The influence of the surface structure on the behavior of arc cathode spots in vacuum

High speed pictures of arc spots on copper cathodes in vacuum have been taken with high time and space resolution (10 ns and 3 μm, respeetively). They reveal that the spots are located at the rim of large craters, sometimes turning around the crater center with velocities in excess of 1000 d s . On a time scale of milliseconds the spots frequently come hack to their previous locations (memory effect). Supplementary experiments with aolyhdenum and tungsten cathodes show that the appearance of the spots can he controlled by small amounts of oxides and carbides, whereas high temperatures strongly reduce the probability the spot ignition. From these observations it is concluded that cold field electron emission is the base for the spot displacement even if the field appears to he relatively low.