ARC VOLTAGE OF A FAST TRIGGERED VACUUM GAP

At high current, the performance of triggered vacuum gaps (TVG) is limited by constriction of the vacuum arc. Several concentrated modes can be defined: foot point, anode spot, and ''intense arc'' mode. In all cases, small, luminous, high temperature spots (from melting to boiling temperature) appear on the anode surface. In accordance with the anodic mode, the arc voltage has different characteristics: quiet and low or with high frequency noise. We have measured the arc voltage in different electrode configurations for a conduction time of 27 mus and for peak current up to 45 kA. For small gap distances (1-1.5 mm), the arc voltage is quiet and low (20 to 30 V) and almost independent of the peak current. For greater distances, the arc voltage increases with the distance and the peak current. If the peak current is higher than the threshold interruption current Ith, the arc voltage is high and noisy. These overvoltages, with a frequency of about one megahertz, can reach more than 1000 V. These overvoltages disappear completely after about 15 mus and the voltage decreases.