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 of <5 μm and <10 ns, respectively. At currents of 30-70 A and sufficiently long time after ignition (3-300 μs), the spots consist of fragments with diameters of 10-20 μm. These fragments appear and disappear in a cyclic way. Formation times <50 ns and residence times < 100 ns have been observed. Apparent fragment merging into one spot is due to the extinction of all of them except one, while apparent spot splitting is due to the formation of a new fragment outside the spot center. Consecutive fragment formation appears as displacement with momentary velocities up to 1000 m/s. The fragment dynamics leads to random displacement of the spot center with a ratio of mean square displacement ²
> to the observation time t of ²
> /t = (2.3±0.6) x 10
^-3
m
²
/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.