Plasma density decay of vacuum discharges after current zero

In vacuum circuit breakers the post-arc current caused by the remaining ions and electrons in the contact gap is an indication of the residual ionization and its decay. It coincides with the formation of a positive space charge sheath in front of the new cathode, which grows toward the new anode. In a vacuum test chamber an arc (1.5-15 kA RMS) is drawn between high current electrodes of the spiral type. At different times after current zero a transient recovery voltage is applied across a separate pair of high voltage electrodes. In contrast to real circuit breakers, where the transient recovery voltage reappears between the arcing contacts, this separation allows the study of residual plasma free from the thermal stress and melting on the contact surfaces. From the post-arc current across these electrodes, in comparison with a mathematical model of sheath growth, the density of the charge carriers can be evaluated. Such values and their temporal decay are presented.