Retrograde arc spot motion in vacuum

Experiments are reported on the retrograde arc spot motion on copper cathodes in vacuum in the presence of a magnetic field. The spots are imaged with time and space resolution of <100 ns and <10 μm, respectively. The magnetic field amounted to B=0.4 T, the arc currents were 5-50 A. For times <1 μs there occurs random displacement on a time scale <100 ns. In intervals of about 4 μs jumps of the spot are observed over distances of 50-300 μm in the retrograde direction, thus yielding macroscopic velocities of about 50 m/s. The jumps are preceded by the ejection of plasma jets in the retrograde direction, having average velocities of about v=4 km/s. New spots are formed exactly in the jet direction. The jets are explained by instabilities of the magnetically confined spot plasma, and the spot formation by electric fields E=vxB within the jets