Wires In Vacuum Research Articles

Numerical investigation of negative polarity electrical explosion of aluminum wire in vacuum

Numerical investigation was carried out on the evolution of inhomogeneous energy deposition of polarity effects in negative polarity electrical explosion of aluminum wire in vacuum. First, radial electric field distribution of the aluminum wire was simulated. The results showed that the initial electric field near electrodes was much smaller than that in the middle of the wire. A model of electrical wire explosion based on ZEUS-EW with artificial limitation on breakdown time of the aluminum wire was used to simulate the behavior of inhomogeneous energy deposition of the polarity effect. The complete evolution process of electrical explosion of the aluminum wire with inhomogeneous energy deposition was further explored and qualitatively divided into three stages according to the simulation results. Finally, the influence of some factors on the inhomogeneous energy deposition simulation was investigated. This study provides some help for a better understanding of the polarity effect in the process of wire explosion.

Laser measurements of static and dynamic dipole polarizability for 11 metal atoms using fast exploding wires in vacuum and integrated-phase technique

The rapid electrical explosion of thin metal wires in a vacuum can completely vaporize the wire and form a fast-expanding gas cylinder of metal atoms. Thin metal wires 12.7 μm in diameter made of W, Mo, Pt, Pd, Ni, Cu, Au, and Al completely evaporated during the first ∼10 ns under the action of a fast-rising current of ∼1 kA/ns. Laser interferometry and the integrated phase technique were used to reconstruct the dynamic atomic polarizability for fully vaporized thin metal wires at 532 and 1064 nm wavelength, calculate the static atomic polarizability, the first dipole-allowed electronic transition energy, and transition wavelengths, and compare our reconstructions with available tabulated values.

ON IMPROVING THE METHOD OF ELECTRON-BEAM DEPOSITION

The paper deals with improvement of the electron-beam additive forming of metal products using a vertically fed filler wire in vacuum with two electron beams as a heating source. We compared the importance of the power of the heat source required for fusing the layers with each other and the calculated power of the heat source required to melt the filler wire and the surface of the product. Within the experimental conditions of the multilayer electron beam deposition using side wire feeding, the electron beam power of 2.4 kW was required to ensure fusion without the defect formation between the layers during the deposition of Ti-6Al-4V titanium alloy. At the same time, approximate calculations of the minimum power of the heat source required to melt the filler wire and the surface of the product showed a level of 730 W.

Instability growth mitigation study of a dielectric coated metallic wire in a low current Z-pinch configuration

An experimental and numerical study of the nanosecond electrical explosion dynamics of bare and dielectric coated metallic wires in vacuum is reported. A table top Z-pinch generator down-tuned to generate a peak current of 40 kA with a rise time of 60 ns is implemented for the experiments in the skin effect mode. Optical probing diagnostics as well as multiphysics–multiphase simulations are used to study the spatiotemporal dynamics of the exploded wire from the solid to the plasma phase. The results show that the inclusion of the dielectric coating mitigates the electro–thermo–mechanical instability growth developed prior to plasma formation, which results in a subsequent reduced magnetohydrodynamic instability growth in the plasma phase. The study offers valuable insights into the understanding of the seeding mechanisms for the generation of plasma instabilities and the efforts for their growth rate suppression.

Manufacturing, assembling and plasma cleaning of new SANAEM RFQ

A new 1.2 m radio frequency quadrupole (RFQ) accelerator cavity, made of oxygen-free copper (OFC) and capable of accelerating protons up to 1.3 MeV energy, is recently manufactured for the Turkish Atomic Energy Authority (TAEA), based on the motivation of establishing a proton beamline with indigenous capabilities. In the present article, the manufacturing and assembling procedures related to the new RFQ copper vanes, especially the novelties applied to a vane type RFQ accelerator cavity, namely plasma cleaning of vane surfaces and utilization of indium wire in vacuum sealing, will be described. The preliminary results, the vacuum level and quality factor, suggest that these applications indicate promising options for future accelerator cavity production.

Laser scattering by submicron droplets formed during the electrical explosion of thin metal wires

This paper presents the results of studying dispersed media formation during the electrical explosion of thin metal wires in vacuum by using low-current generators (∼1–10 kA). Particular attention is paid to the analysis of the composition and structure of the corresponding explosion products as well as to the problem of their visualization using simultaneous laser interferometry and shadow imaging at two wavelengths (1.064 µm and 0.532 µm). Our findings indicate the important role in the visualization of the explosion products that belongs to multiple scattering by submicron droplets of dense condensed matter, which are mixed with metal vapor. The hypothesis on the existence of submicron droplets in the products of exploding metal wires correlates with the results obtained by soft x-ray radiography combined with a laser probing technique. Taking into account the multiple scattering by submicron droplets, it is possible to significantly clarify the parameters of the explosion products visualized via laser probing techniques as well as to gain a deeper insight into the physics behind the electrical wire explosion.

Effect of electrodes on microsecond explosion of thin metal wires in vacuum

An order of magnitude difference was observed in the energy deposition during a microsecond explosion of thin nickel wires in a vacuum, depending on the design of the wire-holding electrodes. In one case, cylindrical polished stainless steel (ss) electrodes with a rounded edge have a small axial hole of 0.3 mm to hold a thin wire. In the second case, these were the same polished electrodes without an axial hole, and the wire was attached to the side of the electrode with a kapton tape. For the electrodes with an axial hole, a voltage breakdown appears when the wire reaches a low temperature of ∼570 K due to the generation of the parasitic plasma discharges from the wire-holder contact. In a long timescale of ∼30 μs, wire resistive heating was interrupted by plasma generations 6 times up to the moment of melting. For the same electrodes without holes, the wire reaches a temperature of ∼1870 K during an ∼2 μs single-stage heating, and the shunting plasma was generated due to the evaporation of impurities from the hot metal surface and avalanche ionization by thermo-emitted electrons from the hot metal surface. We observe similar effects for Cu, Ti, and Pt wires. This effect is important for slow microsecond exploding wires in a vacuum and negligible for fast nanosecond explosions.

Temperature of metal wires for nanosecond and microsecond electrical explosions in vacuum

Experiments show an inverse correlation between temperature and heating time for the electrical explosion of thin metal wires in vacuum. Faster heating of the wire results in higher energy release in a short time, while slow heating lasts longer but releases less energy before breakdown. Breakdown is a natural barrier to the direct release of Joule energy and an increase in the metal temperature to an abnormally high value. An increase in the current rate from 0.07 to 170 A/ns leads to a decrease in the heating time from 5 μs to 5 ns and an increase in the wire temperature from 450 K to 4000 K before the voltage breakdown. This inverse correlation between the heating time and the temperature of the wire in a vacuum can be explained by the inertia of evaporation of the hydrocarbon impurity from the surface of the hot metal.

Wire Explosion in Vacuum

This article presents a review of experimental and theoretical studies devoted to the processes that occur during explosions of wires in vacuum when the current densities in the wire are of the order of 108 A/cm2 and the current density rise rates are no less than 1015 A/(cm $^{2}\cdot \text {s}$ ). The theoretical background is focused on the transformation of the wire metal into ionized plasma. In particular, the basic physical notions used to describe wire explosions (WEs; state diagram, current action integral, and metal conductivity changes in phase transitions) are given; magnetohydrodynamic equations are described which are used to simulate WEs, and the simulation predictions are discussed together with their reliability. Extensive experimental data on WEs in vacuum are presented which made it possible to describe the corona and core formation and the development of electrothermal instabilities in the core. The data on the energy deposited in a wire exploding in vacuum reported by different authors are compared. In conclusion, problems are discussed that require additional experimental investigations, namely, the role of metastable states in a WE and the mechanism by which the core is shunted.

Interferometric measurements of static and dynamic dipole polarizability of titanium atoms using fast electrical explosion of fine metal wires in vacuum

The rapid electrical explosion of fine metal wires in vacuum generates gas cylinders of metal atoms surrounded by low-density and fast-expanding plasma corona. For fully vaporized wires, we utilize the integrated-phase technique, based on laser interferometry, to measure the dynamic dipole polarizability of metal atoms. Titanium wire with a diameter of 20 μm and a length of 1 cm was rapidly vaporized by a fast-rising current at ∼10 ns. We find that the dynamic dipole polarizability of titanium atoms equals 20.5 ± 2 Å3 for 532 nm and 10.6 ± 1 Å3 for 1064 nm. The wire reaches a totally vaporized state when the expansion velocity in a vacuum is ∼5.5 km/s. To vaporize Ti wire, the deposited Joule energy exceeded tabulated enthalpy of atomization by ∼2.8 times. Two-wavelength diagnostic allows reconstruction of the static dipole polarizability 9.13 ± 1.7 Å3 and electron transition energy 3.13 ± 0.2 eV with the corresponding transition wavelength of 396.6 ± 26 nm. This reconstructed wavelength matches to the group of strong dipole-allowed atomic spectral lines for Ti between 394.87 and 402.46 nm for ground-state transitions 3d24s2.

Inverse polarity effect for electrical explosion of fine metal wires in vacuum

It has been experimentally demonstrated that the geometry of a high-voltage electrode strongly affects the magnitude and structure of energy deposition into a thin metal wire before breakdown in vacuum. Additionally, two-dimensional electrostatic simulations provide evidence that the direction and magnitude of the radial electric field on a wire surface are dependent on the geometry of the electrodes. Increasing the size of the ground electrode leads to an increase in the radial electric field, but does not affect the direction of this field. However, modifications of the high-voltage electrode geometry alter the direction and magnitude of radial electric fields on the wire surface. Thus, the hot and homogeneous electrical explosion of a thin metal wire in vacuum can be realized with proper design of the wire's holding electrodes and independent from the polarity of the high-voltage electrode.

Vaporization energy and expansion velocity of electrically exploding aluminum and copper fine wires in vacuum

It has been experimentally shown that the complete vaporization of Al and Cu fine wires due to a fast electrical explosion in vacuum requires 2-3 times the standard atomization enthalpy of ∼4 eV/atom. The expansion speed of a fully evaporated Al wire in vacuum is ∼10 km/s and ∼8 km/s for Cu wire. This excess of evaporation energy over tabulated enthalpy of atomization is related to the “pressure cooker effect,” when a fine metal wire is heated by a fast-rising current with high magnetic and hydrodynamic pressures. In our paper, we demonstrate self-consistent relationships between the deposited energy, expansion rate, and state of the wire core for Al and Cu fine wires.

Explosion symmetry improvement of polyimide-coated tungsten wire in vacuum on negative discharge facility

Tungsten wire explosion is very asymmetric when fast current rate and insulated coatings are both applied on negative discharge facility using a 24-mm-diameter cathode geometry, which is commonly used on mega-ampere facilities. It is inferred, based on an analytical treatment of the guiding center drift and COMSOL simulations, that the large negative radial electric field causes early voltage breakdown and terminates energy deposition into the wire core on the anode side of the wire. After the anode side is short circuited, the radial electric field along the wire surface on the cathode side will change its polarity and thus leading to additional energy deposition into the wire core. This change causes ∼10 times larger energy deposition and ∼14 times faster explosion velocity in the cathode side than the anode side. In order to reduce this asymmetry, a hollow cylindrical cathode geometry was used to reverse the polarity of radial electric field and was optimized to use on multi-MA facilities. In this case, fully vaporized polyimide-coated tungsten wire with great symmetry improvement was achieved with energy deposition of ∼8.8 eV/atom. The atomic and electronic density distributions for the two different load geometries were obtained by the double-wavelength measurement.

Using of fiber-array diagnostic to measure the propagation of fast axial ionization wave during breakdown of electrically exploding tungsten wire in vacuum.

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.

Investigations on stratification structure parameters formed from electrical exploding wires in vacuum

The density distribution and the energy deposition in dark and light transverse layers from stratification structures were measured and analyzed from electrical exploding wire experiments in vacuum. It is demonstrated that the differences in the areal density of the dark layer and light layer regions were about 2 × 1017/cm2 at 296 ns after voltage collapsed. This difference corresponded to an energy deposition difference of ∼2 eV/atom between the dark and light layers, which likely arises from a ±10% resistivity perturbation during the explosion, as suggested by combining a gas column expansion model and a thermodynamic model.

Effect of hydration on the electrical explosion of a fine palladium wire in a vacuum

Experiments with fast electric explosion of a hydrated palladium wire in vacuum show a significant decrease in Joule deposited energy, expansion rate, and voltage amplitude at breakdown. An increase in the density of diffused hydrogen and oxygen at the wire surface leads to an early generation of plasma due to evaporation impurity and rapid development of avalanche breakdown along the wire surface. The non-hydrated Pd wire demonstrates a longer resistive time, higher voltage peak, greater energy and expansion speed. The decrease in the deposited energy was ∼35%, and the expansion rate was ∼18%. The peak of light emission during voltage breakdown was twice higher for a hydrated Pd wire than for a bare one.

Characteristics of the electrical explosion of fine metallic wires in vacuum

The experimental investigations on the electrical explosion of aluminum, silver, tungsten and platinum wires are carried out. The dependence of the parameters related to the specific energy deposition on the primary material properties is investigated. The polyimide coatings are applied to enhance the energy deposition for the exploding wires with percent of vaporized energy less than unit. The characteristics of the exploding wires of different materials with and without insulating coatings are studied. The effect of wire length on the percent of vaporization energy for exploding coated wires is presented. A laser probe is employed to construct the shadowgraphy, schlieren and interferometry diagnostics. The optical diagnostics demonstrate the morphology of the exploding products and structure of the energy deposition. The influence of insulating coatings on different wire materials is analyzed. The expansion trajectories of the exploding wires without and with insulating coatings are estimated from the shadowgram. More specific energy is deposited into the coated wires of shorter wire length, leading to faster expanding velocity of the high-density products.

The formation and evolution of the core-corona structure in the electrical explosion of aluminum wire in vacuum: experimental and numerical investigations

Experimental and numerical investigations on the formation and evolution of the core-corona structure in the electrical explosion of aluminum wire in vacuum are presented. In the experiments of aluminum wire explosion, shadowgraphy, interferometry and schlieren optical diagnostics are constructed with a laser probe to study the morphological evolution and distribution of the core and corona. A set of computational models with cold start conditions, consisting of a thermodynamic calculation and magnetohydrodynamic model combined with a semi-empirical equation of state and transport coefficients, are established to reproduce the behavior of the electrical explosion of aluminum wire. The radial profiles of the density, temperature, current density and magnetic field, addressing the physical scenario of the formation and evolution of the core-corona structure are analyzed. The numerical results are also compared with relevant experimental data.

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.

Nanosecond electrical explosion of bare and dielectric coated tungsten wire in vacuum

Experiments of the electrical explosion of tungsten wire with and without insulating coatings demonstrate that the insulating coatings exert a significant influence on the exploding characteristics. The shadowgraphy and interferometry diagnostics are applied to present the morphology of the exploding products. In the experiments, energy of ∼3.2 eV/atom is deposited into the bare tungsten wire at the instant of voltage breakdown, giving a velocity of 0.38 km/s for the high density core. The value and structure of the energy deposition for the tungsten wire explosions are substantially improved by employing the thin dielectric coatings. Energy of ∼15.2 eV/atom is deposited into the coated tungsten wire transforming the wire into gaseous state and the expanding velocity of the high density core is 5.64 km/s. The interference phase shift and atomic density are reconstructed from the interferogram for the exploding coated tungsten wire.