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. It is somewhat similar to thyratron. The vacuum version is called vacuum krytron, or sprytron.
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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.
The gas used in krytrons is hydrogen;[1] noble gases (usually krypton, or a Penning mixture) can also be used.[2]
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.
The control grid is usually wrapped around the anode, except a small opening on its top.[3]
In place of or in addition to the keep-alive electrode some krytrons may contain a very tiny amount of radioactive material (usually less than 5 microcurie of 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. The radiation source serves to increase the reliability of ignition and formation of the keepalive electrode discharge. The keepalive discharge creates a preionized plasma between the cathode and anode; the time for forming of the arc is considerably shortened as the arc does not have to first create an ionized channel, which achieves switching times 3-4 orders of magnitude lower than with other cold-cathode tubes.[4]
The gas filling provides ions for neutralizing the space charge and allowing high currents at lower voltage.[3] The keepalive discharge populates the gas with ions, forming a preionized plasma; this can shorten the arc formation time by 3-4 orders of magnitude in comparison with non-preionized tubes, as time does not have to be spent on ionizing the medium during formation of the arc path.[4]
The electric arc is self-sustaining; once the tube is triggered, it conducts until the arc is interrupted by the current falling too low for too long (under 10 milliamperes for more than 100 microseconds for the KN22 krytrons).[1]
Krytrons and sprytrons are triggered by a high voltage from a capacitor discharge via a trigger transformer, in a similar way flashtubes for e.g. photoflash applications are triggered. Devices integrating a krytron with a trigger transformer are available.[4]
Sprytron
Sprytron, also known as vacuum krytron, is a vacuum-"filled" version. It 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.)
Sprytrons lack the keepalive electrode and the preionization radioactive source. The trigger pulse for the grid must be stronger than for a krytron. Sprytrons are however able to handle higher currents; krytrons tend to be used for triggering a secondary switch, e.g. a triggered spark gap, sprytrons are usually connected directly to the load.
The trigger pulse has to be much more intense, as there is no preionized gas path for the electric current, and a vacuum arc must form between the cathode and anode. An arc first forms between the cathode and the grid, then a breakdown occurs between the cathode-grid conductive region and the anode.[4]
Sprytrons are evacuated to hard vacuum. As kovar and other metals are somewhat permeable for hydrogen, especially during the 600 °C bakeout before evacuation and sealing, all external metal surfaces have to be plated with thick (25 micrometers or more) of soft gold. Same metallization is used for other switch tubes as well.[5]
Sprytrons can be also triggered by a laser pulse. In 1999, the laser pulse energy needed to trigger a sprytron was reduced to 10 microjoules.[6]
Sprytrons are usually designed as rugged metal/ceramic parts. Sprytrons typically have low inductance (10 nanohenry) and low electrical conductance when switched on (10-30 milliohms). After triggering, just before the sprytron switches on in avalanche mode, it becomes slightly conductive (100-200 amperes) briefly. High-power MOSFET transistors operating in avalanche mode show similar behavior. SPICE models for sprytrons are available.[7]
Performance
This design, dating from the late 1940s, is still capable of pulse-power performance which even the most advanced semiconductors (even IGBTs) cannot match easily. The krytrons and sprytrons are capable of handling high current high voltage pulses, with very fast switching times, constant low time delay between application of the trigger pulse and switching on, and a low jitter of this delay.
Krytrons can switch currents up to about 3000 amperes and voltages up to about 5000 volts. The commutation time of less than 1 nanosecond can be achieved, with the delay between the application of the trigger pulse and switching achievable as low as about 30 nanoseconds. The achievable jitter may be below 5 nanoseconds. The required trigger pulse voltage is about 200-2000 volts. Higher-voltage pulse can to a degree decrease the switching delay. The switching delay is strongly independent on the environment. The commutation time can be somewhat shortened by a faster rise time of the trigger pulse. A given krytron tube, with identical trigger pulse, will show very similar performance (low jitter) between the pulses.[4] The keepalive current ranges in tens to hundreds microamperes. The pulse repetition rate can range from one per minute to tens of thousands per minute.[3]
The switching performance is largely independent on the environment (temperature, acceleration, vibrations, etc.). The formation of the keepalive glow discharge is however more sensitive, which necessitates the use of the radioactive source to aid its ignition.
Krytrons have a limited lifetime, counted in the number of pulses they can switch. The number ranges by type between tens of thousands to tens of millions, but can be as low as few hundreds.[3][4]
Sprytrons have somewhat faster switching times than krytrons.
Hydrogen-filled thyratrons may be used as a replacement in some applications.
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 (sprytrons are usually used in such manner) or by triggering the higher-power spark gap switches. They are also used to trigger thyratrons, 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.
Only some types of krytrons and sprytrons are suitable for nuclear weapon triggers. Typical values considered relevant for nuclear weapons are voltages 2500 V and above, currents 100 A and above, and switching delays of under 10 microseconds.[8]
Krytron in popular culture
A Krytron was the "MacGuffin" in Roman Polanski's 1988 film Frantic.
The krytron, miscalled a 'kryton', also appeared in the Tom Clancy nuclear terrorism novel The Sum of All Fears.
In Larry Collins' book The Road to Armageddon, the plot revolved heavily around American-made krytrons Iranian mullahs wanted for three Russian nuclear artillery shells they had hoped to upgrade to full nuclear weapons.[9]
Further developments
Optically triggered solid-state switches based on diamond are a potential candidate for krytron replacement.[10]
References
- ^ a b "Trapping Low Energy in an Ion Trap" Harvard Ph.D. Thesis of Xiang Fei (Defended 10 May 1990), Chapter 4
- ^ Silicon Investigations Pulse Power Switching & EG&G Krytron Tube Replacement Page. Siliconinvestigations.com (2010-02-22). Retrieved on 2010-06-05.
- ^ a b c d Krytron information on Tube Collector site
- ^ a b c d e f Pulse Power Switching Devices. Electricstuff.co.uk. Retrieved on 2010-06-05.
- ^ Evaluation of non-cyanide gold plating process for switch tubes, Sandia Report, 1996
- ^ Stockpile Stewardship and Management ? United States Nuclear Forces. Globalsecurity.org. Retrieved on 2010-06-05.
- ^ Information Bridge: DOE Scientific and Technical Information - Sponsored by OSTI. Osti.gov (2010-05-28). Retrieved on 2010-06-05.
- ^ Technologies underlying weapons of mass destruction DIANE Publishing ISBN 1428921109 [Amazon-US | Amazon-UK]
- ^ Larry Collins. The Road to Armageddon. New Millennium. 2003. ISBN 193240709X [Amazon-US | Amazon-UK]
- ^ CVD Diamond for Electronic Devices and Sensors by Ricardo S. Sussmann, p. 285 John Wiley and Sons, 2009 ISBN 047006532X [Amazon-US | Amazon-UK]
- EG&G Electronic Components Catalog, 1994.
External links
- John Pasley's article about gas-filled switch tubes, Krytron section
- Photo of a small glass krytron
- 40 month sentence to illegal exporter (though the sentence was definitely related to the 'fugitive' details)
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