Water and propylene carbonate as storage and switching media in pulsed power systems

The electrical characteristics of two polar liquids, water and propylene carbonate, under high electric stress have been studied. The liquids were placed between a hemispherical electrode of 1.7mm diameter and a plane electrode with gap distances of 200 /spl mu/m and 400 /spl mu/n. A voltage pulse of 200 ns duration was applied by means of a Blumlein pulse generator. The increase in current with voltage was recorded up to the breakdown voltage, including the transition phase to breakdown. Schlieren diagnostics, with a temporal resolution of 2 ns, was used to study the changes in the index of refraction of water when it was used as the switch medium, both before and during breakdown. The results of our measurements showed that the electrodes determine, to a large extent, the hold-off voltage. By polishing the stainless steel electrodes, the breakdown strength was increased from 1 MV/cm to 2.2 MV/cm for propylene carbonate as dielectric. Similar increases were found for water as dielectric between polished electrodes. The rate of current rise after breakdown was measured as 4.5-10/sup 11/ A/s for distilled water, a value which is comparable to that measured in gas switches. The Schlieren photographs showed that, depending on the polarity, the breakdown is preceded by the treeing of streamers into the gap.