Restrike Mechanisms of Exploding Wire Discharges

Abstract

The development of the early stages of restrike channels initiated by electrically exploded thin wires was studied, using high-speed Kerr-cell and rotating-mirror streak photographic techniques. The experiments were performed mainly with constantan and copper wires (length: 13.8 cm, diameter: 0.005 cm). Complementary measurements were also made on five lengths between 1.6 and 13.8 cm at voltages up to 23 kV. The results show that at low voltages the restrike is always initiated at the exterior of the exploded wire. At higher voltages, above a critical voltage, the restrike is initiated either at the exterior or in the interior of the exploded wire. The majority of the restrikes are initiated in the interior. Restrikes initiated at the exterior of the exploded wire always have much shorter dwell times than those initiated in the interior and a good dwell-time reproducibility. During the dwell-time which precedes these restrikes, luminous dots appear at the surface of the wire. The dots are distributed helicoidally along the whole wire length. The presence of these dots may indicate an intense local ionization on the wire surface due to the emission of thermal electrons from the wire material. The ionization increases to the point at which a thin helicoidal channel is built up near the surface and along the whole length of the exploded wire. The restrike starts and develops in this channel until it loses its form through instability. For restrikes initiated in the interior of the exploded wire, no local ionization was observed at the surface of the wire during the dwell time. This may have been due to the rapid vaporization of the wire material. During the dwell time the exploded wire shows a diffuse low luminosity, which increases when ionization by impact is initiated in the interior of the wire. The luminosity of the wire remains diffuse until the ionized medium contracts and the restrike starts in a very narrow channel near the axis of the wire. The channel expands with a very high velocity towards the inner shell of the exploded wire. The initial phase of the channel shows an helicoidal form. The channel tends to be straight when its diameter increases. The type of restrike which follows the wire explosion seems to be dictated by the vaporization rate of the wire material.