Decay of residual plasma in a vacuum gap after forced extinction of a 250-ampere arc

The decaying plasma present within an experimental vacuum switch following the forced extinction of a 250-ampere arc across silver electrodes is studied. Ions and electrons are collected by applying a 6-volt potential across the vacuum device at various predetermined delay intervals after arc extinction. The magnitudes of the peak currents collected by this means vary from about ½ ampere a few microseconds after arc extinction to less than a milliampere 200 microseconds later. The results suggest that this current is principally due to ions having an initial density of 4 × 1012/cm3and moving with an average velocity of 2.6 × 105cm/s. The movement of these ions is nearly independent of the applied potential.