Generation of blastwaves from confined electrical discharges

Fast-response pressure transducers have been used to record the initial series of Shockwaves produced by electrical discharges of between 10 and 30 mm in length in argon, air and sulphur hexafluoride contained in a wedge-shaped vessel. The complex series of pressure peaks in the wave train have been identified as arising from the reflection of the original cylindrically expanding front with the upper, lower and side walls at the apex of the vessel and their subsequent interaction with the leading wave propagating towards the base of the wedge. The net result is a steady rise in the pressure in the vessel prior to the later build-up associated with energy transferred from the arc which finally forms. Because of the intense electromagnetic field created by the channel, it was only possible to record pressures at some distance, where the overpressures produced by the incident wave, strengthened by various reflection processes, had fallen to ¿¿ 1. The overpressure of this wave decayed with distance according to an r¿¿3/4-law. At the measuring station closest to the channel, where the sole interaction of the leading cylindrically expanding blastwave was a regular reflection at the side wall, it was possible to deduce from the measurements the strength of the incident wave. Correlation of the overpressures derived in this way for all three gases with the theoretical expression for instantaneous release of energy: log (Z ¿¿ 1) ¿¿ [4E0/b¿¿p0]0.5 was extremely good. The direct comparison of theory with experiment suggests that approximately 10% of the electrical energy dissipated in the early stages of production of the channel is transformed into kinetic energy of the surrounding medium, resulting in the formation of a blastwave.