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 /spl mu/m, respectively. The magnetic field amounted to B=0.4 T, the arc currents were 5-50 A. For times 1 /spl mu/s there occurs random displacement on a time scale 100 ns. In intervals of about 4 /spl mu/s jumps of the spot are observed over distances of 50-300 /spl mu/m 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.
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