Impulse breakdown of water with different conductivities

The complexity of the impulse breakdown of liquid water is reflected by the dependency of pre-breakdown processes on the polarity, rise-time and wave-shape of the applied impulses as well as on physical properties, such as the electrical conductivity of water itself. Further understanding of the mechanisms of formation and propagation of impulse discharges in water and water solutions is therefore required to enable the development of pulsed power and plasma technologies. This paper presents a study of the dielectric behaviour of water stressed with positive and negative high voltage impulses in a point-plane electrode topology. Water with different conductivities including distilled water and tap water was investigated. The volt-time breakdown characteristic of water is discussed and the total pre-breakdown time has been obtained for both positive and negative polarity impulses for 2 mm, 5 mm and 10 mm inter-electrode distances. The Laue statistical analysis has been used in order to calculate the statistical and formative time lags. Using the formative time data the nominal breakdown velocity in water for positive and negative impulses has been obtained. It has been shown that the anode discharges propagate with supersonic velocities, up to ~20×105 cm/s, and the cathode discharges are sub-sonic with a maximal velocity of ~c1x10⁵ cm/s. Discharge velocities are slightly higher in distilled water as compared with tap water. Potential breakdown mechanisms which can explain the observed polarity effects in the transient prebreakdown processes in liquid water stressed with high voltage impulses are discussed.