The retrograde motion of arc cathode spots in vacuum

Abstract

Experiments are reported on the retrograde arc spot motion oncopper and tantalum cathodes in vacuum in the presence of a magnetic field. The spotsare 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 arccurrents to 2-100 A. For times <1 µs random displacement occurs on atime scale <100 ns. At intervals of about 4 µs, jumps of thespot are observed over distances of 50-300 µm in the retrogradedirection, thus yielding macroscopic velocities of about 50 m s-1. The jumps are preceded by the ejection of plasma jets in theretrograde direction, having average velocities of about v = 5 km s-1. New spots are formed exactly in the jet direction. The jets areexplained by instabilities in the magnetically confined spot plasma, and thespot formation by electric fields E⃗ = v⃗×B⃗ within thejets. The jets are ejected in periods of enhanced plasma production caused bythe inner spot processes, i.e., by the dynamics of fragments and cells, havingdiameters of ⩽20 and ⩽10 µm, respectively. No reversal ofthe motion has been observed at elevated temperatures up to 2100 K.