductivity data of water are used, and various physical processes during the plasma channel expansion are considered, including the energy flow and mass exchange between the channel and the surrounding water due to thermal radiation, evaporation, and condensation. The model self-consistently solves the circuit current and voltage, the plasma channel parameters including composition, temperature, conductivity, pressure, etc., and the pressure profile at a certain distance from the discharge channel. The calculated results show good consistency with the experimental measurements, and three sets of experimental results from other literature are tested to further verify the applicability and effectiveness of the model. [ABSTRACT FROM AUTHOR]

The x-ray spectrum of a flash radiography source based on wire-shorted rod-pinch diode (RPD) is measured by an absorption spectrometer and calculated by the Geant4 Monte-Carlo code. The RPD is driven by a two-stage linear transformer driver (charging voltage ±45 kV per stage), which delivers ∼300 kA current within ∼200 ns on a short-circuit inductive load (∼30 nH). The spectrometer consists of LiF thermoluminescence dosimeters and metal filters of different materials and thicknesses, and the spectrum is reconstructed based on the Geant4 simulated dosimeter responses to various photon energies through different filters. The measured spectrum reaches an endpoint photon energy of ∼130 keV. The radiation characteristics of the RPD in the experiment are also modeled in Geant4 using measured voltage and current waveforms, and the results show an overestimation in photon number and endpoint energy (∼147 keV) compared to the measured spectrum, which indicates that during the bremsstrahlung process, the effective electron current is ∼27% of the total current, and the effective voltage is ∼12% lower than the directly reconstructed resistive voltage. A rough agreement is found between the spectra results and the non-relativistic Bethe–Heitler model with Sommerfeld correction; therefore, the analytic result can serve as an initial guess for spectrum reconstruction methods such as the perturbation method. [ABSTRACT FROM AUTHOR]

A rod pinch diode (RPD) is a feasible load configuration to generate a high-brightness, small-size hard x-ray radiation source. In this paper, the radiography performance of a wire-shorted low-impedance RPD on a compactly designed table-top driver (WRPD-1) is demonstrated for the first time. The driver consists of four high-power discharge branches connected in parallel, with each branch consisting of two metal-film capacitors and one multigap field-distortion switch in series. The four branches are triggered synchronously to generate a fast-rising current pulse: the inductance of the load section at the short circuit is ∼10 nH, and the short-circuit current amplitude is ∼325 kA at ±90 kV charging voltage, with a 10%–90% rise time of 110 ns. With a low-impedance RPD shorted by an 18-µm-diameter aluminum wire, a quasi-spherical x-ray focal spot with diameter <0.6 mm (width of the half-maximum grayscale) and a pulse duration of ∼25 ns (half-width of the radiation pulse) is obtained at ±70 kV charging voltage, and the imaging resolution excels 10 lp/mm under 1.56× magnification. According to the transmission–absorption x-ray spectrum estimation, the average emitted photon energy is ∼30 keV with a distinct peak in the 10–15 keV range that corresponds to the L-lines of tungsten, and the total energy of photons >10 keV reaches ∼1.16 J. The present results show that the device can serve well for the flash radiography diagnosis and potentially as an efficient light source for dynamic x-ray diffraction. [ABSTRACT FROM AUTHOR]

Underwater electrical wire explosion is usually used as a source of underwater shock waves, and the generated shock wave intensity can be regulated by adopting a wire array. In this paper, the dynamics of the electrical explosion of two wires with different diameters connected in parallel were studied. The current flowing through each wire was measured to study the current distribution, and the self-emission image and laser backlight shadow image was obtained to observe the evolution of the wires. A numerical model was built to make explanations and predictions. The results showed that due to the presence of wire inductance and contact resistance between the wire and electrode, the wires are with different current densities and undergo asynchronous heating from the beginning of the discharge. Furthermore, the variation of current distribution among the wires was divided into six stages according to the moment the wire melted and exploded, and analyzed stage by stage. The unbalanced energy deposition results in obvious differences in thermal dynamic parameters, which explains the gradual "extinguishment" of the thinner wire in the self-emission image. [ABSTRACT FROM AUTHOR]

In this paper, Fe10Co25Ni34Cu23Al8 high-entropy-alloy nanoparticles have been synthesized by the in situ instantaneous electrical wire explosion method. Based on the thermodynamic parameters of the face-centered cubic phase of high-entropy-alloy, the parameters of the five kinds of wires were calculated and controlled, and the stable face-centered cubic structure with good crystallinity was synthesized in one step. The influence of the energy deposition during the electrical explosion on the nanoparticles and their electromagnetic absorption performance was explored. The results show that the face-centered cubic structure high-entropy-alloy with high crystallinity has better electromagnetic wave absorption performance when the energy deposition of the wires is increased. The minimum reflection loss can reach −39.37 dB at 15.34 GHz when the thickness is 1.9 mm and the effective absorption bandwidth is 6.63 GHz. It can provide a strategy for the microstructure and morphology design of high-entropy-alloy nanoparticles in electromagnetic wave absorption and magnetism in the electronics industry. [ABSTRACT FROM AUTHOR]

The radiation properties including size, intensity, and pulse width of the double-wire X-pinch were optimized using prepulse current. The optimization mechanism was investigated using optical diagnostics. The X-pinch driven by a current with a peak intensity of ∼250 kA and a rise rate of 0.95 kA/ns was difficult to produce strong radiation. The application of prepulse effectively enhances the current rise rate and intensity of the radiation source. The radiation source also exhibited reduced spatial dimensions and radiation duration, thereby improving spatial and temporal resolution while used in x-ray projection shadow radiography. The magnetic field and current distribution were measured using Faraday rotation. During the early stage, the current predominantly flows in the corona plasma at a larger radius generated by the prepulse current, consequently reducing the load inductance. As the time delay between the main and prepulse current increased, the intensity of the radiation source further increased. This study provides an approach for controlling radiation sources and enables different applications of X-pinches with adjustable prepulse current. [ABSTRACT FROM AUTHOR]

As a new method for generating strong underwater shock waves with rapid repetition frequency, the use of underwater electrical-wire explosion (UEWE) to drive insensitive energetic materials has attracted increasing research attention in recent years. Accordingly, equipment based on this new method have been developed. The ability to measure the stress produced by an UEWE on a device plays a very important role in optimizing the device performance. However, in conventional stress measurements, the spatial electromagnetic interference (EMI) produced by the discharge can affect the measurement accuracy or even damage the experimental instruments. In this study, a novel system for measuring stress in a strong electromagnetic field, based on a piezoelectric film and a conditioning circuit, was debugged and evaluated. Shielding was used to eliminate the intense EMI due to the strong electromagnetic field. Our simulation and experimental results demonstrate that the stress generated can be quickly determined by measuring the output voltage of the conditioning circuit. The new system can be used to measure the stress at the fluid–solid interface under a strong electromagnetic field environment. [ABSTRACT FROM AUTHOR]

The influences of the prepulse current on the implosion dynamics of planar wire array were investigated. The time-delay between the prepulse current and the main current (Tdelay) was able to be controlled manually based on the double pulse current generator "Qin-1." In the precondition stage by the prepulse current, the corona plasma, aluminum vapor, and residual wire cores formed during the explosion of the wires, and ∼40% (±10%) mass of the wires was in a gaseous state at ∼425 ns after the prepulse. After the main pulse was applied, the low-density corona plasma was first imploded and then collided with the aluminum vapor and residual dense cores. Then, the further implosion of the preconditoned wires closely related to their mass distribution, which was determined by the duration of Tdelay. The residual dense wire cores had a significant impact on the implosion when Tdelay was ∼200 ns. When Tdelay increased to ∼> 500 ns, the mass distribution gradually became uniform, and the implosion of the preconditioned wires showed no ablation and no trailing mass. [ABSTRACT FROM AUTHOR]

Measurement of the magnetic field distribution in Z-pinch experiments remains an ongoing challenge. We present a method of measuring the radial distribution of the magnetic field around a copper rod using Zeeman splitting of sodium (Na) emission lines, in which an Na layer is formed by the laser ablation of NaCl crystals on a load surface. The load consists of a copper rod of 2 mm diameter and is pre-covered on its surface by the NaCl crystals. An 8 ns pulsed laser with an energy of 1 J and wavelength of 532 nm is focused on the crystals. The Na plasma is produced and expands from the surface of the copper rod into a vacuum. After applying a pulsed current with a peak value of 375 kA to the load, the Na 3s–3p doublet displays significant Zeeman splitting patterns. The self-luminosity of the Na plasma is recorded by a spectrometer coupled with an intensified charge-coupled device camera from an end-on view to eliminate the effects of different observing angles and Doppler shifts. We determine the magnetic field by fitting the measured spectra with the calculated results of the Voigt profile. The measurable range of radial position is 5–7 mm, and the corresponding magnetic field is 5–15 T. The averaged error of curve fitting is less than 12%. [ABSTRACT FROM AUTHOR]

Using laser shadowgraphy and interferometry on a Qin-1 facility, the initial plasma formation and dynamics of an exploding stainless steel liner were investigated. To obtain the absolute electron density distribution inside the liner, we established continuous wave laser interferometry using a streak camera to measure the shift in the fringes over time. Plasma is generated at the interior wall and flows toward the center with a velocity of ∼100 km/s, thus forming a column with higher density by accumulation. Simultaneously, a high-density plasma layer is formed near the interior surface and this layer flows toward the center at approximately 10 km/s. In addition, magnetohydrodynamics (MHD) instabilities were observed at the exterior surface using side-on laser shadow images at a much later time (∼400 ns). The growth in the amplitude and the wavelength of the perturbations were then analyzed. An MHD simulation of this process was then established to demonstrate that the high-density plasma layer carries part of the current and that it flows within the 10 km/s range after comparison with the experimental results. Finally, we measured the voltage and derived the change in the inductance. The results prove that part of the current flows through the center plasma column, which then influences the subsequent plasma flow. [ABSTRACT FROM AUTHOR]

A new approach has been presented herein to prepare nano titanium carbide based on the underwater electrical explosion approach. Scanning electron microscopy and x-ray photoelectron spectroscopy were used to investigate the morphology and composition of the electrical explosion products. A numerical model was established to investigate the nanoparticle formation process. The results show that the average diameter of the formed nanoparticles was ∼60 nm and approximately conformed to a lognormal distribution. Compared with the nanoparticles prepared by electrical explosion in gas, the nanoparticles prepared by the underwater electrical explosion had a smaller size distribution range and better sphericity. During the formation process of nanoparticles, the distribution of nanoparticles formed in a narrow temperature range near the specific temperature directly determined the characteristics of the final electrical explosion products. The specific temperature was ∼3400 K, which was also the specific temperature of the saturation ratio, the nucleation rate, the average diameter of the formed nuclei, the number of monomers, and the number of the formed nanoparticles. The diameters of nanoparticles obtained in the experiment were mainly concentrated between 50 and 70 nm, and the calculated diameters of the nanoparticles were mainly concentrated between 55 and 65 nm; therefore, the data obtained through the model were consistent with the experimental ones. These provide a way to synthesize the nano titanium carbide and a method to estimate their size and distribution, and it is hoped for understanding the evolution of the titanium wire underwater electrical explosion and the formation of nanoparticles. [ABSTRACT FROM AUTHOR]

The main factor that slows down the high-frequency response of a resistive voltage divider (RVD) is the distributed stray capacitance (Cg) between the high-voltage-arm (HVA) and the grounded conductors, due to the charging and discharging of Cg through the high resistance RH of the HVA with characteristic time RHCg. Based on a RVD consisted of ceramic tube resistors, a compensation method utilizing the distributed capacitance between a specially shaped inner conductor and the HVA was proposed in this paper, which is more compact than grading rings and can work well with grounded shielding. The method was verified by electromagnetic simulation, which indicated a bandwidth improvement from 3 MHz to more than 1 GHz for the prototype RVD with grounded shielding. Experimental results showed that the 10%–90% rise time for a step input was improved by the compensating electrode from ∼90 ns to 1.25 ns. The main drawback of the method is the possible degradation of insulation when precise compensation is required. [ABSTRACT FROM AUTHOR]

Underwater electrical explosions of Cu wires were carried out on a microsecond time scale to produce underwater shock waves. Experimental results show that the radial density distribution of the water flow after restrike contains several oscillations, observed as ∼1 mm-spaced layers in the backlit streak images and laser shadowgraphs. The phenomenon is attributed to the partial reheating of the exploding product (EP) by an interior restrike arc, which stimulates a compression wave propagating back and forth radially in the EP. Simulations are used to support the interior breakdown scenario and to demonstrate that each reflection of the compression wave at the EP–water interface launches a weak shock into the water, forming a multilayer structure. As the surrounding metallic vapor is ionized due to radiation and thermal conduction from the arc, the highly conductive plasma channel continues to extend radially and launches the main compression wave, which drowns out the multilayers when the power injection is sufficiently high. [ABSTRACT FROM AUTHOR]

The density distribution is important information in the investigation of electrical exploding wires in the air. In this study, the density profiles of the electrons, tungsten atoms, and air at different instants were reconstructed based on a two-wavelength interferometry method. The experiment was carried out on a 1 kA, 0.1 kA/ns pulsed current generator, with a fine tungsten wire (10 μm in diameter). The laser probing images of the exploding products showed a two-layer structure, exhibiting a shunting discharge scenario. The fitted expanding trajectory of the dense core indicates that the expansion of the wire starts at the instant of the voltage drop. The reconstructed densities show the distribution of particles in the expansion process of the exploding wire. It is found that the wire core has a tube-like structure, and the plasma channel is located around the core boundary. [ABSTRACT FROM AUTHOR]

Electrical explosion of wire provides a practical approach to the experimental measurement of dynamic polarizability of metal atoms with high melting and boiling temperatures. With the help of insulation coating, a section of tungsten wire was transformed to the plasma state while the near electrode region was partially vaporized, which enabled us to locate the “neutral-region” (consisting of gaseous atoms) in the Mach-Zehnder interferogram. In this paper, the polarizability of the tungsten atom at 532 nm was reconstructed based on a technique previously used for the same purpose, and the basic preconditions of the measurement were verified in detail, including the existence of the neutral region, conservation of linear density of tungsten during wire expansion, and neglect of the vaporized insulation coating. The typical imaging time varied from 80 ns to as late as 200 ns and the reconstructed polarizability of the tungsten atom was 16 ± 1 Å3, which showed good statistical consistency and was also in good agreement with the previous results. [ABSTRACT FROM AUTHOR]

The physical process of electrical explosion of wires in vacuum is featured with the surface discharge along the wire, which generates the corona plasma layer and terminates the Joule heating of the wire core. In this paper, a fiber-array probe was designed to directly measure the radiation of surface arc with spatial and temporal resolution. The radiation of the exploding wire was casted to the section of an optical-fiber-array by a lens and transmitted to PIN diodes and finally collected with an oscilloscope. This probe enables direct diagnostics of the evolution of surface discharge with high temporal resolution and certain spatial resolution. The radiation of a tungsten wire driven by a positive current pulse was measured, and results showed that surface discharge initiates near the cathode and propagates toward the anode with a speed of 7.7 ± 1.6 mm/ns; further estimations showed that this process is responsible for the "conical" structure of the exploding wire.