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.
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.
The gas used in krytrons is hydrogen; [2] noble gases (usually krypton), or a Penning mixture can also be used. [3]
A krytron has four electrodes. Two are a conventional anode and cathode. 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. [4]
In place of or in addition to the keep-alive electrode some krytrons may contain a 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.
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]
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, 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 to hydrogen, especially during the 600 °C bake-out before evacuation and sealing, all external metal surfaces must be plated with a 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 (carrying 100–200 amperes); high-power MOSFET transistors operating in avalanche mode show similar behavior. SPICE models for sprytrons are available. [9]
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.
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.
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 in the early 1980s. [10] 469 of these were returned to the United States, 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 actually a Krytron-Pac, which consisted of a Krytron tube along with a trigger transformer encased in black epoxy. [12]
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. [13]
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".
Optically triggered solid-state switches based on diamond are a potential candidate for krytron replacement. [14]
Cathode rays or electron beams (e-beam) are streams of electrons observed in discharge tubes. If an evacuated glass tube is equipped with two electrodes and a voltage is applied, glass behind the positive electrode is observed to glow, due to electrons emitted from the cathode. They were first observed in 1859 by German physicist Julius Plücker and Johann Wilhelm Hittorf, and were named in 1876 by Eugen Goldstein Kathodenstrahlen, or cathode rays. In 1897, British physicist J. J. Thomson showed that cathode rays were composed of a previously unknown negatively charged particle, which was later named the electron. Cathode-ray tubes (CRTs) use a focused beam of electrons deflected by electric or magnetic fields to render an image on a screen.
A vacuum tube, electron tube, valve, or tube, is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied.
A cold cathode is a cathode that is not electrically heated by a filament. A cathode may be considered "cold" if it emits more electrons than can be supplied by thermionic emission alone. It is used in gas-discharge lamps, such as neon lamps, discharge tubes, and some types of vacuum tube. The other type of cathode is a hot cathode, which is heated by electric current passing through a filament. A cold cathode does not necessarily operate at a low temperature: it is often heated to its operating temperature by other methods, such as the current passing from the cathode into the gas.
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.
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 triggered, ionizes and conducts a high-voltage pulse to make light. Flashtubes are used most in photography; they also are used in science, medicine, industry, and entertainment.
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.
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 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.
An ignitron is a type of gas-filled tube used as a controlled rectifier and dating from the 1930s. Invented by Joseph Slepian while employed by Westinghouse, Westinghouse was the original manufacturer and owned trademark rights to the name "Ignitron". Ignitrons are closely related to mercury-arc valves but differ in the way the arc is ignited. They function similarly to thyratrons; a triggering pulse to the igniter electrode turns the device "on", allowing a high current to flow between the cathode and anode electrodes. After it is turned on, the current through the anode must be reduced to zero to restore the device to its nonconducting state. They are used to switch high currents in heavy industrial applications.
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 striking voltage, the gas ionization becomes self-sustaining, and the tube glows with a colored light. The color depends on the gas used.
A vacuum arc can arise when the surfaces of metal electrodes in contact with a good vacuum begin to emit electrons either through heating or in an electric field that is sufficient to cause field electron emission. Once initiated, a vacuum arc can persist, since the freed particles gain kinetic energy from the electric field, heating the metal surfaces through high-speed particle collisions. This process can create an incandescent cathode spot, which frees more particles, thereby sustaining the arc. At sufficiently high currents an incandescent anode spot may also be formed.
A voltage-regulator tube is an electronic component used as a shunt regulator to hold a voltage constant at a pre-determined level.
A mercury-arc valve or mercury-vapor rectifier or (UK) mercury-arc rectifier is a type of electrical rectifier used for converting high-voltage or high-current alternating current (AC) into direct current (DC). It is a type of cold cathode gas-filled tube, but is unusual in that the cathode, instead of being solid, is made from a pool of liquid mercury and is therefore self-restoring. As a result mercury-arc valves, when used as intended, are far more robust and durable and can carry much higher currents than most other types of gas discharge tube. Some examples have been in continuous service, rectifying 50-ampere currents, for decades.
A trigatron is a type of triggerable spark gap switch designed for high current and high voltage. It has very simple construction and in many cases is the lowest cost high energy switching option. It may operate in open air, it may be sealed, or it may be filled with a dielectric gas other than air or a liquid dielectric. The dielectric gas may be pressurized, or a liquid dielectric may be substituted to further extend the operating voltage. Trigatrons may be rated for repeated use, or they may be single-shot, destroyed in a single use.
A TEA laser is a gas laser energized by a high voltage electrical discharge in a gas mixture generally at or above atmospheric pressure. The most common types are carbon dioxide lasers and excimer lasers, both used extensively in industry and research; less common are nitrogen lasers. The acronym "TEA" stands for Transversely Excited Atmospheric.
In electronics, a crossatron is a high-power pulsed modulator device that consists of a cold cathode gas-filled tube that combines features of thyratrons, vacuum tubes, and power semiconductor switches. This switch is capable of operating with voltages in excess of 100 kilovolts by the use of deuterium gas fill to increase the Paschen breakdown voltage, axial molybdenum cathode corrugations to provide a higher current capability, and a Paschen shield that is formed from molybdenum. The terminal curvature of the Paschen shield and of the adjacent portion of the anode are selected to establish a voltage stress at the curved Paschen shield surface within the approximate range of 90–150 kV/cm in response to a 100 kV differential. The cold cathode gives the crossatron an advantage of achievable lifetime and reliability in comparison to a hydrogen-filled thyratron.
The inductive output tube (IOT) or klystrode is a variety of linear-beam vacuum tube, similar to a klystron, used as a power amplifier for high frequency radio waves. It evolved in the 1980s to meet increasing efficiency requirements for high-power RF amplifiers in radio transmitters. The primary commercial use of IOTs is in UHF television transmitters, where they have mostly replaced klystrons because of their higher efficiencies and smaller size. IOTs are also used in particle accelerators. They are capable of producing power output up to about 30 kW continuous and 7 MW pulsed and gains of 20–23 dB at frequencies up to about a gigahertz.
In the years 1942-1944, the Radio Manufacturers Association used a descriptive nomenclature system for industrial, transmitting, and special-purpose vacuum tubes. The numbering scheme was distinct from both the numbering schemes used for standard receiving tubes, and the existing transmitting tube numbering systems used previously, such as the "800 series" numbers originated by RCA and adopted by many others.
The pseudospark switch a gas-filled tube capable of high speed switching. Pseudospark switches are functionally similar to triggered spark gaps.
Electric discharge in gases occurs when electric current flows through a gaseous medium due to ionization of the gas. Depending on several factors, the discharge may radiate visible light. The properties of electric discharges in gases are studied in connection with design of lighting sources and in the design of high voltage electrical equipment.
A noise generator is a circuit that produces electrical noise. Noise generators are used to test signals for measuring noise figure, frequency response, and other parameters. Noise generators are also used for generating random numbers.
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