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 μs by application of a reverse voltage. Near current zero, the ion concentration of 4×10¹⁷ m^-3 is of the same order of magnitude as the atomic tungsten concentration, which is 6×10¹⁷ m^-3. While the concentration of the neutrals remains virtually constant during 20 μs after current zero, the ion concentration decays by three orders of magnitude in the same time. The decay-time constant varies from 1.9 μs close to the postarc cathode to 3.6 μs near the postarc anode. It is concluded that the dielectric recovery of vacuum gaps after diffuse arcs is mainly controlled by residual charge carriers