The retrograde motion of arc cathode spots in vacuum

Experiments are reported on the retrograde are 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 mu m, respectively. The magnetic flux density amounted to B = 0.4 T and the are currents to 2-100 A. For times <1 mu s random displacement occurs on a time scale <100 ns. At intervals of about 4 mu s, jumps of the spot are observed over distances of 50-300 mu m 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 nu = 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 (E) over right arrow = <(nu)over right arrow> x (B) over right arrow 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 less than or equal to 20 and less than or equal to 10 mu m, respectively. No reversal of the motion has been observed at elevated temperatures up to 2100 K.