The structure of the discharge channel during nanosecond wire explosions has been studied using laser probing. Wires of 25μm diameter and 12mm length were exploded in air and vacuum by 10kA current pulse having a 50A∕ns rate of rise. Upon electrical explosion of thin wires in the air, the development of shock waves was observed. The propagation of shock waves was analyzed, and it was possible to draw conclusions on the location of the flow of most of the current in the volume of the discharge channel. This permitted distinguishing between two scenarios (shunting and internal) of the interelectrode gap breakdown development. The scenario depends to a large extent on the properties of the exploding wire material. The same two scenarios are valid upon electrical explosion of wire in vacuum. Moreover, if secondary breakdown develops in the internal scenario, the value of the energy deposition in the wire material during explosion in vacuum may be comparable with that found during explosion in air.

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...

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