INFLUENCE OF ELECTRODE SEPARATION ON ION DENSITY IN THE VACUUM-ARC

Vacuum arcs with peak currents from 200 A to 1 kA were drawn between chromium-copper contacts, 40 mm in diameter. The density of singly charged chromium ions after forced current zeros was measured as a function of contact separation and axial position using laser-induced fluorescence. The ion density at current zero lies between 3 x 10(17) m-3 and 8 X 10(17) m-3 for 2 and 10 mm gaps, respectively, and is distributed uniformly along the axis. After current zero, it decays exponentially, the decay time constant being smallest near the postarc cathode and increasing toward the postarc anode. For a 10-mm gap, time constants of 1.9-mu-s near the postarc anode and 1-mu-s near the postarc cathode are typical, irrespective of the current amplitude. With 2-mm contact separation the time constant is as low as 0.5-mu-s. Results of earlier experiments with copper-tungsten contacts indicate that the density decay is considerably slower for tungsten than for chromium ions. Time constants of 2.6 and 1.3-mu-s, respectively, were measured near the center of the gap. Comparison of the dielectric recovery, neutral copper vapor density, and chromium ion density confirms that early recovery is much more strongly correlated with the ion density than with the concentration of neutral vapor.