Unlike other vacuum tubes, such as a klystron or a traveling-wave tube (TWT), the magnetron cannot function as an amplifier for increasing the intensity of an applied microwave signal; the magnetron serves solely as an oscillator, generating a microwave signal from direct current electricity supplied to the vacuum tube. The use of magnetic fields as a means to control the flow of an electrical current was spurred by the invention of the Audion by Lee de Forest in 1906.
Although efficient, these lamps are much more complex than other methods of lighting and therefore not commonly used. More modern variants use HEMTs or GaN-on-SiC power semiconductor devices to generate the microwaves, which are substantially less complex and can be adjusted to maximize light output using a PID controller. ==History== In 1910 Hans Gerdien (1877–1951) of the Siemens Corporation invented a magnetron.
In 1912, Swiss physicist Heinrich Greinacher was looking for new ways to calculate the electron mass.
Throughout the 1920s, Hull and other researchers around the world worked to develop the magnetron.
He released several papers and patents on the concept in 1921. Hull's magnetron was not originally intended to generate VHF (very-high-frequency) electromagnetic waves.
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.
However, in 1924, Czech physicist August Žáček (1886–1961) and German physicist Erich Habann (1892–1968) independently discovered that the magnetron could generate waves of 100 megahertz to 1 gigahertz.
Habann, a student at the University of Jena, investigated the magnetron for his doctoral dissertation of 1924.
Further research was limited until Okabe's 1929 Japanese paper noting the production of centimeter-wavelength signals, which led to worldwide interest.
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 region. Early conventional tube systems were limited to the [frequency] bands, and although very high frequency systems became widely available in the late 1930s, the ultra high frequency and microwave regions were well beyond the ability of conventional circuits.
Samuel of Bell Telephone Laboratories in 1934, leading to well-known designs by Postumus in 1934 and Hans Hollmann in 1935.
Samuel of Bell Telephone Laboratories in 1934, leading to well-known designs by Postumus in 1934 and Hans Hollmann in 1935.
It was known that a multi-cavity resonant magnetron had been developed and patented in 1935 by Hans Hollmann in Berlin.
By this time the klystron was producing more power and the magnetron was not widely used, although a 300W device was built by Aleksereff and Malearoff in USSR in 1936 (published 1940). The cavity magnetron was a radical improvement introduced by John Randall and Harry Boot at the University of Birmingham, England in 1940.
By this time the klystron was producing more power and the magnetron was not widely used, although a 300W device was built by Aleksereff and Malearoff in USSR in 1936 (published 1940). The cavity magnetron was a radical improvement introduced by John Randall and Harry Boot at the University of Birmingham, England in 1940.
However, his idea was rejected by the Navy, who said their valve department was far too busy to consider it. In 1940, at the University of Birmingham in the UK, John Randall and Harry Boot produced a working prototype of a cavity magnetron that produced about 400 W.
An early 10 kW version, built in England by the General Electric Company Research Laboratories, Wembley, London (not to be confused with the similarly named American company General Electric), was taken on the Tizard Mission in September 1940.
With a flourish, "Taffy" Bowen pulled out a magnetron and explained it produced 1000 times that. Bell Telephone Laboratories took the example and quickly began making copies, and before the end of 1940, the Radiation Laboratory had been set up on the campus of the Massachusetts Institute of Technology to develop various types of radar using the magnetron.
Within weeks, engineers at GEC had improved this to well over a kilowatt, and within months, 25 kilowatts, over 100 by 1941 and pushing towards a megawatt by 1943.
In 1941, the problem of frequency instability was solved by James Sayers coupling ("strapping") alternate cavities within the magnetron which reduced the instability by a factor of 5–6.
By early 1941, portable centimetric airborne radars were being tested in American and British aircraft.
In late 1941, the Telecommunications Research Establishment in the United Kingdom used the magnetron to develop a revolutionary airborne, ground-mapping radar codenamed H2S.
Within weeks, engineers at GEC had improved this to well over a kilowatt, and within months, 25 kilowatts, over 100 by 1941 and pushing towards a megawatt by 1943.
By the end of the war, practically every Allied radar was based on a magnetron. The magnetron saw continued use in radar in the post-war period but fell from favor in the 1960s as high-power klystrons and Travelling-wave tubes emerged.
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