Nanosecond displacement times of arc cathode spots in vacuum

With a high speed camera consisting of a combination of framing and streak channels, arc spots on a copper cathode are imaged in the spectral range 200-800 nm with spatial and time resolution of2> to the observation time t of2> /t = (2.3±0.6) x 10-3m2/s. This holds down to t = 100 ns. Thus, fragments and spots operate on nanosecond time scales. Prior to apparent spot splitting and after apparent fragment merging the spot brightness increases considerably. When analyzing time-integrated pictures, the stages of increased brightness lead to overestimating the average residence time. Because of the short formation time, the fragments do not reach a balance between surface heating and heat conduction into the bulk, i.e., there is no stationary evaporation. A further substructure of the fragments exists with size <5 μm and timescale <10 ns.