Surface Studies of Electrodes Used in Spark

Protrusions on the surfaces of electrodes used in high voltage spark gaps lead to local field enhancements, which lower the self-breakdown voltage of the spark gap. Stainless steel, brass, and copper-tungsten electrodes have been tested in a 5-30 kV, 4-25 kA, .1.6 Caul/shot, unipolar, pulsed spark gap with N2 gas or air. The heights and bases of variously shaped protrusions on the surfaces of these electrodes have been measured using stereo pair micrographs from a Scanning Electron Microscope. For instance, a brass electrode exposed to 5000 shots in N2 has a large number of irregularly shaped protrusions on its surface with base diameters ranging in size from 100 p to 1000 P and heights ranging in size from 250 ~ to 500 ~· Consequently, the self-breakdown voltage statistics for brass electrodes have a very erratic shape. The composition of tnese procrusions has been determined using Auger Electron Spectroscopy. They are composed of a much larger ratio of copper to tin than is normally observed in brass.