Studies on the temporal development of electrical breakdown in water

Pre-breakdown phenomena between a needle point of 5 /spl mu/m curvature and a plate, 100 /spl mu/m apart, in tap water have been studied with a spatial resolution of micrometers and a temporal resolution of 10 ns. The phenomena were observed by means of phase contrast microscopy. The illuminating light source was a flash lamp with a pulse duration of 10 ns. This diagnostic method allowed us to observe changes in the refractive index before the actual electrical breakdown. Voltages of 1.8 kV applied to the gap, generated electric fields of 1.6 MV/cm at the tip of the needle sufficient to initiate breakdown. The results of the measurements showed that after application of the rectangular voltage pulse micro-bubbles with characteristic dimensions of 10 /spl mu/m were formed at the point of the needle. The bubbles seem to serve as centers for local electrical breakdown, which initiate the main breakdown between the electrodes.