magnetron

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magnetron

(măg`nĭtrŏn'), vacuum tube oscillator (see electron tubeelectron tube,
device consisting of a sealed enclosure in which electrons flow between electrodes separated either by a vacuum (in a vacuum tube) or by an ionized gas at low pressure (in a gas tube).
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) that generates high-power electromagnetic signals in the microwavemicrowave,
electromagnetic wave having a frequency range from 1,000 megahertz (MHz) to 300,000 MHz, corresponding to a wavelength range from 300 mm (about 12 in.) to 1 mm (about 0.04 in.). Like light waves, microwaves travel essentially in straight lines.
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 frequency range. Its operation is based on the combined action of a magnetic field applied externally and the electric field between its electrodes. The tube is a diodediode
, two-terminal electronic device that permits current flow predominantly in only one direction. Most diodes are semiconductor devices; diode electron tubes are now used only for a few specialized applications.
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 having a cathode and an anode and is surrounded by an external magnet. Without this external magnetic field, the tube would work much like a simple diode, with the electrons flowing directly from the cathode to the anode. The magnetic field forces the cathode-emitted electrons to assume a curved path and thus creates a rotating electron cloud about the tube axis. The magnetron is noted for its high efficiency (ability to convert electrical power input to microwave power output). Magnetrons are available for generating microwave energies ranging from a few kilowatts to a few megawatts and are used extensively in radarradar,
system or technique for detecting the position, movement, and nature of a remote object by means of radio waves reflected from its surface. Although most radar units use microwave frequencies, the principle of radar is not confined to any particular frequency range.
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 systems and microwave ovensmicrowave oven,
device that uses microwaves to rapidly cook food. The microwaves cause water molecules in the food to vibrate, producing heat, which is distributed through the food by induction. A special electron tube called a magnetron produces the microwaves.
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.

Magnetron

 

in the original, broad meaning of the word, a coaxial cylindrical diode in a magnetic field directed along its axis; in electronic technology, it is an electrovacuum device for generating superhigh frequencies (SHF) through the interaction of electrons with the electric component of the SHF field that takes place in the space where a constant magnetic field is perpendicular to a constant electric field.

The term “magnetron” was introduced by the American physicist A. Hull, who in 1921 first published the results of theoretical and experimental studies on its operation in the steady state and who proposed a number of magnetron designs. In 1924 the Czechoslovak physicist A. £aček discovered that a magnetron generates electromagnetic oscillations in the decimeter wavelength range (λ ≤ 29 cm) and obtained a patent. During the 1920’s the effect of the magnetic field on the generation of SHF oscillations was studied by the physicists E. Habann (1924, Germany), A. A. Slutskin and D. S. Shteinberg (1926-29, USSR), K. Okabe and H. Yagi (1928-29, Japan), and I. Ranzi (1929, Italy).

In the 1930’s research on the magnetron as an SHF oscillator was conducted in many countries. The basic problem of this period was to increase the power output of the generated oscillations. The problem was solved in 1936-37 by the Soviet engineers N. F. Alekseev and D. E. Maliarov under the direction of M. A. Bonch-Bruevich. They increased the power of the magnetron by two orders of magnitude (up to 300 watts [W] at a wavelength of 9 cm) by using as an anode a massive copper block containing a number of resonating cavities. This design, called a multiresonator magnetron, proved to be so advanced that in subsequent years only multiresonator magnetrons were developed and manufactured. The cathode used in the magnetron is in the form of a hollow cylinder with a heater inside. This type of cathode was first proposed for electron tubes by Soviet academician A. A. Chernyshev in 1918.

In the 1930’s many engineers, among them the Americans C. W. Hansell in 1933 (for a magnetron with a cathode that surrounded the anode) and L. Malter, J. Rajchman, and R. Goodrich in 1936 (to use the secondary emission of the cathode in the magnetron) and the Soviet engineer V. P. Iliasov in 1939 (for a multiresonator magnetron), proposed magnetron cathodes in the form of a hollow cylinder.

In the 1940’s to 1970’s, engineers in many countries (USSR, Great Britain, the USA, Japan, and others) made a number of improvements in the multiresonator magnetron, and more than 1,000 types of such magnetrons were developed, primarily for radar. In the late 1960’s there was a sharp increase in the production of magnetrons for the continuous generation of oscillations at a wavelength of about 12 cm to provide heat by means of SHF fields in ovens for household use (with a power of 0.5-3.0 kW) and in industrial equipment (with a power of 5-100 kW). In 1950-70 a number of devices were developed on the basis of the multiresonator magnetron for the generation and amplification of SHF oscillations.

Magnetrons are widespread because of their high efficiency (up to 80 percent), compactness of design, and operational stability with comparatively low anode voltages. In the early 1970’s the industrially developed countries were producing magnetrons for operation at various frequencies from 0.5 to 100 GHz, with a power from several watts up to tens of kilowatts in the continuous mode of oscillation and from 10 W up to 5 MW in the pulsed mode (with pulse durations usually from fractions to tens of microseconds). Magnetrons are made for a fixed frequency (untunable) and for tuning over a small frequency range, usually less than 10 percent (tunable). Manually operated mechanisms are used for slow tuning; rotary and vibrating mechanisms are used for rapid tuning (up to several thousand changes per second).

Figure 1. Multiresonator magnetron of the simplest type (left, external view; right, cross section): (1) anode block with eight resonators of the “hole and slot” type, (2) resonator, (3) vane of anode block, (4) strap in the form of a metal ring (a second such ring is located on the other side of the anode block), (5) cathode, (6) cathode heater leads, (7) cooling fins, (8) coupling loop for SHF power output, (9) SHF power output rod for connection to a coaxial line

In the simplest multiresonator magnetron (Figure 1), the anode block is a massive copper cylinder with a circular opening in its center and eight to 40 symmetrically arranged chambers that act as resonant cavities. Each cavity is connected by a slot with the central hole, in which the cathode is located. The resonators form an annular oscillatory system, which has several resonance frequencies at which a whole number of standing waves from 1 to N/2 (N is the number of resonators) is set up in the system. The most useful mode of oscillation is the type in which the number of half-waves is equal to the number of resonators (called the π mode of oscillations). It is so named because the SHF voltages on two adjacent resonators are out of phase by π. For stable operation of the magnetron (to avoid hopping to other modes of oscillation, which is accompanied by changes in the frequency and output power), the closest resonance frequency of the oscillatory system should differ substantially from the operating frequency (by about 10 percent). Since the difference

Figure 2. Types of magnetron resonator systems (a—unstrapped identical resonators, b—strapped identical resonators, c—rising-sun type) and curves of their resonance frequency separation Δ = (fπ—fπ)/fπ, where fπ is the oscillation frequency corresponding to the π mode and fn is the frequency corresponding to the n-th oscillation number. In an 18-resonator magnetron the ninth oscillation mode is the π mode.

between these frequencies is too small in magnetrons with identical resonators (Figure 2,a), it is increased either by inserting straps in the form of metal rings, one connected to all the even vanes of the anode block and the other to all the odd vanes (Figure 2,b), or by using a rising-sun oscillatory system, in which the even resonators have one size and the odd resonators another (Figure 2,c).

The electrons moving in the space between the cathode and the anode block of a multiresonator magnetron are acted upon by three fields: a constant electric field, a constant magnetic field, and a SHF electric field (from the resonator system). When electrons move radially from the cathode to the anode, energy from the anode voltage source is converted into kinetic energy of the electrons. Under the influence of the constant magnetic field, which is directed along the cathode axis perpendicular to the constant electric field, the electrons change the direction of their motion: their radial velocity goes over into a tangential velocity at right angles to the radial velocity. Since a part of the electric SHF field extends through the resonator slots into the anode-cathode space, electrons moving in a tangential direction are retarded by the tangential component of the SHF electric field, and therefore the energy that they received from the DC voltage source is converted into energy of SHF oscillations. The field of the SHF oscillations changes direction twice in a period. For continuous retarding of the electrons, they must be shifted from one resonator to the next (tangentially) every half-period. This synchronization between the shifting of the electrons and the retarding SHF electric field is the basic operating principle of the multiresonator magnetron. The electrons that enter the accelerating SHF field increase their kinetic energy and fall out of synchronization. They return to the cathode or enter the retarding SHF field and again become synchronized.

Typical characteristics of a magnetron are shown in Figure 3. Operation begins when the anode voltage reaches a value corresponding to the onset of synchronization. As the voltage is increased, the synchronization improves; the current strength, output power, and efficiency of the magnetron increase. Under optimum conditions of synchronization, the efficiency attains a maximum. A further increase in the anode voltage gradually worsens the synchronization, with an accompanying decrease in efficiency, despite increasing current and output power.

Figure 3. Typical operating characteristics of a pulsed magnetron. The shaded areas indicate regions where no generation occurs, the solid curves indicate the pulsed output power Pp and the strength of the constant magnetic field H, and the dotted curves show the efficiency (without taking into account the cathode heating power).

REFERENCES

Alekseev, N. F., and D. E. Maliarov. “Poluchenie moshchnykh kolebanii magnetronom v santimetrovom diapazone voln.” Zhurnal tekhnicheskoi fiziki, 1940, vol. 10, no. 15, pp. 1297-1300.
Fisk, J. B., H. D. Hagstrum, and P. L. Hartman. Magnetrony. Moscow, 1948. (Translated from English.)
Bychkov, S. I. Magnetronnye generatory. Leningrad, 1948.
Magnetrony santimetrovogo diapazona, parts 1-2. Edited by S. A. Zusmanovskii. Moscow, 1950-51. (Translated from English.)
Kovalenko, V. F. Vvedenie v elektroniku sverkhvysokikh chastot, 2nd ed. Moscow, 1955.
Samsonov, D. E. Osnovy rascheta i konstruirovaniia mnogorezonatornykh magnetronov. Moscow, 1966.

V. F. KOVALENKO

magnetron

[′mag·nə‚trän]
(electronics)
One of a family of crossed-field microwave tubes, wherein electrons, generated from a heated cathode, move under the combined force of a radial electric field and an axial magnetic field in such a way as to produce microwave radiation in the frequency range 1-40 gigahertz; a pulsed microwave radiation source for radar, and continuous source for microwave cooking.

Magnetron

The oldest of a family of crossed-field microwave electron tubes wherein electrons, generated from a heated cathode, move under the combined force of a radial electric field and an axial magnetic field. By its structure a magnetron causes moving electrons to interact synchronously with traveling-wave components of a microwave standing-wave pattern in such a manner that electron potential energy is converted to microwave energy with high efficiency. Magnetrons have been used since the 1940s as pulsed microwave radiation sources for radar tracking. Because of their compactness and the high efficiency with which they can emit short bursts of megawatt peak output power, they have proved excellent for installation in aircraft as well as in ground radar stations. In continuous operation, a magnetron can produce a kilowatt of microwave power which is appropriate for rapid microwave cooking.

The magnetron is a device of essentially cylindrical symmetry (see illustration). On the central axis is a hollow cylindrical cathode. The outer surface of the cathode carries electronemitting materials, primarily barium and strontium oxides in a nickel matrix. Such a matrix is capable of emitting electrons when current flows through the heater inside the cathode cylinder.

At a radius somewhat larger than the outer radius of the cathode is a concentric cylindrical anode. The anode serves two functions: (1) to collect electrons emitted by the cathode and (2) to store and guide microwave energy. The anode consists of a series of quarter-wavelength cavity resonators symmetrically arranged around the cathode.

A radial dc electric field (perpendicular to the cathode) is applied between cathode and anode. This electric field and the axial magnetic field (parallel and coaxial with the cathode) introduced by pole pieces at either end of the cathode provide the required crossed-field configuration.

magnetron

A diode-type vacuum tube in which the flow of electrons is controlled by a magnetic field external to the tube. A magnetron is used to produce high-power electromagnetic energy in the frequency range of 1 to 40 GHz. Magnetron tubes find application in radars. Most magnetrons have a central cathode and a surrounding plate. This plate is usually divided into two or more sections by radial barriers called cavities. The RF (radio frequency) output is taken from a wave-guide opening into the anode. The cathode is connected to the negative terminal of a high-voltage source, while the anode is connected to the positive terminal. The electrons start flowing from the cathode to the anode. Application of the magnetic field in a longitudinal direction causes the electrons to travel outward in a spiral instead of along a straight path. The electrons travel in bunches because of the interaction between the electric and magnetic fields, and this results in oscillations, with the frequency being somewhat stabilized by the cavities. Magnetrons can produce continuous power outputs of more than 1 kW at a frequency of 1 GHz. The output drops as the frequency increases. For example, at 10 GHz, a magnetron can produce about 10 to 20 watts of continuous radio frequency output. For pulse modulation, the peak-power values are much higher.

magnetron

an electronic valve with two coaxial electrodes used with an applied magnetic field to generate high-power microwave oscillations, esp for use in radar

magnetron

A type of vacuum tube used as the frequency source in microwave ovens, radar systems and other high-power microwave circuits. In radar, magnetrons can be used as the signal source to feed the power stage of the radar transmitter, typically a klystron tube.

A magnetron is a type of diode vacuum tube in which the filament also serves as the cathode. Rather than being used as a diode, the physical structure of the elements causes the magnetron to act as an oscillator in the UHF or microwave frequency ranges. This is accomplished through the placement of permanent magnets within a series of resonating chambers that are tuned to the center frequency of the oscillation. These resonating chambers comprise the anode of the magnetron.

In operation, as the electrons are released from the cathode, they are directed toward the anode. However, since the electrons are influenced by the magnets in the resonant cavities, they flow circularly through the resonant chambers and set up an oscillation based upon their speed through the chambers. One or more chambers contain collecting antennas that let the generated RF signal flow outside the tube. See klystron and diode.
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