Cathode spot structure and dynamics in low-current vacuum arcs

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A study has been made of cathode spot structure and dynamics in copper vacuum arcs for currents between 25 and 100 A using a high-magnification optical technique. The cathode spot consists of one or more electron emission zones or fragments with areas fluctuating between 10 −10 m 2 and 5 × 10 −8 m 2 . The spot generally exists in one of two types. In the second type the average current density is about 5 × 10 10 A m −2 and in the first type it is about (0·5-1) × 10 10 A m −2 . The cathode spot undergoes continuous rebuilding in the form of fluctuations in the number and size of the fragments such that transitions in spot type, from type 1 to type 2 and vice versa, occur on average every 100 μs. A possible explanation of the observations in terms of a cathode spot model based on temperature-field electron emission is discussed.