Dielectric Strength of Sub-Millimeter Water Gaps Subjected to Microsecond and Sub-Microsecond Voltage Pulses

To move high energy pulsed power systems from the laboratory to practice requires the development of compact lightweight drivers. To reduce the size and weight of high voltage components, an increase in the energy density in dielectrics at high electric stress is required. Water, which has a dielectric constant of epsiv = 80 and high dielectric strength, can be used for both energy storage and for switching. To date, most of the studies of breakdown in water have been conducted for relatively wide gaps. Empirical equations for the breakdown strength under these conditions were developed by J. C. Martin [1]. In this paper, sub-millimeter water gaps were studied for applied pulses of microsecond duration and compared to gaps subjected to sub-microsecond pulses. A pin-plane electrode configuration was used. The pin electrode exhibits a hemispherical geometry with a diameter of 1.7 mm. The gap distance between the electrodes is variable but was set to 200 mum in the experiments reported in this paper. In the case of short pulses (20 ns, 50 ns, 100 ns and 200 ns) a Blumlein type pulse-forming line was used. For the longer pulses of 1 mus, 3 mus and 4.8mus, a Blumlein type pulse forming network (PFN) was employed. It was found that for sub-millimeter gaps with the pin-electrode as anode, and when the pulse width is within the microsecond range, the dielectric strength of water remained almost independent of the pulse width. However, when the pulse width was decreased to the sub- microsecond range, the dielectric strength increased as the pulse width decreased. This trend is consistent with Martin´s empirical formulas except that the breakdown voltage values were about two times higher than predicted. In contrast, when the pin-electrode was used as cathode, the experimental results were in better agreement with those calculated using the empirical formulas.