Impulse breakdown of liquid water

Currently, studies of microsecond and sub-microsecond electrical breakdown in liquid water and water solutions are experiencing their renaissance period due to the development of advanced pulsed power systems and emerging technologies for environmental applications and plasma medicine. The present paper is focused on an investigation of polarity effects in sub-microsecond discharges in water. Distilled water, tap water and water based ionic solutions were stressed with high voltage impulses with a rate of voltage rise exceeding 10¹⁰ V/sec. A needle-plane electrode topology and inter-electrode gaps in the range between 1 and 10 mm were used in these tests. Analysis of the transient current and voltage wave-forms allowed estimation of the average velocity of propagation of pre-breakdown plasma structures (streamers). It has been found that for positive impulses, streamers formed in distilled water propagate with an average super-sonic velocity, 30 km/sec. In the case of negative impulses however, this velocity is significantly lower and depends upon the inter-electrode distance: for 10 mm inter-electrode gaps a subsonic streamer velocity has been registered. Analysed in this paper are possible mechanisms which may result in such differences in the pre-breakdown behaviour of liquid water, amongst which are space charge effects, nucleation, and the formation and breakdown of gas micro-cavities. The results obtained and presented in this paper provide further understanding of the development of sub-microsecond breakdown in water media, which underpins a number of new environmental and medical pulsed plasma technologies.