![KN2KrytronTube](https://upload.wikimedia.org/wikipedia/commons/1/1a/KN2KrytronTube.jpg)
The krytron is a
cold-cathode
A cold cathode is a cathode that is not electrically heated by a filament.A negatively charged electrode emits electrons or is the positively charged terminal. For more, see field emission. A cathode may be considered "cold" if it emits more ele ...
gas-filled tube
A gas-filled tube, also commonly known as a discharge tube or formerly as a Plücker tube, is an arrangement of electrodes in a gas within an insulating, temperature-resistant envelope. Gas-filled tubes exploit phenomena related to electric ...
intended for use as a very high-speed
switch
In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type of ...
, somewhat similar to the
thyratron
A thyratron is a type of gas-filled tube used as a high-power electrical switch and controlled rectifier. Thyratrons can handle much greater currents than similar hard-vacuum tubes. Electron multiplication occurs when the gas becomes ionized, p ...
. It consists of a sealed glass tube with four
electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials d ...
s. A small triggering pulse on the
grid
Grid, The Grid, or GRID may refer to:
Common usage
* Cattle grid or stock grid, a type of obstacle is used to prevent livestock from crossing the road
* Grid reference, used to define a location on a map
Arts, entertainment, and media
* News ...
electrode switches the tube on, allowing a large current to flow between the
cathode
A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction in wh ...
and
anode
An anode is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, an electrode of the device through which conventional current leaves the device. A common mnemonic ...
electrodes. The vacuum version is called a vacuum krytron, or sprytron. The krytron was one of the earliest developments of the
EG&G Corporation.
Description
Unlike most other gas switching tubes, the krytron conducts by means of an
arc discharge
An electric arc, or arc discharge, is an electrical breakdown of a gas that produces a prolonged electrical discharge. The current through a normally nonconductive medium such as air produces a plasma; the plasma may produce visible light. An ...
, to handle very high voltages and currents (reaching several kilovolts and several kiloamperes), rather than the low-current
glow discharge
A glow discharge is a plasma formed by the passage of electric current through a gas. It is often created by applying a voltage between two electrodes in a glass tube containing a low-pressure gas. When the voltage exceeds a value called the st ...
used in other
thyratron
A thyratron is a type of gas-filled tube used as a high-power electrical switch and controlled rectifier. Thyratrons can handle much greater currents than similar hard-vacuum tubes. Electron multiplication occurs when the gas becomes ionized, p ...
s. The krytron is a development of the
triggered spark gap
A spark gap consists of an arrangement of two conducting electrodes separated by a gap usually filled with a gas such as air, designed to allow an electric spark to pass between the conductors. When the potential difference between the conduct ...
s and
thyratron
A thyratron is a type of gas-filled tube used as a high-power electrical switch and controlled rectifier. Thyratrons can handle much greater currents than similar hard-vacuum tubes. Electron multiplication occurs when the gas becomes ionized, p ...
s originally developed for
radar
Radar is a detection system that uses radio waves to determine the distance ('' ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, we ...
transmitters during
World War II
World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposing ...
.
The gas used in krytrons is
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
;
["Trapping Low Energy in an Ion Trap"](_blank)
Harvard Ph.D. Thesis of Xiang Fei (Defended 10 May 1990)
Chapter 4
noble gas
The noble gases (historically also the inert gases; sometimes referred to as aerogens) make up a class of chemical elements with similar properties; under standard conditions, they are all odorless, colorless, monatomic gases with very low ch ...
es (usually
krypton
Krypton (from grc, κρυπτός, translit=kryptos 'the hidden one') is a chemical element with the symbol Kr and atomic number 36. It is a colorless, odorless, tasteless noble gas that occurs in trace amounts in the atmosphere and is often ...
), or a
Penning mixture
A Penning mixture, named after Frans Michel Penning, is a mixture of gases used in electric lighting or displaying fixtures. Although the popular phrase for the most common of these is a neon lamp, it is more efficient to have the glass tube fill ...
can also be used.
Operation
![Krytron](https://upload.wikimedia.org/wikipedia/commons/7/74/Krytron.svg)
A krytron has four
electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials d ...
s. Two are a conventional
anode
An anode is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, an electrode of the device through which conventional current leaves the device. A common mnemonic ...
and
cathode
A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction in wh ...
. One is a keep-alive electrode, placed near 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 fourth is a control grid, usually wrapped around the anode, except for a small opening on its top.
In place of or in addition to the keep-alive electrode some krytrons may contain a tiny amount of radioactive material (usually less than of
nickel-63
Naturally occurring nickel (28Ni) is composed of five stable isotopes; , , , and , with being the most abundant (68.077% natural abundance). 26 radioisotopes have been characterised with the most stable being with a half-life of 76,000 years, ...
), which emits
beta particles (high-speed
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no ...
s) to make
ionization
Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecul ...
easier. The radiation source serves to increase the reliability of ignition and formation of the keep-alive electrode discharge.
The gas filling provides ions for neutralizing the
space charge
Space charge is an interpretation of a collection of electric charges in which excess electric charge is treated as a continuum of charge distributed over a region of space (either a volume or an area) rather than distinct point-like charges. Thi ...
and allowing high currents at lower voltage.
The keep-alive 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.
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).
Krytrons and sprytrons are triggered by a high voltage from a
capacitor
A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals.
The effect of ...
discharge via a trigger
transformer
A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...
, in a similar way
flashtube
A flashtube (flashlamp) is an electric arc lamp designed to produce extremely intense, incoherent, full-spectrum white light for a very short time. A flashtube is a glass tube with an electrode at each end and is filled with a gas that, when tr ...
s for e.g.
photoflash
A flash is a device used in photography that produces a brief burst of light (typically lasting 1/1000 to 1/200 of a second) at a color temperature of about 5500 K to help illuminate a scene. A major purpose of a flash is to illuminate a ...
applications are triggered. Devices integrating a krytron with a trigger transformer are available.
Sprytron
A sprytron, also known as vacuum krytron or triggered vacuum switch (TVS), is a vacuum, rather than a gas-filled, version. It is designed for use in environments with high levels of
ionizing radiation, which might trigger a gas-filled krytron spuriously. It is also more immune to electromagnetic interference than gas filled tubes.
Sprytrons lack the keep alive electrode and the preionization radioactive source. The trigger pulse must be stronger than for a krytron. Sprytrons are able to handle higher currents. Krytrons tend to be used for triggering a secondary switch, e.g., a
triggered spark gap
A spark gap consists of an arrangement of two conducting electrodes separated by a gap usually filled with a gas such as air, designed to allow an electric spark to pass between the conductors. When the potential difference between the conduct ...
, while 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 A vacuum arc can arise when the surfaces of metal electrodes in contact with a good vacuum begin to emit electrons either through heating (thermionic emission) or in an electric field that is sufficient to cause field electron emission. Once initia ...
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.
[Pulse Power Switching Devices](_blank)
Electricstuff.co.uk. Retrieved on 2010-06-05.
Sprytrons are evacuated to
hard vacuum
A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or " void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often di ...
, typically 0.001
Pa. As
kovar
Kovar (trademark of CRS Holdings, inc., Delaware) is a nickel–cobalt ferrous alloy compositionally identical to Fernico 1, designed to have substantially the same thermal expansion characteristics as borosilicate glass (~5 × 10−6 /K betwe ...
and other metals are somewhat permeable to hydrogen, especially during the 600 °C
bake-out
Bake-out, in several areas of technology and fabrication, and in building construction, refers to the process of using high heat temperature (heat), and possibly vacuum, to remove volatile compounds from materials and objects before placing them ...
before evacuation and sealing, all external metal surfaces must be plated with a thick (25 micrometers or more) layer of soft
gold
Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile me ...
. The same metallization is used for other switch tubes as well.
Sprytrons are often designed similar to
trigatron
A trigatron is a type of triggerable spark gap switch designed for high current and high voltage (usually 10–100 kV and 20–100 kA, though devices in the mega-ampere range exist as well). It has very simple construction and in many cases is the ...
s, with the trigger electrode coaxial to the cathode. In one design the trigger electrode is formed as metallization on the inner surface of an
alumina tube. The trigger pulse causes surface
flashover
A flashover is the near-simultaneous ignition of most of the directly exposed combustible material in an enclosed area. When certain organic materials are heated, they undergo thermal decomposition and release flammable gases. Flashover occurs w ...
, which liberates electrons and vaporized surface discharge material into the inter-electrode gap, which facilitates formation of a
vacuum arc A vacuum arc can arise when the surfaces of metal electrodes in contact with a good vacuum begin to emit electrons either through heating (thermionic emission) or in an electric field that is sufficient to cause field electron emission. Once initia ...
, closing the switch. The short switching time suggests electrons from the trigger discharge and the corresponding
secondary electron
Secondary electrons are electrons generated as ionization products. They are called 'secondary' because they are generated by other radiation (the ''primary'' radiation). This radiation can be in the form of ions, electrons, or photons with suffici ...
s knocked from the anode as the initiation of the switching operation; the vaporized material travels too slowly through the gap to play significant role. The repeatability of the triggering can be improved by special coating of the surface between the trigger electrode and the cathode, and the
jitter can be improved by doping the trigger substrate and modifying the trigger probe structures. Sprytrons can degrade in storage, by
outgassing
Outgassing (sometimes called offgassing, particularly when in reference to indoor air quality) is the release of a gas that was dissolved, trapped, frozen, or absorbed in some material. Outgassing can include sublimation and evaporation (which ...
from their components, diffusion of gases (especially hydrogen) through the metal components, and gas leaks through the
hermetic seal
A hermetic seal is any type of sealing that makes a given object airtight (preventing the passage of air, oxygen, or other gases). The term originally applied to airtight glass containers, but as technology advanced it applied to a larger categor ...
s. An example tube manufactured with internal pressure of 0.001 Pa will exhibit spontaneous gap breakdowns when the pressure inside rises to 1 Pa. Accelerated testing of storage life can be done by storing in increased ambient pressure, optionally with added helium for leak testing, and increased temperature storage (150 °C) for outgassing testing. Sprytrons can be made miniaturized and rugged.
Sprytrons can be also triggered by a
laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fi ...
pulse. In 1999 the laser pulse energy needed to trigger a sprytron was reduced to 10 microjoules.
Sprytrons are usually manufactured as rugged metal/
ceramic
A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain ...
parts. They typically have low
inductance (10 nano
henries
The henry (symbol: H) is the unit of electrical inductance in the International System of Units (SI). If a current of 1 ampere flowing through a coil produces flux linkage of 1 weber turn, that coil has a self inductance of 1 henry. The un ...
) and low electrical resistance when switched on (10–30 milli
ohm
Ohm (symbol Ω) is a unit of electrical resistance named after Georg Ohm.
Ohm or OHM may also refer to:
People
* Georg Ohm (1789–1854), German physicist and namesake of the term ''ohm''
* Germán Ohm (born 1936), Mexican boxer
* Jörg Ohm (b ...
s). After triggering, just before the sprytron switches fully on in avalanche mode, it briefly becomes slightly conductive (100–200 amperes); high-power
MOSFET transistors operating in avalanche mode show similar behavior.
SPICE
A spice is a seed, fruit, root, bark, or other plant substance primarily used for flavoring or coloring food. Spices are distinguished from herbs, which are the leaves, flowers, or stems of plants used for flavoring or as a garnish. Spice ...
models for sprytrons are available.
Performance
This design, dating from the late 1940s, is still capable of pulse-power performance that even the most advanced semiconductors (even
IGBTs) cannot match easily. Krytrons and sprytrons are capable of handling high-current high-voltage pulses, with very fast switching times, and constant, low
jitter time delay between application of the trigger pulse and switching on.
Krytrons can switch currents of up to about 3000 amperes and voltages up to about 5000 volts. Commutation time of less than 1 nanosecond can be achieved, with a delay between the application of the trigger pulse and switching 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 voltages decrease the switching delay to some degree. Commutation time can be somewhat shortened by increasing the trigger pulse rise time. A given krytron tube will give very consistent performance to identical trigger pulses (low jitter).
The keep-alive current ranges from tens to hundreds of microamperes. The pulse repetition rate can range from one per minute to tens of thousands per minute.
Switching performance is largely independent of the environment (temperature, acceleration, vibration, etc.). However, the formation of the keep-alive glow discharge is more sensitive, which necessitates the use of a radioactive source to aid its ignition.
Krytrons have a limited lifetime, ranging, according to type, typically from tens of thousands to tens of millions of switching operations, and sometimes only a few hundreds.
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 manufactured by
Perkin-Elmer
PerkinElmer, Inc., previously styled Perkin-Elmer, is an American global corporation focused in the business areas of diagnostics, life science research, food, environmental and industrial testing. Its capabilities include detection, imaging, inf ...
Components and used in a variety of industrial and military devices. They are best known for their use in igniting
exploding-bridgewire and
slapper detonator
A slapper detonator, also called exploding foil initiator (EFI), is a relatively recent kind of a detonator developed by Lawrence Livermore National Laboratory, US Patent No. 4,788,913. It is an improvement of the earlier exploding-bridgewire deto ...
s in
nuclear weapon
A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission (fission bomb) or a combination of fission and fusion reactions ( thermonuclear bomb), producing a nuclear explosion. Both bom ...
s, their original application, either directly (sprytrons are usually used for this) or by triggering higher-power
spark gap
A spark gap consists of an arrangement of two conducting electrodes separated by a gap usually filled with a gas such as air, designed to allow an electric spark to pass between the conductors. When the potential difference between the conduct ...
switches. They are also used to trigger
thyratron
A thyratron is a type of gas-filled tube used as a high-power electrical switch and controlled rectifier. Thyratrons can handle much greater currents than similar hard-vacuum tubes. Electron multiplication occurs when the gas becomes ionized, p ...
s, large
flashlamps in
photocopier
A photocopier (also called copier or copy machine, and formerly Xerox machine, the generic trademark) is a machine that makes copies of documents and other visual images onto paper or plastic film quickly and cheaply. Most modern photocopiers ...
s,
laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fi ...
s and scientific apparatus, and for firing ignitors for industrial
explosives.
Export restrictions in the United States
Because of their potential for use as triggers of nuclear weapons, the export of krytrons is tightly
regulated
Regulation is the management of complex systems according to a set of rules and trends. In systems theory, these types of rules exist in various fields of biology and society, but the term has slightly different meanings according to context. Fo ...
in the United States. 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. One prominent case was that of
Richard Kelly Smyth, who allegedly helped
Arnon Milchan
Arnon Milchan ( he, ארנון מילצ'ן; December 6, 1944) is an Israeli businessman, film producer and spy. He has been involved in over 130 full-length motion pictures and is the founder of production company Regency Enterprises. Regency's ...
smuggle 15 orders of 810 krytrons total to Israel in the early 1980s.
469 of these were returned to the United States, with Israel claiming the remaining 341 were "destroyed in testing".
Krytrons and sprytrons handling voltages of 2,500 V and above, currents of 100 A and above, and switching delays of under 10 microseconds are typically suitable for nuclear weapon triggers.
In popular culture
A krytron was the "
MacGuffin
In fiction, a MacGuffin (sometimes McGuffin) is an object, device, or event that is necessary to the plot and the motivation of the characters, but insignificant, unimportant, or irrelevant in itself. The term was originated by Angus MacPhail for ...
" in Roman Polanski's 1988 film
''Frantic''. The device in the film was actually a Krytron-Pac, which consisted of a Krytron tube along with a trigger transformer encased in black epoxy.
The krytron, incorrectly called a "kryton", also appeared in the
Tom Clancy
Thomas Leo Clancy Jr. (April 12, 1947 – October 1, 2013) was an American novelist. He is best known for his technically detailed espionage and military-science storylines set during and after the Cold War. Seventeen of his novels have ...
nuclear terrorism novel ''
The Sum of All Fears''.
The plot of
Larry Collins' book ''The Road to Armageddon'' revolved heavily around American-made krytrons that Iranian mullahs wanted for three Russian nuclear artillery shells they had hoped to upgrade to full nuclear weapons.
The term "krytron" appeared in the season 3, episode 14 (Provenance) of the television drama ''
Person of Interest''.
In Season 3 of
NCIS episode "Kill Ari, Part 2", it was revealed that Ari Haswari, a rogue Mossad operative, had been tasked with acquiring a krytron trigger. Along with stolen plutonium from Dimona, these were key components for an Israeli sting operation. The krytron was also incorrectly called a "kryton".
Further developments
Optically triggered solid-state switches based on
diamond
Diamond is a solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Another solid form of carbon known as graphite is the chemically stable form of carbon at room temperature and pressure, ...
are a potential candidate for krytron replacement.
CVD Diamond for Electronic Devices and Sensors
by Ricardo S. Sussmann, p. 285 John Wiley and Sons, 2009
Notes
References
* EG&G Electronic Components Catalog, 1994.
CBS
CBS Broadcasting Inc., commonly shortened to CBS, the abbreviation of its former legal name Columbia Broadcasting System, is an American commercial broadcast television and radio network serving as the flagship property of the CBS Entertainm ...
/Hytron second source
In the electronics industry, a second source is a company that is licensed to manufacture and sell components originally designed by another company (the first source).
It is common for engineers and purchasers to avoid components that are only av ...
documentation:
* "''Krytron Trigger Tubes''" spec sheet
E-337
E-337A-1
E-337A-2
"''7229 Cold-Cathode Trigger Tube''" data sheet E287B
"''7230 Reliable Cold-Cathode Trigger Tube''" data sheet E287C
"''7231 Subminiature Cold-Cathode Trigger Tube''" data sheet E287D
"''7232 Reliable Subminiature Cold-Cathode Trigger Tube''" data sheet E287E
External links
* ttp://www.electricstuff.co.uk/sparkgaps.html#krytron Photo of a small glass krytron
40 month sentence to illegal exporter
(though the sentence was definitely related to the 'fugitive' details)
{{Electronic components
Gas-filled tubes
Nuclear weapons
Pulsed power
Switching tubes
Vacuum tubes