Abstract
14 cm. lengths of bare copper wire (diameters 0.0025 cm. and 0.004 cm.) passing through a length of X-band wave guide were exploded by means of a 0.1 uF capacitor charged to various voltages. Measurements of microwave reflection and transmission and simultaneous highspeed framing camera photographs were taken. For the 0.0025 cm. diameter wires the first current pulse was comparatively long (~ 24 microseconds at 5 kV, the threshold for explosion) and the photographs clearly show the formation of striations caused by surface arcs occurring before the current ceased. The formation of these striations (observed previously by various experimenters), and the occurrence of explosion at energies appreciably less than that required for total vapourisation, are attributed to slight variations in the wire along its length. The theory of the effect of variations in cross-section is developed and it is shown that for copper slight cross-section differences cause temperature -'instabilities' such that, if a wire contains a section with radius 10% less than the rest, the temperature of this section reaches the critical temperature (9030 K) whilst the rest of the wire is at about 2000 K. For the 0.004 cm. wires the explosion is much more rapid, the first current pulse being about half a microsecond, and more uniform. The uniformity is attributed to the further development of the break- down at the end of the first current pulse which reverses the growth of instabilities. The plasma formed in these explosions is initially at a high temperature and highly reflecting at X-band frequencies for a period of 5-50 microseconds, after which there is a sharp drop in reflection. The cooling of the plasma and its resultant properties are discussed, and possible reasons for the microwave behaviour are considered. There is insufficient data to reach very definite conclusions, although the results of the calculations performed are in qualitative agreement with the observed phenomena.