The autograph method for investigation arcs in vacuum and in gases at threshold currents

Abstract

The present paper reports an experimental study of craters on the surface of cathodes of pulsed arc under the influence of surface conditions (contaminants and clean), weak transverse magnetic fields in vacuum and ambient gases (argon, nitrogen with high purity introduced with pressures from 50 Torr up to 800 Torr) with cold cathodes. The autograph method was used to investigate the arc spot motion over a cathodes under a magnetic field of H = 0.1-0.7 T in the range of current 2-50 A. Erosion traces have been studied of wires cathodes from Au, Ag, Cu, W. The analysis of arc voltage and arc current fluctuations in vacuum and in gas shows that the spot operates in cycles that include extremely non-stationary periods with time constants about 10-50 ns for all cathodes from Au, Ag, Cu, W. At the surfaces of Au, Ag, Cu cathodes in vacuum and in gases there remain numerous small craters (diameter <; 6 mkm). Such spots are type 1, in contrast to clean surface spots (type 2). Erosion traces clean surfaces of W cathodes in vacuum and in gases consist of overlapping craters which form a coherent chain. It appears that the elementary spot processes (plasma production, electron emission, retrograde movement, average cycle duration, current density) are not changed by outer influences as magnetic field or background gas. The current density measured at the moment of cathode spot death is (1 ± 0.9) × 1011 A/m2. Neither the current density nor the cathode spot lifetime depends on current.