Triggered vacuum gaps

Characteristics of a sealed vacuum gap are described and the difficulties encountered in applying this gap as an overvoltage protection device are discussed. It is shown how these difficulties can be ameliorated by the use of gas-free electrode materials and by triggering the gap when breakdown is required. Several methods of triggering are discussed and some practical triggering devices are described that inject minute quantities of ionized hydrogen into the gap. The hydrogen is eventually recovered by the use of a titanium hydride getter. It is shown that breakdown of the gap can be accomplished in less than one-tenth microsecond by first producing a glow discharge that is rapidly transformed into a metal-vapor arc. Properties of the metal-vapor arc are described which have an effect on the characteristics of the vacuum gap. A number of practical sealed-off triggered vacuum gaps are illustrated. These are used to carry microsecond capacitor discharge currents and 60-cycle power line currents for ½ cycle. The operating voltage range is from a few hundred volts to 100 kV. The advantages of vacuum gaps over gas-filled gaps are given and a number of overvoltage protection and switching applications are listed.