Electrical explosion of a wire (EEW) is performed by passing a pulsed high current through a metallic wire.
Matter and Radiation at Extremes, Volume. 4, Issue 1, 017201(2019)
Research at Tsinghua University on electrical explosions of wires
Electrical explosion of a wire (EEW) has been investigated for more than ten years at Tsinghua University, and the main results are reviewed in this paper. Based on EEW in vacuum, an X-pinch was used as an x-ray source for phase-contrast imaging of small insects such as mosquitoes and ants in which it was possible to observe clearly their detailed internal structures, which can never be seen with conventional x-ray radiography. Electrical explosion of a wire array (EEWA) in vacuum is the initial stage in the formation of a wire-array Z-pinch. The evolution of EEWA was observed with x-ray backlighting using two X-pinches as x-ray sources. It was found that each wire in an EEWA exhibits a core–corona structure instead of forming a fully vaporized metallic vapor. This structure is detrimental to the plasma implosion of a Z-pinch. By inserting an insulator as a flashover switch into the cathode, formation of a core–corona structure was suppressed and core-free EEWA was realized. EEW in gases was used for nanopowder production. Three parameters (vaporization rate, gas pressure, and energy deposited in the exploding plasma) were found to influence the nanoparticle size. EEW in water was used for shock-wave generation. The shock wave generated by melting could be recorded with a piezoelectric gauge only in underheat EEW. For EEW with a given stored energy but different energy-storage capacitor banks, the small capacitor bank produced a rapidly rising current that deposited more energy into the wire and generated a stronger shock wave.
I. INTRODUCTION
Electrical explosion of a wire (EEW) is performed by passing a pulsed high current through a metallic wire.
Figure 1.Current waveforms for the three discharge modes.
EEW can be conducted in a variety of surrounding media (vacuum, gas, liquid, or solid) and behaves differently in each case, allowing different applications. In this paper, we present experimental results on EEW obtained at Tsinghua University, including EEW in vacuum for X- and Z-pinches, in gases for nanopowder production, and in liquids for the generation of shock waves.
II. EEW IN VACUUM FOR X- AND Z-PINCHES
A. EEW in vacuum for an X-pinch
For use as the load of a pulsed power generator, an X-pinch is formed in vacuum from two fine wires in the form of an “X” and touching at a single point. As a result of compression by the magnetic field generated by the current in the wires, the plasma at the point of contact forms an extremely hot and dense point (X-pinch) that emits intense pulsed x-rays.
Figure 2.Sketch of an X-pinch.
Figure 3.(a) Typical image of x-ray point source and (b) waveform of an x-ray pulse from an X-pinch.
The X-pinch device used in our laboratory is a compact and portable device, PPG-2, with five components: a Marx generator, a pulse forming line (PFL), a V/N switch, a pulse transmission line (PTL), and an X-pinch load. The Marx generator consists of six 0.15-μF capacitors charged in parallel to a maximum voltage of 60 kV and then discharged in series to the PFL. The PFL uses deionized water as insulating dielectric and is 1 m long, forming a 60 ns pulse. The characteristic impedance of the PFL is 1.25 Ω. The parameters of the PTL are the same as those of the PFL, except that its length is only 0.3 m for the sake of compactness. PPG-2 is capable of delivering to the X-pinch load a current of amplitude 100 kA and pulse width 60 ns.
There are two kinds of x-ray radiography: conventional radiography and phase-contrast radiography (PCR). While conventional radiography relies on x-ray absorption as the sole source of contrast, PCR uses phase variations as an additional source of contrast by recording the interference between x-rays passing through neighboring parts of the sample.
Since PCR is based on the interference of x-rays, it requires from the x-ray source a long spatial coherence length that is proportional to the ratio d/σ, where d is the distance from the source to the observation point and σ is the source size. Thus, high spatial coherence may be achieved by using a source of small size or by observing the beam at a large distance from the source. The intense and small x-ray source makes an X-pinch perfect for PCR. We used such an X-pinch x-ray source and took PCR images of small insects such as ants and mosquitoes.
Figure 4.PCR image of a 3-mm-long ant.
Based on our experimental results, the X-pinch has proved to be an ideal x-ray source for PCR of weakly x-ray-absorbing samples, which makes it promising for clinical and biological applications. For example, in mammography, there is a need to distinguish between different kinds of soft tissue, namely, tumors and normal tissue. Because absorption in such cases is small, and the differences in density and composition between the different tissues are slight, conventional x-ray radiography is not very successful at this task.
B. EEW in vacuum for a wire-array Z-pinch
A wire-array Z-pinch is constructed by using a cylindrical array of wires in vacuum as the discharge load. The wire array is several centimeters in diameter and made of hundreds of fine wires.
Figure 5.Physical process involved in the formation of a wire-array Z-pinch.
When a pulsed high current flows through a wire array, all the wires are electrically exploded into a cylindrical plasma shell in a process called electrical explosion of a wire array (EEWA). Under compression by the magnetic field generated by the current flowing through the plasma shell, plasma implosion occurs. As the radius of the plasma shell becomes smaller and smaller owing to the implosion, the plasma density and temperature become higher and higher. When the magnetic compressive force is balanced by the thermal pressure inside the compressed plasma shell, the plasma stagnates near the axis of the plasma shell, and intensive pulsed x-rays are emitted by the extremely hot pinched plasma.
In 1997, a breakthrough was made in Z-pinch research at Sandia Laboratories,
The experiment was performed on a pulsed power generator, PPG-1, capable of delivering to the load a 400 kA current in 100 ns.
The experimental arrangement for backlighting of the wire-array Z-pinch using two X-pinches as the x-ray sources is shown in
Figure 6.Experimental arrangement for x-ray backlighting of wire-array Z-pinch.
Figure 7.Typical images of EEWA with two molybdenum wires of diameter 50 μm spaced 2 mm apart: (a) 61 ns, 172 kA; (b) 67 ns, 188 kA. (c) Waveforms of the current and x-ray pulses.
From
Much effort has been expended on suppressing the formation of a core–corona structure.
A method for realizing core-free EEW has indeed been found, by inserting an insulator as a flashover switch into the cathode, as shown in
Figure 8.An insulator as a flashover switch inserted in the cathode to realize core-free EEW.
The flashover switch has two favorable effects on EEW. First, it steepens the voltage pulse applied to the wire and makes flashover along the wire surface more difficult. Second, it raises the electric potential of the wire relative to the cathode, because there is still a potential drop across the insulator in the process of surface flashover over the insulator, which causes the radial electric field on the wire surface to change from negative to positive in the case where a negative pulsed voltage is applied to the cathode and the anode is grounded. Here, a positive radial field is defined as one in which the direction of the field is outward from the wire surface.
The radial fields on the wire surface at the time at which wire melting begins were calculated with electrostatic simulation using COMSOL software. The results are shown in
Figure 9.Radial electric fields on the wire surface.
The validity of the method was verified by calculation of the deposition energy before the flashover along the wire surface and by taking interferograms of EEWs with and without the flashover switch. Here, we take as an example EEW of a tungsten wire of length 1 cm and diameter 12.5 μm. For tungsten, the deposition energy required for the whole wire to completely vaporize is 8.8 eV/atom; i.e., the vaporization energy Ev for tungsten is 8.8 eV/atom.
The deposition energy can be calculated as follows:
For EEW without the flashover switch, the deposition energy is only 3.4 eV/atom and is thus significantly lower than Ev. Correspondingly, a dense core is visible in the center of the interferogram, as shown in
Figure 10.Interferograms of EEW (a) without and (b) with a flashover switch.
In summary, then, we have developed a new method of inserting a flashover switch into the cathode to realize core-free EEW of a single wire in vacuum. Although the flashover switch can also be considered as a peaking switch, the effect of which in steepening a voltage pulse is well known, a second effect of this flashover switch in changing the radial field of the wire surface from negative to positive has been reported here for the first time. This second effect reduces the electron emission from the wire surface, which plays a more important role in realizing a core-free EEW. We are now applying this method to wire arrays and hope that it will similarly be successful in realizing core-free EEWA.
III. EEW IN GASES FOR NANOPOWDER PRODUCTION
When EEW is performed in a gas at pressures from 0.1 to 1 atm, the metallic vapor is cooled by collisions with gas molecules, with nanoparticles being formed from the condensed vapor.
Figure 11.Experimental setup of EEW for nanopowder production.
We investigated how different experimental conditions affect nanoparticle size by changing the wire (material and diameter), the gas (gas type and pressure), and the charging voltage of the energy-storage capacitors.
As can be seen from the results in
Figure 12.Dependences of deposition rate
Figure 13.TEM images of nanopowders obtained with charging voltages of (a) 9 kV and (b) 24 kV.
The specific surface area S of the nanoparticles was measured using the Brunauer–Emmett–Teller (BET) method of multilayer gas absorption. The average diameter of the nanoparticles was calculated fromwhere ρ is the density of the nanoparticles.
Figure 14.Dependences of specific surface area and average diameter of nanoparticles on deposition rate
The dependence of nanoparticle size on nitrogen pressure was also investigated, with the charging voltage being kept at 18 kV.
Figure 15.Dependences of specific surface area and average diameter of nanoparticles on nitrogen pressure at a charging voltage of 80 kV.
It was found that there are three parameters that determine the size of the nanoparticles produced by EEW. The first parameter is the deposition ratio. A higher η favors complete vaporization of the wire, without any residual liquid drops, which is good for obtaining smaller nanoparticles. The second parameter is the gas pressure. A lower pressure is favors faster vapor expansion and cooling, leading to smaller nanoparticles. The final parameter is the plasma energy. A lower Wp will not heat the surrounding gas to a high temperature, and thus will favor faster vapor cooling and smaller nanoparticles.
In summary, EEW in different gases is an easy way to get different nanopowders. However, there are two improvements that need to be made before this method can move to industrial application. First, there must be greater uniformity in nanoparticle size. Second, the method must be adapted to allow automatic and continuous wire feeding.
IV. EEW IN LIQUID FOR SHOCK-WAVE GENERATION
When EEW is performed in a liquid, shock waves are generated by the rapid volume expansions accompanied by fast phase transitions.
A. Underheat experiment
EEW was performed in a water chamber of diameter 560 mm and height 500 mm. The copper wire to be exploded was immersed in the water inside the chamber. A 1-μF energy-storage capacitor was placed inside an oil tank outside the water chamber. In the underheat experiment, the energy-storage capacitor was charged to a relatively low energy so that the energy deposited into the wire was not large enough to completely vaporize the wire. Through a field distortion switch and a high-voltage cable outside the water chamber, the energy-storage capacitor discharged to the wire inside the water chamber. The waveforms of the discharge current i(t) and the wire voltage u(t) were directly measured with a Pearson 101 Rogowski coil (with a bandwidth of 4 MHz) and a Tektronix P6015A voltage divider (with a bandwidth of 75 MHz), respectively. A Piezotronics PCB138 pressure probe was placed 100 mm away from the wire and used to record the waveform of the shock wave.
It was found that the shock waves had poor reproducibility owing to only partial vaporization of the exploding wire. Although a partial vaporization rate as low as 6% is enough to generate a shock wave, the amplitude of the latter depends strongly on the vaporized mass of the wire. The greater the vaporized mass, the stronger is the shock wave.
It is widely accepted that the generation of shock waves by EEW in water is due to rapid volume expansion accompanied by phase transitions, such as melting of solid, vaporization of liquid, and formation of plasma. In the past, the formation of shock waves by wire melting could only be observed using expensive streak cameras that are not available in most laboratories. The reason why shock waves generated by melting have never been observed with commonly used piezoelectric gauges is as follows. To avoid damage by the wire explosion, a piezoelectric gauge must be placed some distance from the wire. By the time it has reached such a distance, a shock wave generated by melting will have been overtaken by and immersed within the stronger and faster shock wave generated by vaporization and therefore will not be detected by the piezoelectric gauge. However, it was found that in an underheat experiment, the partial vaporization generates a significantly weaker and slower shock wave that is unable to overtake the shock wave generated by melting, and the latter can therefore be detected by a piezoelectric gauge.
Figure 16.Typical shock-wave pressure waveform showing two shock waves (SW): one generated by melting and the other by vaporization.
The pressure–time curves in
Figure 17.Process by which the shock wave generated by vaporization overtakes the shock wave generated by melting as
B. EEW at a given storage energy for two different capacitor banks
In the experiment to investigate the effect of the current rise rate on EEW, the experimental setup was almost the same as that used in the underheat experiment. Two capacitor banks of 1 μF and 200 μF were used. They were charged to different voltages for the same storage energy in the range from 70 J to 1.8 kJ. In this case, the rise rate of the current using the small capacitor bank of 1 μF was much greater than that using the large capacitor bank of 200 μF. It was found that the resulting EEWs were significantly different. Here we take as an example the EEW of a copper wire using the two capacitor banks, each charged to an energy of 121 J. The copper wire was 0.12 mm in diameter and 50 mm in length, corresponding to a complete vaporization energy of 31 J. A comparison of the parameters for EEW using these two capacitor banks is given in
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The discharge mode of EEW was the cutoff current mode when the 200-μF capacitor bank was used, but the restrike mode when the 1-μF bank was used, as shown in
Figure 18.Comparison of discharge modes: (a) cutoff current mode with the 200-μF capacitor bank; (b) restrike mode with the 1-μF capacitor bank.
In summary, in previous work, the focus has usually been on experiments with overheat, in order to produce as strong as possible a shock wave. In our underheat experiment, we hoped to find new phenomena and investigate the underlying physics. In this experiment, we observed a phenomenon in which the shock wave generated by vaporization overtook the shock wave generated by melting. By comparing the results of an overheat experiment using a 1-μF capacitor bank with those of an underheat experiment using a 200-μF bank, we found significant differences in the rise rates of the current and in the discharge modes, leading to a great difference in the amplitudes of the shock waves generated. We are now trying to use shock waves generated by this technique to increase production and enhance recovery in oil wells.
V. CONCLUSIONS
EEW in vacuum was used to produce an X-pinch that proved to be an ideal point x-ray source for phase-contrast imaging of weakly x-ray absorbing samples such as small insects like mosquitoes and ants. The images show clearly the internal structures of these samples, which cannot be seen with conventional x-ray radiography, indicating the potential biological and clinical applications of this technique. We are now examining the possibility of using this technique in mammography, in which there is a need to distinguish between different kinds of soft tissue, namely, tumors and normal tissue. Because the absorption is small, and the differences in density and composition are slight, conventional x-ray radiography is not very successful at this task.
EEW in vacuum was used to produce a wire-array Z-pinch. Electrical explosion of a wire array (EEWA) often exhibits a core–corona structure that is detrimental to the plasma implosion of a Z-pinch. By inserting an insulator as a flashover switch into a negatively biased cathode, the formation of the core–corona structure was suppressed and a core-free EEWA was realized. Although the flashover switch can also be considered as a peaking switch, with a well-known effect in steepening the voltage pulse, a second effect of this flashover switch in changing the radial field of the wire surface from negative to positive was reported here for the first time. This second effect suppresses electron emission from the wire surface and thereby plays a more important role in realizing a core-free EEW. We are now applying this method to wire arrays in the hope that it will be similarly successful in realizing core-free EEWA.
EEW in gases was used for nanopowder production. Three parameters (vaporization rate, gas pressure, and energy deposited in the exploding plasma) were found to influence the nanoparticle size. Although EEW in different gases is an easy route for the production of different nanopowders, there are two obstacles to be overcome before this method can be applied in industry: the uniformity of nanoparticle size needs to be improved, and techniques for automatic and continuous wire feeding need to be developed.
In previous studies of the generation of shock waves by EEW in liquids, the focus was usually on experiments with overheat, in order to obtain shock waves that were as strong as possible. The aim of our underheat experiment, in contrast, was to discover new phenomena and the underlying physics. We observed a phenomenon in which the shock wave generated by vaporization overtook the shock wave generated by melting. By comparing the results of an overheat experiment using a 1-μF capacitor bank with those of an underheat experiment using a 200-μF bank, we found significant differences in the rise rates of the current and in the discharge modes, leading to a great difference in the amplitudes of the shock waves generated.
[1] W. G. Chance, H. K. Moore. Exploding Wires(1962).
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Xinxin Wang. Research at Tsinghua University on electrical explosions of wires[J]. Matter and Radiation at Extremes, 2019, 4(1): 017201
Category: Pulsed Power Technology and High Power Electromagnetics
Received: Jun. 26, 2018
Accepted: Sep. 24, 2018
Published Online: Nov. 14, 2019
The Author Email: Wang Xinxin (wangxx@tsinghua.edu.cn)