Random and directed components of arc spot motion in vacuum

Sequences of photographic frames with resolution down to 10 ns and 2 μm pixel −1 were obtained for vacuum arcs (30–40 A) with and without external magnetic field (0.36 T). In both cases extensive statistical studies confirm the diffusion character of the motion of cathode spot fragments on 10 ns timescale. In addition to such a diffusion, the magnetic field provokes fragment steps of length ∼5 μm in the retrograde direction, approximately once in every 0.3 μs, which account for the directed spot movement in magnetic field. Thus two mechanisms are responsible for spot dynamics. Episodes of immobile spot fragments are also observed.