Electrical breakdown of sub-millimeter water gaps

We have studied the temporal development of the electrical breakdown in water by means of electrical and optical diagnostic methods. Distilled water with a resistivity of 200 kΩcm was placed between a 1.7 mm diameter sphere steel electrode and a plane stainless steel electrode. The gap distance was varied from 50 μm to 400 μm. Up to 50 kV high voltage pulses with duration of 200 ns, provided by a 50 Ω Blumlein generator, were applied to the electrodes. The temporal development of voltage current in the prebreakdown and the breakdown phase was recorded. Laser Schlieren diagnostics and Mach-Zehnder interferometry with a temporal resolution of 1 ns was used to observe the temporal development of plasma formation and changes in the index of refraction before and during breakdown. The electrical breakdown, with highest values of almost 1.2 MV/cm between a sphere and a plane with a gap distance of 50 μm, is preceded by a phase of slightly increased current, indicating streamer development. The rate of current rise during breakdown was measured as 2.5×10¹¹ A/s. According to the time resolved observation, the discharges are initiated by streamer development between the electrodes. The streamer development in quasi-homogeneous fields is compared to that in inhomogeneous fields or large gap distance electrode configurations.