Krytron

The Krytron is a cold-cathode gas filled tube intended for use as a very high-speed switch and was one of the earliest developments of the EG&G Corporation.

Description

Unlike most other gas switch tubes, the krytron uses arc discharge to handle very high voltages and currents (several kV and several kA peak), rather than the usual low-current glow discharge. The krytron is a development of the triggered spark gaps and thyratrons originally developed for radar transmitters during World War II.

Operation

There are four electrodes in a krytron. Two are conventional anode and cathode. One is a keep-alive electrode, arranged to be close to the cathode. The keep-alive has a low positive voltage applied, which causes a small area of gas to ionize near the cathode. High voltage is applied to the anode, but primary conduction does not occur until a positive pulse is applied to the trigger electrode ("Grid" in the image above). Once started, arc conduction carries a considerable current. In place of or in addition to the keep-alive electrode some krytrons may contain a very tiny amount of radioactive material (usually nickel-63) which emits beta particles (high-speed electrons) to make ionization easier. The amount of radiation in a krytron is very small and not harmful.

Performance

This design, dating from the late 1940s, is still capable of pulse-power performance which even the most advanced semiconductors (even IGBT transistors) cannot match easily. The vacuum-filled version is called a Sprytron and is designed for use in environments where high levels of ionizing radiation are present (because the radiation might cause the gas-filled krytron to trigger inadvertently.)

Applications

Krytrons and their variations are still manufactured by Perkin-Elmer Components, and used in a variety of industrial and military devices. They are best known for their use in igniting the exploding-bridgewire detonators and slapper detonators in nuclear weapons, their original application, either directly or by triggering the higher-power spark gap switches. They are also used to trigger large flashlamps in photocopiers, lasers and scientific apparatus, as well as firing ignitors for industrial explosives.

Export restrictions

Because of the potential for use as nuclear triggers, the export of krytrons is tightly regulated. A number of cases involving the smuggling or attempted smuggling of krytrons have been reported, as countries seeking to develop nuclear weapons have attempted to procure supplies of krytrons for igniting their weapons.

Krytron in popular culture

A Krytron was the "MacGuffin" in Roman Polanski's 1988 film "Frantic".

References

* EG&G Electronic Components Catalog, 1994.

External links

* [http://www.tubecollector.org/krytron.htm Krytron information on "Tube Collector" site]
* [http://www.electricstuff.co.uk/pulse.html#Krytron John Pasley's article about gas-filled switch tubes, Krytron section]
* [http://www.electricstuff.co.uk/sparkgaps.html#krytron Photo of a small glass krytron]
* [http://www.usdoj.gov/usao/cac/news/pr2002/070.html 40 month sentence to illegal exporter] (though the sentence was definitely related to the 'fugitive' details)