Impact of surface conditioning of large area electrodes on the dielectric strength of water

Due to water's high dielectric constant, 6r = 81, there is considerable interest in breakdown phenomena in water, particularly as an insulator and/or switching media in pulsed power systems. In numerous research efforts conducted over the last several decades, it has been documented that electrical breakdown in water is initiated from localized field enhancements on the electrode surface caused from asperities or microprotrusions on the electiode surface. The research conducted here attempts to determine the impact of electiode surface conditions on the holdoff voltage of a water gap.

In this document, experimental results are presented on the impact electrode material and surface treatment has on the dielectric strength of water. A 4mm water gap was tested under pulsed conditions with pulse widths greater than 1 /is, peak electric fields over 1 MV/cm and peak currents over 20 kA. Stainless steel electrodes machined with a Bruce profile and an effective area of 5cm^ were tested with surface roughnesses ranging from .34 /.im to 1.41 /xm. Results comparing electiodes that have additionally been electropohshed are compared to those without an additional tieatinent. These various surface tieatments remove microprotrusions from the electiode surface and reduce localized field enhancements. It is believed that this technique will minimize the number of breakdown initiation points, thereby, increasing tiie dielectiic stiength. Results are given on the degree to which each finish improves the dielectiic stiength of water.