Abstract
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 m2 and 5×10−8 m2. The spot generally exists in one of two types. In the second type the average current density is about 5×1010 A m−2 and in the first type it is about (0·5-1)×1010 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.
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