Measurement of the tungsten ion concentration after forced extinction of a vacuum arc

The concentrations of singly ionized and neutral tungsten atoms were measured by laser-induced fluorescence after the forced extinction of vacuum arcs between tungsten-copper butt contacts, 28-mm in diam. and 10-mm apart. The 50-Hz current was forced to zero at its maximum of 200 A in 1.3 mu s by application of a reverse voltage. Near current zero, the ion concentration of 4*10/sup 17/ m/sup -3/ is of the same order of magnitude as the atomic tungsten concentration, which is 6*10/sup 17/ m/sup -3/. While the concentration of the neutrals remains virtually constant during 20 mu s after current zero, the ion concentration decays by three orders of magnitude in the same time. The decay-time constant varies from 1.9 mu s close to the postarc cathode to 3.6 mu s near the postarc anode. It is concluded that the dielectric recovery of vacuum gaps after diffuse arcs is mainly controlled by residual charge carriers.>