Different mechanisms of shock wave generation and scenarios of second breakdown development upon electrical explosion of wires

Abstract

The structure of the discharge channel upon nanosecond wire explosion has been studied using laser Schlieren probing. Wires of 25 µm diameter and 12 mm length were exploded in air and vacuum by 10 kA current pulse having a 50 A/ns rise time. The development of shock waves in the air was observed. The propagation of shock waves was analyzed using a simple model of flat piston. It became possible to draw conclusions the dislocation of the flow of the main part of the current in the volume of the discharge channel. This permitted to distinguish two scenarios of development of secondary breakdown of the interelectrode gap. The scenario (shunting or internal) in accordance with which secondary breakdown develops in each concrete case depends to a large extent on the properties of the material of the exploding conductor. I. EXPERIMENTAL SETUP AND RESULTSIn experiments on electrical explosion of wire, it is not possible to investigate the distribution of current by direct means. Therefore, as a rule, it is implicitly assumed that until the instant of secondary breakdown current flows in the wire. Then, it is shunted along the boundary between the wire and the medium or plasma formed by matter polluting the wire surface. In wire explosions in vacuum as well as in media, after secondary breakdown the resistive deposit of energy in the dense layers of wire explosion products practically ceases. We would show that such scenario not always is valid.Experiments were performed on a set-up having the following parameters: capacitance C = 100 nF, maximum charged voltage U 0 = 20 kV and circuit inductance L = 340 nH with length of interelectrode gap l = 12 mm and initial wire diameter d = 25 µm. This set of parameters secures an EEW with an initial pre-breakdown stage (resistive heating of wire and phase explosion) without development of MHD-instability. The analysis considered mainly data for explosion of tungsten and copper wire because it is known that the characteristic appearance of the discharge channel for these materials cardinally differs. A reduction of varying parameters permits to reveal the main regularities for the selected EEW conditions. Data on the localization of the channel of current flow will help when constructing a model of secondary breakdown for EEW. Fig. 1 shows examples of optical images when exploding 25-micron copper and tungsten wires in air. The images have been selected to demonstrate that discharge channels of exploding copper wires greatly differ from those of tungsten wires. The most noticeable difference is that when exploding a copper wire the region of dense products of explosion occupies practically all of the disturbed volume, i.e., their outer boundary, denoted by arrow 4, do not greatly lag from the position of the shock wave, denoted by arrow 1. On the other hand, when exploding a tungsten wire in air the region of dense products of explosion (arrow 2) occupies barely a third of the radius of the region disturbed by the shock wave. We analyzed the data at our disposal to show possible ca...

A Study of Energy Partition During Arc Initiation

“…Several publications report the dynamics of the arc plasma expansion, with emphasis given to the pressure and velocity fields produced in liquids, [7,[12][13][14][15][16]. Moreover, several studies have investigated the dynamics of arc plasma expansion in gases, [10,11,17,18]. Since the electrical discharge involves complex electronic and hydrodynamic processes, the development of the discharge can be described with the help of certain parameters: plasma channel resistance and radius, temperature, pressure and energy dissipation.…”

“…For the development of this model, a number of necessary assumptions were made. Atmospheric air was considered as a Newtonian fluid (non-viscous, thermally non-conducting, ideal gas) that expands only in the radial direction, [18]. The gas density, ρ 0, was considered constant at 1.2 kg/m 3 , and the atmospheric pressure, P 0 , was considered constant at 101 kPa.…”

Optical Examination of Shockwave Propagation Induced by an Underwater Wire Explosion

“…When pulse power is applied to the wire, the electrical energy rapidly sublimates the wire, thus ___________________________________ *Corresponding Author: osamu@okinawa-ct.ac.jp converting it into plasma along with the surrounding water. At this instant, an underwater shockwave is generated around the rapidly expanding plasma [11] [14][15][16]. A high-voltage pulse generator is used for supplying high-voltage, large-current electrical discharge to the wire (Figure 2A).…”

The initiation of electric arcs and the possible impact in industrial environments

“…[Dra51], [Bra58], [Mar60], [TBI+07]. A well-established, previously-developed theory describes the arc channel expansion based on the concepts of hydrodynamics (fluid dynamics) and considers the plasma channel as a piston that pushes the surrounding medium, establishing a shock-wave [Dra51], [Bra58].…”

“…For the development of this model, a number of assumptions were made, which have been described in Subchapter 6.1.4. Atmospheric air was considered as a Newtonian fluid (nonviscous, thermally non-conducting, ideal gas) that expands only in the radial direction, [TBI+07]. The non-disturbed gas density, ρ0, was considered constant at 1.2 kg/m 3 , and the atmospheric pressure, P0, was considered constant at 101 kPa.…”