Trigger Discharge Tube

Trigger Discharge Tube 

a cold-cathode ion device in which the electrical discharge between the main electrodes is initiated by a voltage pulse applied to a trigger (control) electrode. Trigger discharge tubes may have three (triode) or four (tetrode) electrodes. In triodes, the trigger electrode is located either between the main electrodes or in a cavity within one of these electrodes. In tetrodes, the fourth electrode is used to produce the trigger discharge (a current of tens of microamperes being passed from a high-voltage, direct-current source through a limiting resistor). This discharge stabilizes the time lag of the main discharge with respect to the instant of the triggering pulse.

The electrodes of trigger discharge tubes are made of refractory metals and their alloys and are enclosed in glass, metal-and-glass, or cermet envelopes filled with gas at a pressure of 10–10³ kilonewtons/m². In trigger tubes known as vacuum spark gap relays, the space inside the envelope is pumped to a high vacuum; the spark discharge, initiated in the vacuum, is subsequently maintained by metal vapors emanating from the electrodes.

Trigger discharge tubes are used in pulse engineering as fast-acting switches and in devices designed to protect electric circuits and equipment from excessive voltages and currents. Trigger discharge tubes are capable of switching currents from tens of amperes to tens of kiloamperes at voltages from hundreds of volts to hundreds of kilovolts and a pulse duration from tenths of a microsecond to several milliseconds. Their service life is rated at 10³ breakdowns at a current of several tens of kiloamperes and at 10⁷ breakdowns at a current of several kiloamperes. Trigger discharge tubes are usually operated either in a single-pulse mode or at pulse frequencies that do not exceed several tens of hertz. Multichamber designs are used for higher pulse frequencies—up to several kilohertz at voltages up to several kilovolts.

Trigger discharge tubes are distinguished from other switching devices having the same purpose by the absence of a heating filament, by their instantaneous readiness to operate, their high overload stability, their small size and weight, and the simplicity of their design.

V. V. NIKITIN and L. M. TIKHOMIROV