Krytron

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KN2 "Krytron" switch tube, made by EG&G (about 25 mm tall) KN2KrytronTube.jpg
KN2 "Krytron" switch tube, made by EG&G (about 25 mm tall)

The krytron is a cold-cathode gas-filled tube intended for use as a very high-speed switch, somewhat similar to the thyratron. It consists of a sealed glass tube with four electrodes. A small triggering pulse on the grid electrode switches the tube on, allowing a large current to flow between the cathode and anode electrodes. The vacuum version is called a vacuum krytron, or sprytron. The krytron was one of the earliest developments of the EG&G Corporation.

Gas-filled tube arrangement of electrodes in a gas within an insulating, temperature-resistant envelope

A gas-filled tube, also known as a discharge tube, is an arrangement of electrodes in a gas within an insulating, temperature-resistant envelope. Gas-filled tubes exploit phenomena related to electric discharge in gases, and operate by ionizing the gas with an applied voltage sufficient to cause electrical conduction by the underlying phenomena of the Townsend discharge. A gas-discharge lamp is an electric light using a gas-filled tube; these include fluorescent lamps, metal-halide lamps, sodium-vapor lamps, and neon lights. Specialized gas-filled tubes such as krytrons, thyratrons, and ignitrons are used as switching devices in electric devices.

In electrical engineering, a switch is an electrical component that can "make" or "break" an electrical circuit, interrupting the current or diverting it from one conductor to another. The mechanism of a switch removes or restores the conducting path in a circuit when it is operated. It may be operated manually, for example, a light switch or a keyboard button, may be operated by a moving object such as a door, or may be operated by some sensing element for pressure, temperature or flow. A switch will have one or more sets of contacts, which may operate simultaneously, sequentially, or alternately. Switches in high-powered circuits must operate rapidly to prevent destructive arcing, and may include special features to assist in rapidly interrupting a heavy current. Multiple forms of actuators are used for operation by hand or to sense position, level, temperature or flow. Special types are used, for example, for control of machinery, to reverse electric motors, or to sense liquid level. Many specialized forms exist. A common use is control of lighting, where multiple switches may be wired into one circuit to allow convenient control of light fixtures.

Thyratron type of gas filled tube

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, producing a phenomenon known as Townsend discharge. Gases used include mercury vapor, xenon, neon, and hydrogen. Unlike a vacuum tube (valve), a thyratron cannot be used to amplify signals linearly.

Contents

Description

Unlike most other gas switching tubes, the krytron conducts by means of an arc discharge, to handle very high voltages and currents (reaching several kilovolts and several kiloamperes), rather than the low-current glow discharge used in other thyratrons. The krytron is a development of the triggered spark gaps and thyratrons originally developed for radar transmitters during World War II.

Electric arc electrical breakdown of a gas that produces an ongoing electrical 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 arc discharge is characterized by a lower voltage than a glow discharge and relies on thermionic emission of electrons from the electrodes supporting the arc. An archaic term is voltaic arc, as used in the phrase "voltaic arc lamp".

Spark gap arrangement of two conducting electrodes separated by a 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 conductors exceeds the breakdown voltage of the gas within the gap, a spark forms, ionizing the gas and drastically reducing its electrical resistance. An electric current then flows until the path of ionized gas is broken or the current reduces below a minimum value called the "holding current". This usually happens when the voltage drops, but in some cases occurs when the heated gas rises, stretching out and then breaking the filament of ionized gas. Usually, the action of ionizing the gas is violent and disruptive, often leading to sound, light and heat.

Radar object detection system based on radio waves

Radar is a detection system that uses radio waves to determine the range, angle, or velocity of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. A radar system consists of a transmitter producing electromagnetic waves in the radio or microwaves domain, a transmitting antenna, a receiving antenna and a receiver and processor to determine properties of the object(s). Radio waves from the transmitter reflect off the object and return to the receiver, giving information about the object's location and speed.

The gas used in krytrons is hydrogen; [2] noble gases (usually krypton), or a Penning mixture can also be used. [3]

Hydrogen Chemical element with atomic number 1

Hydrogen is a chemical element with symbol H and atomic number 1. With a standard atomic weight of 1.008, hydrogen is the lightest element in the periodic table. Hydrogen is the most abundant chemical substance in the Universe, constituting roughly 75% of all baryonic mass. Non-remnant stars are mainly composed of hydrogen in the plasma state. The most common isotope of hydrogen, termed protium, has one proton and no neutrons.

Noble gas group of chemical elements

The noble gases make up a group of chemical elements with similar properties; under standard conditions, they are all odorless, colorless, monatomic gases with very low chemical reactivity. The six noble gases that occur naturally are helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and the radioactive radon (Rn). Oganesson (Og) is variously predicted to be a noble gas as well or to break the trend due to relativistic effects; its chemistry has not yet been investigated.

Krypton Chemical element with atomic number 36

Krypton is a chemical element with symbol Kr and atomic number 36. It is a member of group 18 elements. A colorless, odorless, tasteless noble gas, krypton occurs in trace amounts in the atmosphere and is often used with other rare gases in fluorescent lamps. With rare exceptions, krypton is chemically inert.

Operation

Diagram of a Krytron Krytron.svg
Diagram of a Krytron

A krytron has four electrodes. Two are a 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.

Electrode electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte or a vacuum)

An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit. The word was coined by William Whewell at the request of the scientist Michael Faraday from two Greek words: elektron, meaning amber, and hodos, a way.

Anode electrode through which conventional current flows into a polarized electrical device

An anode is an electrode through which the conventional current enters into a polarized electrical device. This contrasts with a cathode, an electrode through which conventional current leaves an electrical device. A common mnemonic is ACID for "anode current into device". The direction of conventional current in a circuit is opposite to the direction of electron flow, so electrons flow out the anode into the outside circuit. In a galvanic cell, the anode is the electrode at which the oxidation reaction occurs.

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 which positive charges move. Electrons have a negative electrical charge, so the movement of electrons is opposite to that of the conventional current flow. Consequently, the mnemonic cathode current departs also means that electrons flow into the device's cathode from the external circuit.

The fourth is a control grid, usually wrapped around the anode, except for a small opening on its top. [4]

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 microcuries (180  kBq ) of nickel-63), which emits beta particles (high-speed electrons) to make ionization easier. The radiation source serves to increase the reliability of ignition and formation of the keep-alive electrode discharge.

Curie non-SI unit of radioactivity

The curie is a non-SI unit of radioactivity originally defined in 1910. According to a notice in Nature at the time, it was named in honour of Pierre Curie, but was considered at least by some to be in honour of Marie Curie as well.

The becquerel is the SI derived unit of radioactivity. One becquerel is defined as the activity of a quantity of radioactive material in which one nucleus decays per second. The becquerel is therefore equivalent to an inverse second, s−1. The becquerel is named after Henri Becquerel, who shared a Nobel Prize in Physics with Pierre and Marie Curie in 1903 for their work in discovering radioactivity.

Naturally occurring nickel (28Ni) is composed of five stable isotopes; 58
Ni
, 60
Ni
, 61
Ni
, 62
Ni
and 64
Ni
with 58
Ni
being the most abundant. 26 radioisotopes have been characterised with the most stable being 59
Ni
with a half-life of 76,000 years, 63
Ni
with a half-life of 100.1 years, and 56
Ni
with a half-life of 6.077 days. All of the remaining radioactive isotopes have half-lives that are less than 60 hours and the majority of these have half-lives that are less than 30 seconds. This element also has 1 meta state.

The gas filling provides ions for neutralizing the space charge and allowing high currents at lower voltage. [4] 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. [5]

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). [2]

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. [5]

Sprytron

A sprytron, also known as vacuum krytron or triggered vacuum switch (TVS), is a vacuum, rather than 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 keepalive 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, 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 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. [5]

Sprytrons are evacuated to hard vacuum, typically 0.001 Pa. As kovar and other metals are somewhat permeable for hydrogen, especially during the 600 °C bake-out before evacuation and sealing, all external metal surfaces have to be plated with thick (25 micrometers or more) layer of soft gold. The same metallization is used for other switch tubes as well. [6]

Sprytrons are often designed similar to trigatrons, 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, which liberates electrons and vaporized surface discharge material into the inter-electrode gap, which facilitates formation of a vacuum arc, closing the switch. The short switching time suggests electrons from the trigger discharge and the corresponding secondary electrons 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 from their components, diffusion of gases (especially hydrogen) through the metal components, and gas leaks through the hermetic seals; 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. [7]

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. [8]

Sprytrons are usually manufactured as rugged metal/ceramic parts. They typically have low inductance (10 nanohenries) and low electrical resistance when switched on (10–30 milliohms). 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 models for sprytrons are available. [9]

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). [5] 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. [4]

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. [4] [5]

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 Components and used in a variety of industrial and military devices. They are best known for their use in igniting exploding-bridgewire and slapper detonators in nuclear weapons, their original application, either directly (sprytrons are usually used for this) or by triggering higher-power spark gap switches. They are also used to trigger thyratrons, large flashlamps in photocopiers, lasers and scientific apparatus, and for firing ignitors for industrial explosives.

Export restrictions in USA

Because of their potential for use as triggers of nuclear weapons, the export of krytrons is tightly regulated 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 smuggle 15 orders of 810 krytrons total to Israel. [10] 469 of these were returned to America, with Israel claiming the remaining 341 were "destroyed in testing". [10]

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. [11]

A krytron was the "MacGuffin" in Roman Polanski's 1988 film Frantic. The device in the film was either a high-tech updated version or simply a fictionalized version made up for the story.

The krytron, incorrectly called a "kryton", also appeared in the Tom Clancy 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. [12]

The term "krytron" appeared in the season 3, episode 14 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 are a potential candidate for krytron replacement. [13]

Notes

  1. "Krytrons - Cold Cathode Switch Tube data sheet K5500B-1" (PDF). EG&G Electro-Optics Division, Salem, Massachusetts, USA. September 1973. Retrieved 11 September 2016.
  2. 1 2 "Trapping Low Energy in an Ion Trap" Harvard Ph.D. Thesis of Xiang Fei (Defended 10 May 1990), Chapter 4
  3. Silicon Investigations Pulse Power Switching & EG&G Krytron Tube Replacement Page. Siliconinvestigations.com (2010-02-22). Retrieved on 2010-06-05.
  4. 1 2 3 4 Krytron information on Tube Collector site
  5. 1 2 3 4 5 Pulse Power Switching Devices. Electricstuff.co.uk. Retrieved on 2010-06-05.
  6. Evaluation of non-cyanide gold plating process for switch tubes, Sandia Report, 1996
  7. , Miniature triggered vacuum switches for precise initiation of insensitive loads in demanding environments, e2v 2012
  8. Stockpile Stewardship and Management ? United States Nuclear Forces. Globalsecurity.org. Retrieved on 2010-06-05.
  9. Information Bridge: DOE Scientific and Technical Information – Sponsored by OSTI. Osti.gov (2010-05-28). Retrieved on 2010-06-05.
  10. 1 2 "Israel's Nuclear Weapons". fas.org.
  11. Technologies underlying weapons of mass destruction DIANE Publishing ISBN   1-4289-2110-9
  12. Larry Collins. The Road to Armageddon. New Millennium. 2003. ISBN   1-932407-09-X
  13. CVD Diamond for Electronic Devices and Sensors by Ricardo S. Sussmann, p. 285 John Wiley and Sons, 2009 ISBN   0-470-06532-X

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References

CBS/Hytron second source documentation: