Investigations on the series connection of two switching gaps in one tube in vacuum

Presently, there is a wide interest to study the extension of the vacuum switching principle toward higher voltages. This work studies the behavior of two vacuum arc gaps in series within the same vacuum vessel. The arrangement investigated uses a common center shield for both gaps, yielding asymmetrical conditions in the high-current, as well as in the post-arc phase. The voltage distribution during arcing is nearly equal at low and at high currents, while in an intermediate range, a strong asymmetry exists. The arc where the shield has anodic polarity stays diffuse and remains at a lower voltage, while the other, already constricted arc is characterized by a considerably higher, fluctuating voltage. Investigations with only one gap show that the post-arc current is higher when the shield is post-arc cathode because this current is determined by the growth of a space-charge sheath in front of the cathode. Consequently, in a double-break arrangement where the post-arc current is the same for both gaps, the voltage distribution is uneven in the sense that during the post-arc current flow the gap with the cathodic shield takes over a lower portion of the total transient recovery voltage (TRV). Despite the uneven voltage distribution, the breaking capacity of a double-break arrangement more than doubles in comparison with that of a single break. In case of the breakdown of the first gap, the other gap often takes over the complete TRV temporarily without final failure of both gaps.