Cathode spots on arc-cleaned copper and molybdenum electrodes in vacuum were studied by fast image converter framing and streak camera photography with high temporal and spatial resolution. The frame exposure time was 20 ns and the interframe period was 200 ns; the streak sweep time was between 200 ns and 1 ps. Spatial structures with a resolution of 5 pm could be determined by observing the spot movement with a small slit at the streak camera and a high sweep speed. Strong fluctuations of the

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... ight emission of the spot were found with characteristic times of 50-100 ns. When the spot moved out of the slit field of view a stepwise decrease occurred in the measured light, indicating an internal substructure of the spot with distances between fragments less than 10 pm and even smaller fragment diameters. The current per fragment was estimated to be 20-40 A. The frames confirm the short time constants of the spot. From frame to frame a spot motion was observed in most cases, yielding spot residence times < 200 ns. 4763