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Spark Gaps

A triggered spark gap is a high voltage switch capable of switching tens of thousands of volts in microseconds with peak currents in the range of thousands to hundreds of thousand of amperes. A typical spark gap consists of two refractory metal electrodes held at a fixed separation inside an electrically insulating envelope. A third small electrode is fixed in close proximity to one of the main electrodes. The pressure of the gas within the envelope is adjusted so that when the operating voltage is applied to the main electrodes no conduction occurs. Upon applying a low current, high voltage pulse to the third (trigger) electrode a plasma is formed which causes the gas to become conductive and allows the passage of a high current discharge between the main electrodes.

Each discharge causes some metal to be sputtered off the main electrodes due to ion bombardment of the negative electrode (cathode). Ultimately a thin metal film coats the inside of the insulating envelope and creates a short circuit between the main electrodes. At this point the gap must be replaced.

We have found that cryogenic treatment of triggered spark gaps retards the rate of formation of the metal film by a factor of four. That is, treated spark gaps have a service life which is four times that of the identical, but untreated, gap! We believe this is due to the reduced rate of sputtering under ion bombardment of the cathode in treated, and hence stress relieved electrodes. By the way, the electrodes are typically non-ferrous. There is no austenite to martensite transformation involved here.