Displacement times of arc cathode spots in vacuum

With a combination of fast framing and streak channels, arc spots on a copper cathode are imaged by glass and UV-optics with /spl mu/m-resolution. At currents of about 70 A and sufficiently long after ignition (3-100 /spl mu/s), the spots reveal an inner fragment structure with sizes <5 /spl mu/m. The fragments change their position with velocities up to 1000 m/s. This leads to random displacement of the spot on a time scale <100 ns. The ratio of the mean square displacement to the observation time amounts to (R/sup 2/)/t=(2.3/spl plusmn/0.6)/spl times/10/sup -3/ m/sup 2//s. Thus, spot and fragments are nanosecond phenomena. Splitting and merging of the fragments are associated with strong brightness fluctuations. When imaging the fragments with increasing exposure times, the fluctuations lead to overestimating the centre of the brightness profile, resulting in smaller displacement values and not correctly reflecting the fragment dynamics.