Photographic Study on Spark Discharge Generated by a Nanosecond High-Voltage Pulse over a Water Surface

Spark discharge generated by a nanosecond positive high-voltage pulse over a water surface at atmospheric pressure in air was studied using a high speed camera system. Faint streamers form near the pin electrode and propagate towards the water surface. The time for the streamer propagating across the air gap was estimated to be about 50 ns to 60 ns with a propagation velocity of ∼ 1.3 × 10 ⁵ m/s. It was found that the water conductivity and the gap distance have no significant effect on the propagation velocity of the streamer. After the streamers touch the water surface a brilliant spark channel forms across the air gap. The maximum diameter at the middle of the spark channel is about 1 mm, and approximately contracts with a radical velocity of about 2.0×10 ³ m/s. No significant dependence of the maximum diameter and decay velocity of the spark channel on the water conductivity and the gap distance were recognized in the present work. The maximum conduction current for a gap distance of 5 mm is significantly larger than that for a gap distance of 10 mm at the same water conductivity, and shows an increasing tendency with increasing water conductivity for a fixed gap distance. Based on the maximum conduction current, the effect of water conductivity and gap distance on the electron density of the spark discharge plasma at the peak current was investigated. Within the range studied, the electron density in the spark channel is about 10 15 cm ⁻³ and increases with water conductivity at a fixed gap distance.