Single-wire explosion experiments relevant to the initial stages of wire array z pinches

This article reviews the status of experiments directed to understanding the initial phases of exploding wire plasma formation, especially those relevant to the initiation of wire explosions and plasma formation in wire array z-pinch experiments. Thus, although we discuss experiments with similar to 100 kA per wire, in which magnetic forces play a major role, our emphasis is on experiments with similar to1 kA per wire, in which magnetic forces appear to be unimportant. With high current (similar to 100 kA) per wire, the exploding wire consists of a rapidly expanding (1-3 cm/mus) coronal plasma surrounding a dense core that expands much more slowly. The coronal plasma exhibits strong, azimuthally symmetric instabilities driven by the high current, but the dense core appears to be stable, suggesting that it is carrying little of the current. In low-current experiments, the initial wire core expansion rate depends upon the material, the wire size, and whether or not it is coated with an insulator. For bare wires, the core diameter expands at rates which range from less than or equal to0.03 cm/mus (for 25-mum-diameter W) to 0.46 cm/mus (for 25-mum-diameter Ag). This expansion rate increases with the energy deposited resistively in the wire before coronal plasma formation. Furthermore, expansion is more uniform as well as faster for wires in which the deposited energy is comparable to or larger than the vaporization energy. Insulating coatings increase the energy deposition, evidently by forestalling the formation of plasma around a wire. Therefore, wires coated with 1-mum thickness of plastic expand faster (e.g., by a factor of 2 for Ag) than bare wires for all wires tested so far.