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A thyratron is a type of gas-filled tube used as a high-power electrical switch and controlled
rectifier A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. The reverse operation (converting DC to AC) is performed by an inve ...
. 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 The Townsend discharge or Townsend avalanche is a gas ionisation process where free electrons are accelerated by an electric field, collide with gas molecules, and consequently free additional electrons. Those electrons are in turn accelerated an ...
. Gases used include mercury vapor,
xenon Xenon is a chemical element with the symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...
, neon, and (in special high-voltage applications or applications requiring very short switching times)
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-to ...
. Unlike a
vacuum tube A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied. The type known as ...
(valve), a thyratron cannot be used to amplify signals linearly. In the 1920s, thyratrons were derived from early vacuum tubes such as the UV-200, which contained a small amount of argon gas to increase its sensitivity as a
radio Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30  hertz (Hz) and 300  gigahertz (GHz). They are generated by an electronic device called a tr ...
signal detector, and the German LRS relay tube, which also contained argon gas. Gas
rectifier A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. The reverse operation (converting DC to AC) is performed by an inve ...
s, which predated vacuum tubes, such as the argon-filled General Electric " Tungar bulb" and the Cooper-Hewitt mercury-pool rectifier, also provided an influence.
Irving Langmuir Irving Langmuir (; January 31, 1881 – August 16, 1957) was an American chemist, physicist, and engineer. He was awarded the Nobel Prize in Chemistry in 1932 for his work in surface chemistry. Langmuir's most famous publication is the 1919 ar ...
and G. S. Meikle of GE are usually cited as the first investigators to study controlled rectification in gas tubes, about 1914. The first commercial thyratrons appeared circa 1928. The term "thyratron" is derived from Ancient Greek "θύρα" ("thyra"), meaning "door" or "valve". The term "
thyristor A thyristor () is a solid-state semiconductor device with four layers of alternating P- and N-type materials used for high-power applications. It acts exclusively as a bistable switch (or a latch), conducting when the gate receives a current ...
" was further derived from a combination of "thyratron" and "
transistor upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink). A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch ...
". Since the 1960s thyristors have replaced thyratrons in most low- and medium-power applications.


Description

Thyratrons resemble
vacuum tubes A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied. The type known as a ...
both in appearance and construction but differ in behavior and operating principle. In a vacuum tube, conduction is dominated by free electrons because the distance between
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 is ...
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 whi ...
is small compared to the
mean free path In physics, mean free path is the average distance over which a moving particle (such as an atom, a molecule, or a photon) travels before substantially changing its direction or energy (or, in a specific context, other properties), typically as ...
of electrons. A thyratron, on the other hand, is intentionally filled with gas so that the distance between anode and cathode is comparable with the mean free path of electrons. This causes conduction in a thyratron to be dominated by plasma conductivity. Due to the high conductivity of plasma, a thyratron is capable of switching higher currents than vacuum tubes which are limited by space charge. A vacuum tube has the advantage that conductivity may be modulated at any time whereas a thyratron becomes filled with plasma and continues to conduct as long as a
voltage Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to ...
exists between the anode and cathode. A pseudospark switch operates in a similar regime of the Paschen curve as a thyratron and is sometimes called a cold cathode thyratron. A thyratron consists of a hot cathode, an anode, and one or more control grids between the anode and cathode in an airtight glass or ceramic envelope that is filled with gas. The gas is typically
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-to ...
or
deuterium Deuterium (or hydrogen-2, symbol or deuterium, also known as heavy hydrogen) is one of two stable isotopes of hydrogen (the other being protium, or hydrogen-1). The nucleus of a deuterium atom, called a deuteron, contains one proton and one ...
at a pressure of 300 to 500 m
Torr The torr (symbol: Torr) is a unit of pressure based on an absolute scale, defined as exactly of a standard atmosphere (). Thus one torr is exactly (≈ ). Historically, one torr was intended to be the same as one " millimeter of merc ...
(40 to 70  Pa). Commercial thyratrons also contain a titanium hydride reservoir and a reservoir heater that together maintain gas pressure over long periods regardless of gas loss. Conductivity of a thyratron remains low as long as the control grid is negative relative to the cathode because the grid repels electrons emitted by the cathode. Space charge limited electron current flows from the cathode through the control grid toward the anode if the grid is made positive relative to the cathode. Sufficiently high space charge limited current initiates
Townsend discharge The Townsend discharge or Townsend avalanche is a gas ionisation process where free electrons are accelerated by an electric field, collide with gas molecules, and consequently free additional electrons. Those electrons are in turn accelerated an ...
between anode and cathode. The resulting plasma provides high conductivity between anode and cathode and is not limited by space charge. Conductivity remains high until the current between anode and cathode drops to a small value for a sufficiently long time that the gas ceases to be ionized. This recovery process takes 25 to 75 μ s and limits thyratron repetition rates to a few k Hz.


Applications

Low-power thyratrons (''relay tubes'' and ''trigger tubes'') were manufactured for controlling incandescent lamps, electromechanical relays or solenoids, for bidirectional counters, to perform various functions in Dekatron calculators, for voltage threshold detectors in RC timers, etc. ''Glow thyratrons'' were optimized for high gas-discharge light output or even
phosphor A phosphor is a substance that exhibits the phenomenon of luminescence; it emits light when exposed to some type of radiant energy. The term is used both for fluorescent or phosphorescent substances which glow on exposure to ultraviolet or v ...
ized and used as self-displaying shift registers in large-format, crawling-text dot-matrix displays. Another use of the thyratron was in relaxation oscillators. Since the plate turn-on voltage is much higher than the turn-off voltage, the tube exhibits
hysteresis Hysteresis is the dependence of the state of a system on its history. For example, a magnet may have more than one possible magnetic moment in a given magnetic field, depending on how the field changed in the past. Plots of a single component of ...
and, with a capacitor across it, it can function as a sawtooth oscillator. The voltage on the grid controls the breakdown voltage and thus the period of oscillation. Thyratron relaxation oscillators were used in power inverters and oscilloscope sweep circuits. One miniature thyratron, the triode 6D4, found an additional use as a potent noise source, when operated as a diode (grid tied to cathode) in a transverse magnetic field. Sufficiently filtered for "flatness" ("
white noise In signal processing, white noise is a random signal having equal intensity at different frequencies, giving it a constant power spectral density. The term is used, with this or similar meanings, in many scientific and technical disciplines ...
") in a band of interest, such noise was used for testing radio receivers, servo systems and occasionally in analog computing as a random value source. The miniature RK61/2 thyratron marketed in 1938 was designed specifically to operate like a vacuum triode below its ignition voltage, allowing it to amplify analog signals as a self-quenching superregenerative detector in
radio control Radio control (often abbreviated to RC) is the use of control signals transmitted by radio to remotely control a device. Examples of simple radio control systems are garage door openers and keyless entry systems for vehicles, in which a smal ...
receivers, and was the major technical development which led to the wartime development of radio-controlled weapons and the parallel development of radio controlled modelling as a hobby.George Honnest-Redlich ''Radio Control for Models (1950)'' p. 7 Some early television sets, particularly British models, used thyratrons for vertical (frame) and horizontal (line) oscillators. Medium-power thyratrons found applications in machine tool motor controllers, where thyratrons, operating as phase-controlled rectifiers, are utilized in the tool's armature regulator (zero to "base speed", "constant torque" mode) and in the tool's field regulator ("base speed" to about twice "base speed", "constant horsepower" mode). Examples include
Monarch Machine Tool A monarch is a head of stateWebster's II New College DictionarMonarch Houghton Mifflin. Boston. 2001. p. 707. for life or until abdication, and therefore the head of state of a monarchy. A monarch may exercise the highest authority and power ...
10EE lathe, which used thyratrons from 1949 until solid-state devices replaced them in 1984.http://www.lathes.co.uk/monarch/page2.html Lathes.co.uk, retrieved 2012 July 27 High-power thyratrons are still manufactured, and are capable of operation up to tens of kiloamperes (kA) and tens of
kilovolts The volt (symbol: V) is the unit of electric potential, electric potential difference (voltage), and electromotive force in the International System of Units (SI). It is named after the Italian physicist Alessandro Volta (1745–1827). Defini ...
(kV). Modern applications include pulse drivers for pulsed
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, Marine radar, ships, spacecraft, guided missiles, motor v ...
equipment, high-energy gas lasers,
radiotherapy Radiation therapy or radiotherapy, often abbreviated RT, RTx, or XRT, is a therapy using ionizing radiation, generally provided as part of cancer treatment to control or kill malignant cells and normally delivered by a linear accelerator. Rad ...
devices, particle accelerators and in Tesla coils and similar devices. Thyratrons are also used in high-power
UHF Ultra high frequency (UHF) is the ITU designation for radio frequencies in the range between 300 megahertz (MHz) and 3 gigahertz (GHz), also known as the decimetre band as the wavelengths range from one meter to one tenth of a meter (on ...
television Television, sometimes shortened to TV, is a telecommunication medium for transmitting moving images and sound. The term can refer to a television set, or the medium of television transmission. Television is a mass medium for advertising, ...
transmitter In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna. The transmitter itself generates a radio frequency alternating current, which is applied to the ...
s, to protect
inductive output tube 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-po ...
s from internal shorts, by grounding the incoming high-voltage supply during the time it takes for a
circuit breaker A circuit breaker is an electrical safety device designed to protect an electrical circuit from damage caused by an overcurrent or short circuit. Its basic function is to interrupt current flow to protect equipment and to prevent the ris ...
to open and reactive components to drain their stored charges. This is commonly called a ''
crowbar circuit A crowbar, also called a wrecking bar, pry bar or prybar, pinch-bar, or occasionally a prise bar or prisebar, colloquially, in Britain and Australia sometimes called a jemmy or jimmy (also called jemmy bar), gooseneck, or pig foot, is a tool ...
''. Thyratrons have been replaced in most low and medium-power applications by corresponding semiconductor devices known as
thyristor A thyristor () is a solid-state semiconductor device with four layers of alternating P- and N-type materials used for high-power applications. It acts exclusively as a bistable switch (or a latch), conducting when the gate receives a current ...
s (sometimes called silicon-controlled rectifiers, or SCRs) and triacs. However, switching service requiring voltages above 20 kV and involving very short risetimes remains within the domain of the thyratron. Variations of the thyratron idea are the krytron, the sprytron, the ignitron, and 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 conductor ...
, all still used today in special applications, such as nuclear weapons (krytron) and AC/DC-AC power transmission (ignitron).


Example of a small thyratron

The ''885'' is a small thyratron tube, using
argon Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice a ...
gas. This device was used extensively in the timebase circuits of early oscilloscopes in the 1930s. It was employed in a circuit called a relaxation oscillator. 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 World War II by country, vast majority of the world's countries—including all of the great power ...
, small thyratrons similar to the 885 were utilized in pairs to construct bistables, the "memory" cells used by early
computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations ( computation) automatically. Modern digital electronic computers can perform generic sets of operations known as programs. These prog ...
s and
code breaking Cryptanalysis (from the Greek ''kryptós'', "hidden", and ''analýein'', "to analyze") refers to the process of analyzing information systems in order to understand hidden aspects of the systems. Cryptanalysis is used to breach cryptographic sec ...
machines. Thyratrons were also used for phase angle control of
alternating current Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in whic ...
(AC) power sources in
battery charger A battery charger, recharger, or simply charger is a device that stores energy in a battery by running an electric current through it. The charging protocol (how much voltage or current for how long, and what to do when charging is complete) depe ...
s and
light dimmer A dimmer is a device connected to a light fixture and used to lower the brightness of the lighting, light. By changing the voltage waveform applied to the lamp, it is possible to lower the luminous intensity, intensity of the light output. Alt ...
s, but these were usually of a larger current handling capacity than the 885. The 885 is a 2.5 volt, 5-pin based variant of the 884/6Q5.


Notes


References

*Stokes, John, ''70 Years of Radio Tubes and Valves,'' Vestal Press, NY, 1982, pp. 111–115. *Thrower, Keith, ''History of the British Radio Valve to 1940,'' MMA International, 1982, p. 30, 31, 81. * Hull, A. W., "Gas-Filled Thermionic Valves", Trans. AIEE, 47, 1928, pp. 753–763. *Data for 6D4 type, "Sylvania Engineering Data Service", 1957 *J.D. Cobine, J.R. Curry, "Electrical Noise Generators", Proceedings of the I.R.E., 1947, p. 875 * Radio and Electronic Laboratory Handbook, M.G. Scroggie 1971,


External links


tpub.comArticle on gas filled thyratrons
{{Authority control Products introduced in 1928 Gas-filled tubes Switching tubes Rectifiers