Mechanism of dielectric recovery in uniform- and nonuniform-field gaps after high-current high-pressure spark discharges

Abstract

Previous experiments on the dielectric recovery of high-current spark channels have been conducted with small gap spacings; so it has not been possible to separate the cooling effect of the electrodes from other electrode effects; such as changes in the Townsend secondary-ionisation coefficient γ. In the paper, measurements of gas-temperature decay and dielectric recovery rate are made for an 8 cm gap and 16 mm-diameter or smaller electrodes following a 190 kA spark discharge at a pressure of 570 torr; the electrode cooling effect here should be negligible. This electrode configuration constitutes a nonuniform field, in contrast with measurements on 5 mm gaps where the field is assumed to be uniform. The form of the recovery curves at later recovery times depends on the degree of uniformity of the field when measured curves are compared with those derived from gas temperatures. Increased attachment due to the formation of impurity products appears to be dominant in the uniform-field case; in the nonuniform-field configuration, negative-ion detachment facilitating the growth of space-charge-distorted fields, causing lower breakdown voltages, seems to be an important process. At early recovery times, increases in γ seem to occur, and, with tungsten electrodes, field-enhanced thermionic emission may be important. A model for the gas-temperature decay in the 8 cm-gap case demonstrates that the major mechanism for heat removal is by radial thermal conduction; the temperature predicted by this model shows good agreement with the measured values of gas temperature.