Plasma density decay of vacuum discharges after current zero

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