Measurement of the Neutral Copper Vapor Density around Current Zero of a 500-A Vacuum Arc Using Laser-Induced Fluorescence

The technique of laser-induced fluorescence was applied to measure the time dependence of the copper vapor density from 280 ¿s before to 400 ¿s after current zero of a vacuum arc driven by a sinusoidal current of 500-A rms. Atomic densities between 5 × 1017 m-3 and 5 × 1014 m-3 were observed at the center of the gap with a local resolution of better than 1 mm3. The observed densities were more than an order of magnitude smaller than those predicted by model calculations according to Rich and Farrall [16], and the density decay after current zero occurred much more slowly than expected. Both findings indicate that the correlation of recovery of dielectric strength to metal vapor density after current zero is different from the recovery criterion based on the ratio of the mean free path for atomic collisions to the gap length.