The retrograde motion of arc cathode spots in vacuum

Abstract

Experiments are reported on the retrograde arc spot motion on copper and tantalum cathodes in vacuum in the presence of a magnetic field. The spots are imaged with time and space resolutions of <100 ns and <10 µm, respectively. The magnetic flux density amounted to B = 0.4 T and the arc currents to 2-100 A. For times <1 µs random displacement occurs on a time scale <100 ns. At 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-1. The jumps are preceded by the ejection of plasma jets in the retrograde direction, having average velocities of about v = 5 km s-1. New spots are formed exactly in the jet direction. The jets are explained by instabilities in the magnetically confined spot plasma, and the spot formation by electric fields = × within the jets. The jets are ejected in periods of enhanced plasma production caused by the inner spot processes, i.e., by the dynamics of fragments and cells, having diameters of ⩽20 and ⩽10 µm, respectively. No reversal of the motion has been observed at elevated temperatures up to 2100 K.