Wikipedia

Cavity magnetron

(Redirected fromMagnetron)
"Magnetron" redirects here. Not to be confused withMegatron,Metatron,orMagneton (disambiguation).

Thecavity magnetronis a high-powervacuum tubeused in earlyradarsystems and subsequently inmicrowave ovensand inlinear particle accelerators.A cavity magnetron generatesmicrowavesusing the interaction of a stream ofelectronswith amagnetic field,while moving past a series ofcavity resonators,which are small, open cavities in a metal block. Electrons pass by the cavities and cause microwaves to oscillate within, similar to the functioning of a whistle producing a tone when excited by an air stream blown past its opening. Theresonant frequencyof the arrangement is determined by the cavities' physical dimensions. Unlike other vacuum tubes, such as aklystronor atraveling-wave tube(TWT), the magnetron cannot function as anamplifierfor increasing the intensity of an applied microwave signal; the magnetron serves solely as anelectronic oscillatorgenerating a microwave signal from direct current electricity supplied to the vacuum tube.

Magnetron with section removed to exhibit the cavities. The cathode in the center is not visible. The antenna emitting microwaves is at the left. The magnets producing a field parallel to the long axis of the device are not shown.
A similar magnetron with a different section removed. Central cathode is visible; antenna conducting microwaves at the top; magnets are not shown.
Obsolete 9 GHz magnetron tube and magnets from a Soviet aircraft radar. The tube is embraced between the poles of two horseshoe-shapedalnicomagnets(top, bottom),which create a magnetic field along the axis of the tube. The microwaves are emitted from the waveguide aperture(top)which in use is attached to a waveguide conducting the microwaves to the radar antenna. Modern tubes userare-earth magnets,electromagnets orferrite magnetswhich are much less bulky.

The use of magnetic fields as a means to control the flow of an electric current was spurred by the invention of theAudionbyLee de Forestin 1906.Albert HullofGeneral Electric Research Laboratory,USA, began development of magnetrons to avoid de Forest's patents,[1]but these were never completely successful. Other experimenters picked up on Hull's work and a key advance, the use of two cathodes, was introduced by Habann in Germany in 1924. Further research was limited until Okabe's 1929 Japanese paper noting the production of centimeter-wavelength signals, which led to worldwide interest. The development of magnetrons with multiple cathodes was proposed by A. L. Samuel ofBell Telephone Laboratoriesin 1934, leading to designs by Postumus in 1934 andHans Hollmannin 1935. Production was taken up byPhilips,General Electric Company(GEC),Telefunkenand others, limited to perhaps 10 W output. By this time the klystron was producing more power and the magnetron was not widely used, although a 300 W device was built by Aleksereff and Malearoff in the USSR in 1936 (published in 1940).[1]

Thecavitymagnetron was a radical improvement introduced byJohn RandallandHarry Bootat theUniversity of Birmingham,England in 1940.[2]: 24–26 [3]Their first working example produced hundreds of watts at 10 cm wavelength, an unprecedented achievement.[4][5]Within weeks, engineers at GEC had improved this to well over akilowatt(kW), and within months 25 kW, over 100 kW by 1941 and pushing towards a megawatt by 1943. The high power pulses were generated from a device the size of a small book and transmitted from an antenna only centimeters long, reducing the size of practical radar systems by orders of magnitude.[6]New radars appeared fornight-fighters,anti-submarine aircraftand even the smallest escort ships,[6]and from that point on theAllies of World War IIheld a lead in radar that their counterparts in Germany and Japan were never able to close. By the end of the war, practically every Allied radar was based on the magnetron.

The magnetron continued to be used in radar in the post-war period but fell from favour in the 1960s as high-power klystrons andtraveling-wave tubesemerged. A key characteristic of the magnetron is that its output signal changes from pulse to pulse, both in frequency and phase. This renders it less suitable for pulse-to-pulse comparisons for performingmoving target indicationand removing "clutter"from the radar display.[7]The magnetron remains in use in some radar systems, but has become much more common as a low-cost source for microwave ovens. In this form, over one billion magnetrons are in use today.[7][8]

Construction and operation

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Conventional tube design

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In a conventional electron tube (vacuum tube),electronsare emitted from a negatively charged, heated component called thecathodeand are attracted to a positively charged component called theanode.The components are normally arranged concentrically, placed within a tubular-shaped container from which all air has been evacuated, so that the electrons can move freely (hence the name "vacuum" tubes, called "valves" in British English).

If a third electrode (called acontrol grid) is inserted between the cathode and the anode, the flow of electrons between the cathode and anode can be regulated by varying the voltage on this third electrode. This allows the resulting electron tube (called a "triode"because it now has three electrodes) to function as anamplifierbecause small variations in the electric charge applied to the control grid will result in identical variations in the much larger current of electrons flowing between the cathode and anode.[9]

Hull or single-anode magnetron

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The idea of using a grid for control was invented byPhilipp Lenard,who received theNobel Prize for Physicsin 1905. In the USA it was later patented byLee de Forest,resulting in considerable research into alternate tube designs that would avoid his patents. One concept used a magnetic field instead of an electrical charge to control current flow, leading to the development of the magnetron tube. In this design, the tube was made with two electrodes, typically with the cathode in the form of a metal rod in the center, and the anode as a cylinder around it. The tube was placed between the poles of ahorseshoe magnet[10][better source needed]arranged such that the magnetic field was aligned parallel to the axis of the electrodes.

With no magnetic field present, the tube operates as a diode, with electrons flowing directly from the cathode to the anode. In the presence of the magnetic field, the electrons will experience a force at right angles to their direction of motion (theLorentz force). In this case, the electrons follow a curved path between the cathode and anode. The curvature of the path can be controlled by varying either the magnetic field using anelectromagnet,or by changing the electrical potential between the electrodes.

At very high magnetic field settings the electrons are forced back onto the cathode, preventing current flow. At the opposite extreme, with no field, the electrons are free to flow straight from the cathode to the anode. There is a point between the two extremes, the critical value or Hull cut-off magnetic field (and cut-off voltage), where the electrons just reach the anode. At fields around this point, the device operates similar to a triode. However, magnetic control, due tohysteresisand other effects, results in a slower and less faithful response to control current than electrostatic control using a control grid in a conventional triode (not to mention greater weight and complexity), so magnetrons saw limited use in conventional electronic designs.

It was noticed that when the magnetron was operating at the critical value, it would emit energy in theradio frequencyspectrum. This occurs because a few of the electrons, instead of reaching the anode, continue to circle in the space between the cathode and the anode. Due to an effect now known ascyclotron radiation,these electrons radiate radio frequency energy. The effect is not very efficient. Eventually the electrons hit one of the electrodes, so the number in the circulating state at any given time is a small percentage of the overall current. It was also noticed that the frequency of the radiation depends on the size of the tube, and even early examples were built that produced signals in the microwave regime.

Early conventional tube systems were limited to thehigh frequencybands, and althoughvery high frequencysystems became widely available in the late 1930s, the ultra high frequency and microwave bands were well beyond the ability of conventional circuits. The magnetron was one of the few devices able to generate signals in the microwave band and it was the only one that was able to produce high power at centimeter wavelengths.

Split-anode magnetron

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Split-anode magnetron (c. 1935).(left)The bare tube, about 11 cm high.(right)Installed for use between the poles of a strongpermanent magnet

The original magnetron was very difficult to keep operating at the critical value, and even then the number of electrons in the circling state at any time was fairly low. This meant that it produced very low-power signals. Nevertheless, as one of the few devices known to create microwaves, interest in the device and potential improvements was widespread.

The first major improvement was thesplit-anode magnetron,also known as anegative-resistance magnetron.As the name implies, this design used an anode that was split in two—one at each end of the tube—creating two half-cylinders. When both were charged to the same voltage the system worked like the original model. But by slightly altering the voltage of the twoplates,the electrons' trajectory could be modified so that they would naturally travel towards the lower voltage side. The plates were connected to an oscillator that reversed the relative voltage of the two plates at a given frequency.[10]

At any given instant, the electron will naturally be pushed towards the lower-voltage side of the tube. The electron will then oscillate back and forth as the voltage changes. At the same time, a strong magnetic field is applied, stronger than the critical value in the original design. This would normally cause the electron to circle back to the cathode, but due to the oscillating electrical field, the electron instead follows a looping path that continues toward the anodes.[10]

Since all of the electrons in the flow experienced this looping motion, the amount of RF energy being radiated was greatly improved. And as the motion occurred at any field level beyond the critical value, it was no longer necessary to carefully tune the fields and voltages, and the overall stability of the device was greatly improved. Unfortunately, the higher field also meant that electrons often circled back to the cathode, depositing their energy on it and causing it to heat up. As this normally causes more electrons to be released, it could sometimes lead to a runaway effect, damaging the device.[10]

Cavity magnetron

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The great advance in magnetron design was theresonant cavitymagnetronorelectron-resonance magnetron,which works on entirely different principles. In this design the oscillation is created by the physical shape of the anode, rather than external circuits or fields.

A cross-sectional diagram of aresonant cavitymagnetron. Magnetic lines of force are parallel to the geometric axis of this structure.

Mechanically, the cavity magnetron consists of a large, solid cylinder of metal with a hole drilled through the centre of the circular face. A wire acting as the cathode is run down the center of this hole, and the metal block itself forms the anode. Around this hole, known as the "interaction space", are a number of similar holes ( "resonators" ) drilled parallel to the interaction space, connected to the interaction space by a short channel. The resulting block looks something like the cylinder on arevolver,with a somewhat larger central hole. Early models were cut usingColtpistol jigs.[11]Remembering that in an AC circuit the electronstravel along the surface,not the core, of the conductor, the parallel sides of the slot act as acapacitorwhile the round holes form aninductor:anLC circuitmade of solid copper, with the resonant frequency defined entirely by its dimensions.

The magnetic field is set to a value well below the critical, so the electrons follow curved paths towards the anode. When they strike the anode, they cause it to become negatively charged in that region. As this process is random, some areas will become more or less charged than the areas around them. The anode is constructed of a highly conductive material, almost always copper, so these differences in voltage cause currents to appear to even them out. Since the current has to flow around the outside of the cavity, this process takes time. During that time additional electrons will avoid the hot spots and be deposited further along the anode, as the additional current flowing around it arrives too. This causes an oscillating current to form as the current tries to equalize one spot, then another.[12]

The oscillating currents flowing around the cavities, and their effect on the electron flow within the tube, cause large amounts of microwave radiofrequency energy to be generated in the cavities. The cavities are open on one end, so the entire mechanism forms a single, larger, microwave oscillator. A "tap", normally a wire formed into a loop, extracts microwave energy from one of the cavities. In some systems the tap wire is replaced by an open hole, which allows the microwaves to flow into awaveguide.

As the oscillation takes some time to set up, and is inherently random at the start, subsequent startups will have different output parameters. Phase is almost never preserved, which makes the magnetron difficult to use inphased arraysystems. Frequency also drifts from pulse to pulse, a more difficult problem for a wider array of radar systems. Neither of these present a problem forcontinuous-wave radars,nor for microwave ovens.

Common features

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Cutaway drawing of a cavity magnetron of 1984. Part of the righthand magnet and copper anode block is cut away to show the cathode and cavities. This older magnetron uses two horseshoe shapedalnicomagnets, modern tubes userare-earth magnets.

All cavity magnetrons consist of a heated cylindricalcathodeat a high (continuous or pulsed) negative potential created by a high-voltage, direct-current power supply. The cathode is placed in the center of anevacuated,lobed, circular metal chamber. The walls of the chamber are the anode of the tube. Amagnetic fieldparallel to the axis of the cavity is imposed by apermanent magnet.The electrons initially move radially outward from the cathode attracted by the electric field of the anode walls. The magnetic field causes the electrons to spiral outward in a circular path, a consequence of theLorentz force.Spaced around the rim of the chamber are cylindrical cavities. Slots are cut along the length of the cavities that open into the central, common cavity space. As electrons sweep past these slots, they induce a high-frequency radio field in each resonant cavity, which in turn causes the electrons to bunch into groups. A portion of the radio frequency energy is extracted by a short coupling loop that is connected to awaveguide(a metal tube, usually of rectangular cross section). The waveguide directs the extracted RF energy to the load, which may be a cooking chamber in a microwave oven or a high-gainantennain the case of radar.

The size of the cavities determine the resonant frequency, and thereby the frequency of the emitted microwaves. However, the frequency is not precisely controllable. The operating frequency varies with changes in loadimpedance,with changes in the supply current, and with the temperature of the tube.[13]This is not a problem in uses such as heating, or in some forms ofradarwhere the receiver can be synchronized with an imprecise magnetron frequency. Where precise frequencies are needed, other devices, such as theklystronare used.

The magnetron is a self-oscillating device requiring no external elements other than a power supply. A well-defined threshold anode voltage must be applied before oscillation will build up; this voltage is a function of the dimensions of the resonant cavity, and the applied magnetic field. In pulsed applications there is a delay of several cycles before the oscillator achieves full peak power, and the build-up of anode voltage must be coordinated with the build-up of oscillator output.[13]

Where there are an even number of cavities, two concentric rings can connect alternate cavity walls to prevent inefficient modes of oscillation. This is called pi-strapping because the two straps lock the phase difference between adjacent cavities at π radians (180°).

The modern magnetron is a fairly efficient device. In a microwave oven, for instance, a 1.1-kilowatt input will generally create about 700 watts of microwave power, an efficiency of around 65%. (The high-voltage and the properties of the cathode determine the power of a magnetron.) LargeS bandmagnetrons can produce up to 2.5 megawatts peak power with an average power of 3.75 kW.[13]Some large magnetrons are water cooled. The magnetron remains in widespread use in roles which require high power, but where precise control over frequency and phase is unimportant.

Applications

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Radar

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9.375 GHz 20 kW (peak) magnetron assembly for an early commercial airport radar in 1947. In addition to the magnetron (right), it contains a TR (transmit/receive) switch tube and thesuperheterodynereceiver front end, a 2K25reflex klystrontubelocal oscillatorand a 1N21germanium diodemixer. The waveguide aperture (left) would be connected to a waveguide going to the antenna.
Main article:History of radar (Centimetric radar)

In aradarset, the magnetron's waveguide is connected to anantenna.The magnetron is operated with very short pulses of applied voltage, resulting in a short pulse of high-power microwave energy being radiated. As in all primary radar systems, the radiation reflected from a target is analyzed to produce a radar map on a screen.

Several characteristics of the magnetron's output make radar use of the device somewhat problematic. The first of these factors is the magnetron's inherent instability in its transmitter frequency. This instability results not only in frequency shifts from one pulse to the next, but also a frequency shift within an individual transmitted pulse. The second factor is that the energy of the transmitted pulse is spread over a relatively wide frequency spectrum, which requires the receiver to have a correspondingly wide bandwidth. This wide bandwidth allows ambient electrical noise to be accepted into the receiver, thus obscuring somewhat the weak radar echoes, thereby reducing overall receiversignal-to-noise ratioand thus performance. The third factor, depending on application, is the radiation hazard caused by the use of high-power electromagnetic radiation. In some applications, for example, amarine radarmounted on a recreational vessel, a radar with a magnetron output of 2 to 4 kilowatts is often found mounted very near an area occupied by crew or passengers. In practical use these factors have been overcome, or merely accepted, and there are today thousands of magnetron aviation and marine radar units in service. Recent advances in aviation weather-avoidance radar and in marine radar have successfully replaced the magnetron withmicrowave semiconductor oscillators,which have a narrower output frequency range. These allow a narrower receiver bandwidth to be used, and the higher signal-to-noise ratio in turn allows a lower transmitter power, reducing exposure to EMR.

Heating

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Magnetron from amicrowave ovenwith magnet in its mounting box. The horizontal plates form aheat sink,cooled by airflow from a fan. The magnetic field is produced by two powerful ring magnets, the lower of which is just visible. Almost all modern oven magnetrons are of similar layout and appearance.

Inmicrowave ovens,the waveguide leads to a radio-frequency-transparent port into the cooking chamber. As the fixed dimensions of the chamber and its physical closeness to the magnetron would normally create standing wave patterns in the chamber, the pattern is randomized by a motorized fan-likemode stirrerin the waveguide (more often in commercial ovens), or by a turntable that rotates the food (most common in consumer ovens). An early example of this application was when British scientists in 1954 used a microwave oven to resurrectcryogenicallyfrozenhamsters.[14]

Lighting

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In microwave-excited lighting systems, such as asulfur lamp,a magnetron provides the microwave field that is passed through awaveguideto the lighting cavity containing the light-emitting substance (e.g.,sulfur,metal halides,etc.). Although efficient, these lamps are much more complex than other methods of lighting and therefore not commonly used. More modern variants useHEMTsor GaN-on-SiCpower semiconductor devicesinstead of magnetrons to generate the microwaves, which are substantially less complex and can be adjusted to maximize light output using aPID controller.

History

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In 1910, Hans Gerdien (1877–1951) of theSiemens Corporationinvented a magnetron.[15][16]In 1912, Swiss physicistHeinrich Greinacherwas looking for new ways to calculate theelectron mass.He settled on a system consisting of a diode with a cylindrical anode surrounding a rod-shaped cathode, placed in the middle of a magnet. The attempt to measure the electron mass failed because he was unable to achieve a good vacuum in the tube. However, as part of this work, Greinacher developed mathematical models of the motion of the electrons in the crossed magnetic and electric fields.[17][18]

In the US,Albert Hullput this work to use in an attempt to bypassWestern Electric's patents on the triode. Western Electric had gained control of this design by buyingLee De Forest's patents on the control of current flow using electric fields via the "grid". Hull intended to use a variable magnetic field, instead of an electrostatic one, to control the flow of the electrons from the cathode to the anode. Working atGeneral Electric's Research Laboratories inSchenectady, New York,Hull built tubes that provided switching through the control of the ratio of the magnetic and electric field strengths. He released several papers and patents on the concept in 1921.[19]

Hull's magnetron was not originally intended to generate VHF (very-high-frequency) electromagnetic waves. However, in 1924, Czech physicist August Žáček[20](1886–1961) and German physicist Erich Habann[21](1892–1968) independently discovered that the magnetron could generate waves of 100 megahertz to 1 gigahertz. Žáček, a professor at Prague'sCharles University,published first; however, he published in a journal with a small circulation and thus attracted little attention.[22]Habann, a student at theUniversity of Jena,investigated the magnetron for his doctoral dissertation of 1924.[23]Throughout the 1920s, Hull and other researchers around the world worked to develop the magnetron.[24][25][26]Most of these early magnetrons were glass vacuum tubes with multiple anodes. However, the two-pole magnetron, also known as a split-anode magnetron, had relatively low efficiency.

Whileradarwas being developed duringWorld War II,there arose an urgent need for a high-powermicrowavegenerator that worked at shorterwavelengths,around 10 cm (3 GHz), rather than the 50 to 150 cm (200 MHz) that was available from tube-based generators of the time. It was known that a multi-cavity resonant magnetron had been developed and patented in 1935 byHans HollmanninBerlin.[27]However, the German military considered the frequency drift of Hollman's device to be undesirable, and based their radar systems on theklystroninstead. But klystrons could not at that time achieve the high power output that magnetrons eventually reached. This was one reason that Germannight fighterradars, which never strayed beyond thelow-UHF band to start withfor front-line aircraft, were not a match for their British counterparts.[24]: 229 Likewise, in the UK,Albert Beaumont Woodproposed in 1937 a system with "six or eight small holes" drilled in a metal block, differing from the later production designs only in the aspects of vacuum sealing. However, his idea was rejected by the Navy, who said their valve department was far too busy to consider it.[28]

Sir John RandallandHarry Boot's original cavity magnetron developed in 1940 at theUniversity of Birmingham,England, now in theScience Museum, London.
The electromagnet used in conjunction with Randall and Boot's original magnetron, in the Science Museum, London.
The anode block which is part of the cavity magnetron developed by Randall and Boot

In 1940, at theUniversity of Birminghamin the UK,John RandallandHarry Bootproduced a working prototype of a cavity magnetron that produced about 400 W.[5]Within a week this had improved to 1 kW, and within the next few months, with the addition of water cooling and many detail changes, this had improved to 10 and then 25 kW.[5]To deal with its drifting frequency, they sampled the output signal and synchronized their receiver to whatever frequency was actually being generated. In 1941, the problem of frequency instability was solved byJames Sayerscoupling ( "strapping" ) alternate cavities within the magnetron, which reduced the instability by a factor of 5–6.[29](For an overview of early magnetron designs, including that of Boot and Randall, see[30].)

GECat Wembley made 12 prototype cavity magnetrons in August 1940, and No 12 was sent to America with Bowen on theTizard Mission,where it was shown on 19 September 1940 in Alfred Loomis’ apartment. The American NDRC Microwave Committee was stunned at the power level produced. However Bell Labs' director was upset when it was X-rayed and had eight holes rather than the six holes shown on the GEC plans. After contacting (via the transatlantic cable) Dr Eric Megaw, GEC’s vacuum tube expert Megaw recalled that when he had asked for 12 prototypes he said make 10 with 6 holes, one with 7 and one with 8; there was no time to amend the drawings. And No 12 with 8 holes was chosen for the Tizard Mission. So Bell Labs chose to copy the sample; and while early British magnetrons had six cavities the American ones had eight cavities.[31]

According to Andy Manning from theRAF Air Defence Radar Museum,Randall and Boot's discovery was "a massive, massive breakthrough" and "deemed by many, even now [2007], to be the most important invention that came out of the Second World War", while professor of military history at theUniversity of Victoriain British Columbia, David Zimmerman, states:

The magnetron remains the essential radio tube for shortwave radio signals of all types. It not only changed the course of the war by allowing us to develop airborne radar systems, it remains the key piece of technology that lies at the heart of your microwave oven today. The cavity magnetron's invention changed the world.[5]

Because France had just fallen to theNazisand Britain had no money to develop the magnetron on a massive scale,Winston Churchillagreed thatSir Henry Tizardshould offer the magnetron to the Americans in exchange for their financial and industrial help.[5]An early 10 kW version, built in England by theGeneral Electric CompanyResearch Laboratories inWembley,London,was taken on theTizard Missionin September 1940. As the discussion turned to radar, the US Navy representatives began to detail the problems with their short-wavelength systems, complaining that their klystrons could only produce 10 W. With a flourish,"Taffy" Bowenpulled out a magnetron and explained it produced 1000 times that.[5][32]

Bell Telephone Laboratoriestook the example and quickly began making copies, and before the end of 1940, theRadiation Laboratoryhad been set up on the campus of theMassachusetts Institute of Technologyto develop various types of radar using the magnetron. By early 1941, portable centimetric airborne radars were being tested in American and British aircraft.[5]In late 1941, theTelecommunications Research Establishmentin the United Kingdom used the magnetron to develop a revolutionary airborne, ground-mapping radar codenamed H2S. TheH2S radarwas in part developed byAlan BlumleinandBernard Lovell.

The cavity magnetron was widely used duringWorld War IIin microwave radar equipment and is often credited with giving Allied radar a considerable performance advantage overGermanandJapaneseradars, thus directly influencing the outcome of the war. It was later described by American historianJames Phinney Baxter IIIas "[t]he most valuable cargo ever brought to our shores".[33]

Centimetric radar, made possible by the cavity magnetron, allowed for the detection of much smaller objects and the use of much smaller antennas. The combination of small-cavity magnetrons, small antennas, and high resolution allowed small, high quality radars to be installed in aircraft. They could be used by maritime patrol aircraft to detect objects as small as a submarine periscope, which allowed aircraft to attack and destroy submerged submarines which had previously been undetectable from the air. Centimetric contour mapping radars likeH2Simproved the accuracy of Allied bombers used in thestrategic bombing campaign,despite the existence of the GermanFuG 350Naxosdevice to specifically detect it. Centimetric gun-laying radars were likewise far more accurate than the older technology. They made the big-gunned Allied battleships more deadly and, along with the newly developedproximity fuze,made anti-aircraft guns much more dangerous to attacking aircraft. The two coupled together and used by anti-aircraft batteries, placed along the flight path of GermanV-1 flying bombson their way toLondon,are credited with destroying many of the flying bombs before they reached their target.

Since then, many millions of cavity magnetrons have been manufactured; while some have been for radar the vast majority have been formicrowave ovens.The use in radar itself has dwindled to some extent, as more accurate signals have generally been needed and developers have moved toklystronandtraveling-wave tubesystems for these needs.

Health hazards

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ISO 7010Warning sign: Non-ionizing radiation

At least one hazard in particular is well known and documented. As thelensof theeyehas no cooling blood flow, it is particularly prone to overheating when exposed to microwave radiation. This heating can in turn lead to a higher incidence ofcataractsin later life.[34]

There is also a considerable electrical hazard around magnetrons, as they require a high voltage power supply.

Most magnetrons contain a small amount ofberyllium oxide[citation needed],andthoriummixed withtungstenin theirfilament.Exceptions to this are higher power magnetrons that operate above approximately 10,000 volts where positive ion bombardment becomes damaging to thorium metal, hence pure tungsten (potassium doped) is used. While thorium is a radioactive metal, the risk of cancer is low as it never gets airborne in normal usage. Only if the filament is taken out of the magnetron, finely crushed, and inhaled can it pose a health hazard.[35][36][37]

See also

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References

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  1. ^abRedhead, Paul A., "The Invention of the Cavity Magnetron and its Introduction into Canada and the U.S.A.",La Physique au Canada,November 2001
  2. ^Fine, Norman (2019).Blind Bombing: How Microwave Radar brought the Allies to D-Day and Victory in World War II.Nebraska: Potomac Books/University of Nebraska Press. pp. 24–26.ISBN978-1640-12279-6.
  3. ^"The Magnetron".Bournemouth University. 1995–2009.Archivedfrom the original on 26 July 2011.Retrieved23 August2009.
  4. ^Fine 2019,pp. 24–26.
  5. ^abcdefgAngela Hind (February 5, 2007)."Briefcase 'that changed the world'".BBC News.Archivedfrom the original on November 15, 2007.Retrieved2007-08-16.
  6. ^abSchroter, B. (Spring 2008)."How important was Tizard's Box of Tricks?"(PDF).Imperial Engineer.8:10.Archived(PDF)from the original on 2011-06-17.Retrieved2009-08-23.
  7. ^abBrookner, Eli (19–20 April 2010). "From $10,000 magee to $7 magee and $10 transmitter and receiver (T/R) on single chip".2010 International Conference on the Origins and Evolution of the Cavity Magnetron.pp. 1–2.doi:10.1109/CAVMAG.2010.5565574.ISBN978-1-4244-5609-3.
  8. ^Ma, L. "3D Computer Modeling of MagnetronsArchived2008-10-10 at theWayback Machine."University of London Ph.D. Thesis.December 2004. Accessed 2009-08-23.
  9. ^White, Steve."Electric Valves: Diodes, Triodes, and Transistors".zipcon.net.Archivedfrom the original on 25 August 2017.Retrieved5 May2018.
  10. ^abcd"The Magnetron".electriciantraining.tpub.Archivedfrom the original on 3 March 2016.Retrieved5 May2018.
  11. ^J. Brittain (1985). "The Magnetron and the Beginnings of the Microwave Age".Physics Today.38(7): 60–67.Bibcode:1985PhT....38g..60B.doi:10.1063/1.880982.
  12. ^"Magnetron Operation".hyperphysics.phy-astr.gsu.edu.Archivedfrom the original on 11 September 2017.Retrieved5 May2018.
  13. ^abcL.W. Turner,(ed),Electronics Engineer's Reference Book, 4th ed.Newnes-Butterworth, London 1976ISBN9780408001687,pp. 7-71 to 7-77
  14. ^Smith, A. U.; Lovelock, J. E.; Parkes, A. S. (June 1954)."Resuscitation of Hamsters after Supercooling or Partial Crystallization at Body Temperatures Below 0°C".Nature.173(4415): 1136–37.Bibcode:1954Natur.173.1136S.doi:10.1038/1731136a0.ISSN0028-0836.PMID13165726.S2CID4242031.
  15. ^See:
  16. ^Goerth, Joachim (2010). "Early magnetron development especially in Germany".International Conference on the Origins and Evolution of the Cavity Magnetron (CAVMAG 2010), Bournemouth, England, UK, 19–20 April 2010.Piscataway, New Jersey: IEEE. pp. 17–22.
  17. ^Greinacher, H. (1912)."Über eine Anordnung zur Bestimmung von e/m"[On an apparatus for the determination of e/m].Verhandlungen der Deutschen Physikalischen Gesellschaft(in German).14:856–64.
  18. ^Wolff, Dipl.-Ing. (FH) Christian."Radar Basics".radartutorial.eu.Archivedfrom the original on 23 December 2017.Retrieved5 May2018.
  19. ^See:
  20. ^Biographical information about August Žáček:
  21. ^Biographical information about Erich Habann:
    • Günter Nagel, "Pionier der Funktechnik. Das Lebenswerk des Wissenschaftlers Erich Habann, der in Hessenwinkel lebte, ist heute fast vergessen" (Pioneer in Radio Technology. The life's work of scientist Erich Habann, who lived in Hessenwinkel, is nearly forgotten today.),Bradenburger Blätter(supplement of theMärkische Oderzeitung,a daily newspaper of the city of Frankfurt in the state of Brandenburg, Germany), 15 December 2006, page 9.
    • Karlsch, Rainer; Petermann, Heiko, eds. (2007).Für und Wider "Hitlers Bombe": Studien zur Atomforschung in Deutschland[For and Against "Hitler's Bomb": Studies on atomic research in Germany] (in German). New York: Waxmann Publishing Co. p. 251 footnote.
  22. ^See:
  23. ^Habann, Erich (1924). "Eine neue Generatorröhre" [A new generator tube].Zeitschrift für Hochfrequenztechnik(in German).24:115–20, 135–41.
  24. ^abKaiser, W. (1994). "The Development of Electron Tubes and of Radar technology: The Relationship of Science and Technology". In Blumtritt, O.; Petzold, H.; Aspray, W. (eds.).Tracking the History of Radar.Piscataway, NJ: IEEE. pp. 217–36.
  25. ^Brittain, James E. (1985). "The magnetron and the beginnings of the microwave age".Physics Today.38(7): 60–67.Bibcode:1985PhT....38g..60B.doi:10.1063/1.880982.
  26. ^See for example:
    • Soviet physicists:
    • Slutskin, Abram A.; Shteinberg, Dmitry S. (1926). "[Obtaining oscillations in cathode tubes with the aid of a magnetic field]".Журнал Русского Физико-Химического Общества [Zhurnal Russkogo Fiziko-Khimicheskogo Obshchestva, Journal of the Russian Physico-Chemical Society](in Russian).58(2): 395–407.
    • Slutskin, Abram A.; Shteinberg, Dmitry S. (1927). "[Electronic oscillations in two-electrode tubes]".Український фізичний журнал [Ukrainski Fizychni Zapysky, Ukrainian Journal of Physics](in Ukrainian).1(2): 22–27.
    • Slutzkin, A. A.; Steinberg, D. S. (May 1929). "Die Erzeugung von kurzwelligen ungedämpften Schwingungen bei Anwendung des Magnetfeldes" [The generation of undamped shortwave oscillations by application of a magnetic field].Annalen der Physik(in German).393(5): 658–70.Bibcode:1929AnP...393..658S.doi:10.1002/andp.19293930504.
    • Japanese engineers:
    • Yagi, Hidetsugu (1928). "Beam transmission of ultra-short waves".Proceedings of the Institute of Radio Engineers.16(6): 715–41.Magnetrons are discussed in Part II of this article.
    • Okabe, Kinjiro (March 1928). "[Production of intense extra-short radio waves by a split-anode magnetron (Part 3)]".Journal of the Institute of Electrical Engineering of Japan(in Japanese): 284ff.
    • Okabe, Kinjiro (1929). "On the short-wave limit of magnetron oscillations".Proceedings of the Institute of Radio Engineers.17(4): 652–59.
    • Okabe, Kinjiro (1930). "On the magnetron oscillation of new type".Proceedings of the Institute of Radio Engineers.18(10): 1748–49.
  27. ^Hollmann, Hans Erich,"Magnetron,"Archived2018-01-14 at theWayback MachineU.S. patent no. 2,123,728 (filed: 1936 November 27; issued: 1938 July 12).
  28. ^Lythall, B. W. (1995). "Basic science and research for naval radar, 1935-1945". In Kingsley, F. A. (ed.).The Development of Radar Equipments for the Royal Navy, 1935–45.London, England: Macmillan Press Ltd. pp. 68–69.
  29. ^Barrett, Dick."M.J.B.Scanlan; Early Centimetric Ground Radars – A Personal Reminiscence".radarpages.co.uk.Archivedfrom the original on 4 March 2016.Retrieved5 May2018.
  30. ^Willshaw, W. E.; L. Rushforth; A. G. Stainsby; R. Latham; A. W. Balls; A. H. King (1946)."The high-power pulsed magnetron: development and design for radar applications".Journal of the Institution of Electrical Engineers - Part IIIA: Radiolocation.93(5): 985–1005.doi:10.1049/ji-3a-1.1946.0188.Archived fromthe originalon May 5, 2018.Retrieved22 June2012.
  31. ^Fine 2019,pp. 56–64.
  32. ^Harford, Tim (9 October 2017)."How the search for a 'death ray' led to radar".BBC World Service.Archivedfrom the original on 9 October 2017.Retrieved9 October2017.The magnetron stunned the Americans. Their research was years off the pace.
  33. ^Baxter, James Phinney (III) (1946).Scientists Against Time.Boston, Massachusetts: Little, Brown, and Co. p. 142.(Baxter was the official historian of the Office of Scientific Research and Development.)
  34. ^Lipman, R. M.; Tripathi, B. J.; Tripathi, R. C. (1988). "Cataracts induced by microwave and ionizing radiation".Survey of Ophthalmology.33(3): 200–10.doi:10.1016/0039-6257(88)90088-4.OSTI6071133.PMID3068822.
  35. ^Australian Nuclear Science and Technology Organisation."In the home – ANSTO".ansto.gov.au.Archived fromthe originalon 5 September 2017.Retrieved5 May2018.
  36. ^"EngineerGuy Video: microwave oven".engineerguy.Archivedfrom the original on 5 September 2017.Retrieved5 May2018.
  37. ^EPA,OAR,ORIA,RPD, US (2014-07-16)."Radiation Protection".US EPA.Archivedfrom the original on 1 October 2006.Retrieved5 May2018.{{cite web}}:CS1 maint: multiple names: authors list (link)
  38. ^Jr. Raymond C. Watson (25 November 2009).Radar Origins Worldwide: History of Its Evolution in 13 Nations Through World War II.Trafford Publishing. pp. 315–.ISBN978-1-4269-2110-0.Retrieved24 June2011.
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Wikimedia Commons has media related toMagnetrons.
Information
Patents
  • US 2123728Hans Erich Hollmann/Telefunken GmbH: „Magnetron “filed November 27, 1935
  • US 2315313Buchholz, H. (1943).Cavity resonator
  • US 2357313Carter, P.S. (1944).High frequency resonator and circuit therefor
  • US 2357314Carter, P.S. (1944).Cavity resonator circuit
  • US 2408236Spencer, P.L. (1946).Magnetron casing
  • US 2444152Carter, P.S. (1948).Cavity resonator circuit
  • US 2611094Rex, H.B. (1952).Inductance-capacitance resonance circuit
  • GB 879677Dexter, S.A. (1959).Valve oscillator circuits; radio frequency output couplings
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