Huantong Shi

Fully vaporized electrical explosion of bare tungsten wire in vacuum

A flashover switch inserted between the cathode and wire end was used to enhance the deposition energy into an exploding wire in vacuum. It was found that this flashover switch can play two roles. The first one is to reduce the rise time of the current pulse, the second and more important one to greatly improve the surface radial electric field and suppress the electronic emission that is responsible for triggering the surface breakdown of the exploding wire. Only when the flashover switch is on the cathode, the second one can take effect and result in a much higher deposition energy. In this case, the deposition energy could increase by ∼three times (3.4 eV/atom–12 eV/atom) under the negative current pulse and by ∼two times (5.7 eV/atom–13 eV/atom) under the positive one, and fully vaporized wire core was observed with laser interferometry.

Study of the Relationship Between Maximum Specific Energy and Wire Diameter During Electrical Explosion of Tungsten Wires

Driven by a pulsed current of 1-3 kA in amplitude and 15-30 ns in risetime, electrical explosions of tungsten wires of different diameters (25/18/15/12.5/10 μm) were carried out. Shadowgraphs as well as interferograms of exploding tungsten wires were obtained with a YAG Laser. The experimental results showed that with the decrease in wire diameter, the maximum specific energy Esm (the maximum deposition energy per atom), as well as the wire expansion speed has an increase-decrease tendency under both negative and positive current pulses. In order to understand this phenomenon, the specific power Pv at the start moment of vaporization was simulated, and the changing tendency of Pv versus diameter was found to be similar to that of Esm versus diameter. Based on the experiments and simulations, we propose that the specific power Pv at the evaporating point must also be regarded as an important factor leading to the change in the maximum specific energy Esm.

Measuring the Evolution of Mass Density Distribution of Wire Explosion

Wire explosion is the initial stage of the development of wire array Z-pinch load driven by high pulsed current. Based on the pulsed power generator PPG-1 (400 kA/100 ns), the process of 10-μm-W wire explosion at tens of kiloamperes was investigated. The time sequence images of wire explosion were obtained by X-ray backlighting with X-pinch X-ray point source, and the mass density distribution of the exploding wire at a certain time was calibrated by method of grayscale contrast based on a particular step wedge filter. By image processing of the time sequence images of wire explosion, the evolution of mass density distribution of wire explosion was presented.

Mass Density Evolution of Wire Explosion Observed Using X-Ray Backlight

The evolution of single- and dual-wire Z-pinch of 8-μm W was investigated by X-ray backlighting using an X-pinch X-ray source. The experiments were carried out on the pulsed-power generator PPG-I (400 kA/500 kV/100 ns), which was designed and constructed by the Department of Electrical Engineering of Tsinghua University. To scale the mass density distributions at different explosion time, a mass step wedge including eight tungsten layers was fabricated and inserted between the object Z-pinch and the X-ray film. By a large number of imaging experiments, the physical images of coronal plasma formation and interwire plasma merging were obtained. Based on the X-ray photos of 8-μm W and mass step wedge, the mass density distributions at different explosion time were drawn, and the conductance curve of time dependence of 8-μm W was also calculated using waveform of voltage and current.

Effect of thickness of insulation coating on temperature of electrically exploded tungsten wires in vacuum

This paper reports an interesting observation of great differences in the temperature of exploded wires with insulation coating of different thicknesses. Two kinds of polyimide-coated tungsten wires were used with the same conductive diameter 12.5 μm but a different thickness of coating, 0.75–2.25 μm and 2.25–4.25 μm, respectively. The specific energy reconstructed from the current and voltage signals was quite close for the tested wires. However, the exploding scenario, obtained from Mach-Zehnder interferograms, showed great differences: a neutral outer-layer was observed around the thick-coated wire, which was absent for the thin-coated wire; and the calculated electron density and local thermal equilibrium temperature were much higher for thick-coated wires. The heat-preserving neutral layer formed by the decomposition of the insulation was supposed to be the cause of this phenomenon.

Using fast moving electrode to achieve overvoltage breakdown of gas switch stressed with high direct voltages

A small-scale fast risetime gas switch attached to a 50 Ω pulse forming line is tested. It includes a fast moving electrode and a fixed electrode. For the applied direct voltages, such as 2.8 kV, 2.0 kV, and 1.0 kV, the risetimes of this switch are tested to be ∼3.8 ns, ∼2.3 ns, and ∼1.1 ns, respectively, while the risetimes of a switch with two fixed electrodes are about ∼10.1 ns, ∼9.0 ns, and ∼3.6 ns. The results of high-speed photography and laser interferometry reveal that the moving electrode will obviously shorten discharge spark length but almost will not change the inter-electrodes gas pressure. The reason of shortening spark length is the existence of the discharge time delay of gas switch. After moving to the static breakdown spacing, the fast moving electrode will move on for an additional distance within the discharge time delay, which makes gas switch achieve overvoltage breakdown under high direct voltages and therefore leads to shorter spark length and faster switch risetime.

The development of coated and non-coated wire explosions observed by X-ray backlighting

Based on an X-pinch X-ray point source, the electrical explosion of coated and non-coated wires with 25 μm diameter was backlit. The experiments were performed on the pulsed power device PPG-1 (500 kV/400 kA/100 ns) which was designed and constructed by the Department of Electrical Engineering of Tsinghua University. The source X-pinch was installed between the main output electrodes, while the object coated and non-coated wires were placed at the positions of the left and right current return rods, respectively. The backlighting images were recorded by X-ray films with high resolution and sensitivity. By a large number of backlighting experiments, the exploding physical images and expansion ratio curves of the coated and non-coated wires were obtained, and the results showed that coating can make the wire expand to be larger and more uniform.

Measuring the dynamic polarizability of tungsten atom via electrical wire explosion in vacuum

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 A3, which showed good statistical consistency and was also in goo...

Effect of high-voltage electrode geometry on energy deposition into exploding wire in vacuum

Based on electrical and optical diagnostics and electrostatic calculations, the relationship between distribution of radial electric field and energy deposition of electrically exploded wires in vacuum was investigated. Copper plates with different diameters were used to modify the distribution of radial electric field along wire surface in the experiments. The calculated radial electric field showed good consistency with the wire core outline in optical images, and there is a saturated positive correlation between maximum specific energy (Esm) and averaged radial field strength over the wire length. It can be concluded that the direction and amplitude of radial electric field significantly affects Esm and dominates the structure of energy deposition.

Study on electrical explosion of bare and insulation coated tungsten wires

Summary form only given. Electrical explosion of single wire is the initial stage of wire array z-pinches. In order to study the physical process of single wire explosion, a small-scale pulsed power generator was established, which can provide positive and negative polarity current with the peak value of 3kA and the rise-time from 20ns to hundreds of nanoseconds. The single wire loads are bare and insulation coated tungsten wires with a dozen microns in diameter. The electrical behavior (i.e. the voltage and current waveforms) of the wire explosion was measured by a capacitive divider and a Rogowski coil, respectively. The evolution images of wire explosion were photographed with laser shadowgraph, interferometry and schlieren. A large number of experiments of wire explosion have been carried out on the small pulsed power generator. The results show that for the bare tungsten wire, due to the earlier flashover breakdown along the wire surface, the energy deposited into the wire is less than the energy needed for vaporizing the whole wire, which always results in a heterogeneous structure of exploding wire, i.e. high-density wire core surrounded by low-density coronal plasma. While for the insulation coated tungsten wires, the flashover can be delayed and thus the deposited energy can be close to or greater than the vaporization energy, which leads to a higher wire expansion ratio, and wire core free explosion (i.e. wire material totally vaporized) can be achieved before flashover occurs with suitable current waveforms.