Reverse current and arc-back in a single-gap mercury-arc valve

Arc-backs produced by continuously raising the stress on a high-voltage single-gap single-anode mercury-arc valve are preceded by a change in reverse current before failure occurs. On the cycle at which arc-back occurs the reverse current rises rapidly to about 10 A at peak reverse voltage (140 kV maximum). Subsequently, the reverse current decreases and then rises again steadily for several hundred microseconds. Failure occurs during this second rise of current. These changes in reverse current prior to arc-back are reproducible. The peak reverse voltage reached on the cycle at which arc-back sets in is also reproducible (to ±10 percent). This type of arc-back is interpreted as a sequential process that is initiated by ignition, at peak voltage, of a stable high-voltage discharge in the valve. The subsequent rise of reverse current which leads to failure is attributed to a rise in gas pressure within the valve due to ion bombardment from the high-voltage discharge. The measured delay of 60 µs from the point of rapid rise in reverse current to the start of the final growth of current suggests that mercury vapor is evolved from the negatively biased main anode to cause a rise in gas pressure. Detection of precursor currents might allow suspect valves to be blocked in time to prevent the occurrence of a full arc-back.