Influence of electrode separation on ion density in the vacuum arc

The density of singly ionized chromium shortly before and after forced extinction of vacuum arcs between chromium-copper electrodes was measured by laser-induced fluorescence for 2- and 10-mm contact gaps and currents between 200 A and 1 kA. In all cases studied, the ion density was constant before ramping down to the current and decayed exponentially after current zero. The ion density at current zero was found to be lower and to decay faster for a short gap than for a longer one, clearly indicating the effect of the contact separation on the charge carrier density. The variation of the time constant for the ion density decay with contact separation is closely analogous to the influence of contact separation on the recovery time of a switch gap. Furthermore, the recovery of dielectric strength of a chromium-copper gap proceeds on the same timescale as the decay of the density of singly ionized chromium. Both of these findings confirm that the ion density has a strong impact on the recovery of a vacuum gap