Electrical breakdown in polar liquids

The electrical breakdown in water and propylene carbonate was studied with respect to its application in fast rise-time, high repetition rate pulse power switches. A rectangular voltage pulse of 200 ns duration was applied to hemispherical-plane electrodes with a gap distance of 200-400 μm. Electrical measurements and optical methods, such as photography, interferometry, Schlieren photography, and shadowgraphy have been used to study the temporal development from prebreakdown to recovery. The electrical breakdown was found to develop from a tree-like, non-luminous structure, which initiates at the hemispherical electrode (cathode) into a single luminous filament. The maximum breakdown strength depends strongly on the electrode surface structure. For highly polished electrodes, 1.6 MV/cm for water and 2.2 MV/cm for propylene carbonate, were measured. The experimental results indicate that, for submicrosecond pulses, electron emission caused by the high electric fields in the cathode boundary layer is the cause for breakdown in polar liquids. The expanding plasma column generates, first shockwaves and, at a later stage, a vapor bubble which expands for 200 μs and then decays with a time constant of 1 ms. This recovery time, which limits the switch repletion rate to 1 kHz, can be reduced by flowing the polar liquid through the gap.