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A cavity magnetron is a high-powered vacuum tube that generates coherent microwaves. This article is about the electronic device not an evacuated pipe used for experiments in Free-fall. In Physics, coherence is a property of waves that enables stationary (i Microwaves are electromagnetic waves with Wavelengths ranging from 1 mm to 1 m or frequencies between 0 They are commonly found in microwave ovens, as well as various radar applications. A microwave oven, or a microwave, is a Kitchen appliance that cooks or heats Food by Dielectric heating. Radar is a system that uses electromagnetic waves to identify the range altitude direction or speed of both moving and fixed objects such as Aircraft, ships

Contents

Construction and operation

Magnetron with section removed (magnet is not shown)
Magnetron with section removed (magnet is not shown)

All cavity magnetrons consist of a hot filament (cathode) kept at, or pulsed to, a high negative potential by a high-voltage, direct-current power supply. A cathode is an Electrode through which (positive Electric current flows out of a polarized electrical device The cathode is built into the center of an evacuated, lobed, circular chamber. A magnetic field parallel to the filament is imposed by a permanent magnet. The magnetic field causes the electrons, attracted to the (relatively) positive outer part of the chamber, to spiral outward in a circular path rather than moving directly to this anode. An anode is an Electrode through which Electric current flows into a polarized electrical device Spaced around the rim of the chamber are cylindrical cavities. The cavities are open along their length and connect the common cavity space. As electrons sweep past these openings, they induce a resonant, high-frequency radio field in the cavity, which in turn causes the electrons to bunch into groups. A portion of this field is extracted with a short antenna that is connected to a waveguide (a metal tube usually of rectangular cross section). In Electromagnetics and communications engineering, the term waveguide may refer to any linear structure that guides electromagnetic waves The waveguide directs the extracted RF energy to the load, which may be a cooking chamber in a microwave oven or a high-gain antenna in the case of radar. An antenna is a Transducer designed to transmit or Receive electromagnetic waves In other words antennas convert electromagnetic waves into

A cross-sectional diagram of a resonant cavity magnetron. Magnetic field is perpendicular to the plane of the diagram.
A cross-sectional diagram of a resonant cavity magnetron. A resonator is a device or system that exhibits Resonance or resonant behavior that is it naturally oscillates at some frequencies, called its resonance Magnetic field is perpendicular to the plane of the diagram.

The sizes of the cavities determine the resonant frequency, and thereby the frequency of emitted microwaves. However, the frequency is not precisely controllable. This is not a problem in many uses such as heating or some forms of radar where the receiver can be synchronized with an imprecise output. Radar is a system that uses electromagnetic waves to identify the range altitude direction or speed of both moving and fixed objects such as Aircraft, ships Where precise frequencies are needed, other devices such as the Klystron are used. A klystron is a specialized linear-beam Vacuum tube (evacuated electron tube The voltage applied and the properties of the cathode determine the power of the device.

The magnetron is a fairly efficient device. In a microwave oven, for instance, a 1,100 Watt input will generally create about 700 Watts of microwave energy, an efficiency of around 65%. Modern, solid-state, microwave sources at this frequency typically operate at around 25 to 30% efficiency and are used primarily because they can generate a wide range of frequencies. Thus, the magnetron remains in widespread use in roles which require high power, but where precise frequency control is unimportant.

Applications

Magnetron with magnet in its mounting box. The horizontal plates form a Heatsink, cooled by airflow from a fan
Magnetron with magnet in its mounting box. The horizontal plates form a Heatsink, cooled by airflow from a fan

Radar

Main article: History of radar (Centimetric radar)

In radar devices the waveguide is connected to an antenna. A heat sink (or heatsink) is an environment or object that absorbs and dissipates heat from another object using Thermal contact (either direct or radiant The history of radar began in the 1900s when engineers invented simple uni-directional ranging devices Radar is a system that uses electromagnetic waves to identify the range altitude direction or speed of both moving and fixed objects such as Aircraft, ships An antenna is a Transducer designed to transmit or Receive electromagnetic waves In other words antennas convert electromagnetic waves into The magnetron is operated with very short pulses of applied voltage, resulting in a short pulse of microwave energy being radiated. As in all radar systems, the radiation reflected off a target is analyzed to produce a radar map on a screen.

Heating

In microwave ovens the waveguide leads to a radio frequency-transparent port into the cooking chamber. A microwave oven, or a microwave, is a Kitchen appliance that cooks or heats Food by Dielectric heating. It is important that there is food in the oven when it is operated so that these waves are absorbed, rather than reflecting into the waveguide where the intensity of standing waves can cause arcing. A standing wave, also known as a stationary wave, is a Wave that remains in a constant position The arcing, if allowed to occur for long periods, will destroy the magnetron. If a very small object is being microwaved, it is recommended that a glass of water be added as an energy sink, although care must be taken not to "superheat" the water. See Superheater for the device used in Steam engines In Physics, superheating (sometimes referred to as boiling retardation

History

The oscillation of magnetrons was first observed and noted by Augustin Žáček, professor at the Charles University, Prague in the Czech Republic, although the first simple, two-pole magnetrons were developed in the 1920s by Albert Hull at General Electric's Research Laboratories (Schenectady, New York), as an outgrowth of his work on the magnetic control of vacuum tubes in an attempt to work around the patents held by Lee De Forest on electrostatic control. Albert W Hull ( 19 April[[ 880]] &ndash 22 January[[ 966]] is most remembered for his early invention of the Magnetron. This article is about the electronic device not an evacuated pipe used for experiments in Free-fall. Lee De Forest, ( August 26, 1873 – June 30, 1961) was an American Inventor with over 300 patents to his credit The two-pole magnetron, also known as a split-anode magnetron, had relatively low efficiency. The cavity version (properly referred to as a resonant-cavity magnetron) proved to be far more useful.

There was an urgent need during radar development in World War II for a high-power microwave generator that worked in shorter wavelengths—around 10 cm (3 GHz) rather than 150 cm—(200 MHz) available from tube-based generators of the time. Radar is a system that uses electromagnetic waves to identify the range altitude direction or speed of both moving and fixed objects such as Aircraft, ships World War II, or the Second World War, (often abbreviated WWII) was a global military conflict which involved a majority of the world's nations, including Microwaves are electromagnetic waves with Wavelengths ranging from 1 mm to 1 m or frequencies between 0 In Physics wavelength is the distance between repeating units of a propagating Wave of a given Frequency. It was known that a multi-cavity resonant magnetron had been developed in 1935 by Hans Hollmann in Berlin. Hans Erich (Eric Hollmann ( 4 November, 1899 — 19 November 1960) was a German electronic specialist who made several breakthroughs in the development Berlin is the capital city and one of sixteen states of Germany. However, the German military considered its frequency drift to be undesirable and based their radar systems on the klystron instead. A klystron is a specialized linear-beam Vacuum tube (evacuated electron tube It was primarily for this reason that German night fighter radars were not a match for their British counterparts. A night fighter (also all-weather fighter) is a Fighter aircraft adapted for use at night or in other times of bad visibility

In 1940, at the University of Birmingham in the UK, John Randall and Dr. Harry Boot produced a working prototype similar to Hollman's cavity magnetron, but added liquid cooling and a stronger cavity. The University of Birmingham (informally Birmingham University) is a British red brick University located in the city of Birmingham The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom, the UK or Britain,is a Sovereign state located Sir John Randall, FRSE, ( March 23, 1905 &ndash June 16, 1984) was a British Physicist, credited with radical Henry Albert Howard "Harry" Boot ( 29 July, 1917 - 8 February, 1983) was an English physicist who with Sir John Randall Randall and Boot soon managed to increase its power output 100 fold. Instead of giving up on the magnetron due to its frequency inaccuracy, they sampled the output signal and synced their receiver to whatever frequency was actually being generated.

Because France had just fallen to the Nazis and Britain had no money to develop the magnetron on a massive scale, Churchill agreed that Sir Henry Tizard should offer the magnetron to the Americans in exchange for their financial and industrial help. This article is about the country For a topic outline on this subject see List of basic France topics. Nazism, which was a short name for National Socialism (Nationalsozialismus refers primarily to the Ideology and practices of the National Socialist German Sir Henry Thomas Tizard ( 23 August 1885 in Gillingham, Kent – 9 October 1959 in Fareham, Hampshire By September, the Massachusetts Institute of Technology had set up a secret laboratory to develop the cavity magnetron into a viable radar. Ernest Lawrence's laboratory at UC Berkeley now known as Lawrence Berkeley National Laboratory, was also known as the Radiation Laboratory Two months later, it was in mass production, and by early 1941, portable airborne radar were being installed into American and British planes. [1]

An early 6 kW version, built in England by the GEC Research Laboratories, Wembley, London, was given to the US government in September 1940. The watt (symbol W) is the SI derived unit of power, equal to one Joule of energy per Second. The General Electric Company or GEC was a major UK company involved in consumer and defence electronics communications and engineering Wembley is an area located in North-West London, England which forms the northern part of the London Borough of Brent. London ( ˈlʌndən is the capital and largest urban area in the United Kingdom. The federal government of the United States is the central United States Governmental body established by the United States Constitution. It was later described as "the most valuable cargo ever brought to our shores" (see Tizard Mission). The Tizard Mission officially the British Technical and Scientific Mission was a British delegation that visited the United At the time the most powerful equivalent microwave producer available in the US (a klystron) had a power of only ten watts. A klystron is a specialized linear-beam Vacuum tube (evacuated electron tube The cavity magnetron was widely used during World War II in microwave radar equipment and is often credited with giving Allied radar a considerable performance advantage over German and Japanese radars, thus directly influencing the outcome of the war. World War II, or the Second World War, (often abbreviated WWII) was a global military conflict which involved a majority of the world's nations, including Germany, officially the Federal Republic of Germany ( ˈbʊndəsʁepuˌbliːk ˈdɔʏtʃlant is a Country in Central Europe. For a topic outline on this subject see List of basic Japan topics.

Short-wave, centimetric radar, which was made possible by the cavity magnetron, allowed for the detection of much smaller objects and the use of much smaller antennas. The combination of the small-cavity magnetron, 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 like H2S improved the accuracy of Allied bombers used in the strategic bombing campaign. The H2S Radar was used in bombers of RAF Bomber Command. It was designed to identify targets on the ground for night and all-weather bombing Strategic bombing during World War II was greater in scale than any wartime attack the world had previously witnessed 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 developed proximity fuse, made anti-aircraft guns much more dangerous to attacking aircraft. A proximity fuze (also called a VT fuze for "variable time" is a fuze that is designed to detonate an explosive device automatically when the distance The two coupled together and used by anti-aircraft batteries, placed along the flight path of German V-1 flying bombs on their way to London, are credited with destroying many of the flying bombs before they reached their target. The Fieseler Fi 103, better known as V-1 (German Vergeltungswaffe 1 was an early Cruise missile used during World War Two London ( ˈlʌndən is the capital and largest urban area in the United Kingdom.

Since then, many millions of cavity magnetrons have been manufactured; some for radar, but the vast majority for microwave ovens. A microwave oven, or a microwave, is a Kitchen appliance that cooks or heats Food by Dielectric heating. The use in radar itself has dwindled to some extent, as more accurate signals have generally been needed and developers have moved to klystron and traveling wave tube systems for these needs. A klystron is a specialized linear-beam Vacuum tube (evacuated electron tube A traveling-wave tube (TWT is an electronic device used to amplify Radio frequency signals to high power usually in an electronic assembly known as a traveling-wave

Health hazards

Caution: radiowaves hazard
Caution: radiowaves hazard

Among more speculative hazards, at least one in particular is well known and documented. As the lens of the eye has no cooling blood flow, it is particularly prone to overheating when exposed to microwave radiation. The lens is a transparent biconvex structure in the Eye that along with the Cornea, helps to Refract Light to be focused Eyes are organs that detect Light, and send signals along the Optic nerve to the visual areas of the brain This heating can in turn lead to a higher incidence of cataracts in later life. A cataract is a clouding that develops in the crystalline lens of the Eye or in its envelope varying in degree from slight to complete opacity A microwave oven with a warped door or poor microwave sealing can be hazardous.

There is also a considerable electrical hazard around magnetrons, as they require a high voltage power supply. Operating a magnetron with the protective covers and interlocks bypassed should therefore be avoided.

Some magnetrons have ceramic insulators with a bit of beryllium oxide (beryllia) added—these ceramics often appear somewhat pink or purple-colored (see the photos above). Beryllium oxide ( BeO) is a white crystalline oxide It is notable as it is an electrical insulator with a thermal conductivity higher than any other non-metal that actually Note that beryllium oxide is white, so relying on the color to identify its presence would be unwise. The beryllium in this ceramic is a serious chemical hazard if crushed and inhaled, or otherwise ingested. Beryllium (bəˈrɪliəm is a Chemical element with the symbol Be and Atomic number 4 Single or chronic exposure can lead to berylliosis, an incurable lung condition. Berylliosis or chronic beryllium disease (CBD is an Occupational lung disease. In addition, beryllia is listed as a confirmed human carcinogen by the IARC; therefore, broken ceramic insulators or magnetrons should not be directly handled.

See also

References

  1. ^ Angela Hind. "Briefcase 'that changed the world'", BBC News, February 5, 2007. Events 1576 - Henry of Navarre converts to Roman Catholicism in order to ensure his right to the throne of France. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Retrieved on 2007-08-16. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 1384 - The Hongwu Emperor of Ming China, Emperor Dong hears a case of a couple who tore paper money bills while fighting  

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