Nanosecond interferometric measurements of the electric field distribution in pulse charged water gaps

The high dielectric strength of water allows its use as a switch medium in compact, low inductance, and therefore fast, high power closing switches. We have investigated the breakdown strength of short gaps in water by applying rectangular voltage pulses of 200 ns duration to a pin-to-plane electrode configuration with a gap distance of 400 μm. Voltage pulses of up to 52 kV amplitude were applied by means of a 50 Ω. Blumlein pulse generator. During the charging phase, preceding the breakdown, the electric field and field-related changes in water density cause changes in the index of refraction. Consequently, the temporal and spatial variations of the electric field and density (caused by changes in temperature and pressure, respectively) can be derived from the corresponding changes in the refractive index. These changes were visualized by using a Mach-Zehnder interferometer. A high-speed camera with a temporal resolution of 3 ns recorded the fringe patterns. The Kerr effect, which is related to the electric field, is dependent on the polarization of incident laser light, while the density effect is independent of the polarization. Temporally resolved contour maps of the refractive index were obtained using Abel inversion, and the corresponding electric field distribution was derived using Kerr´s law. The results showed that the refractive index was not only changed by the Kerr effect, but also by a build-up of a high-pressure layer at the hemispherical anode.