page_title: Cavity magnetron
section_title: History
hierarchical_section_title: Cavity magnetron / History
caption_attribution_description: English: Microwave cavities in original cavity magnetron tube, developed by John Randall (1905-1984) and Harry Boot (1917-1983) at Birmingham University in 1940. This was contained inside a vacuum tube. A heated cathode extended up the central axis, which emitted electrons, and a permanent magnet embracing the tube created a magnetic field along the axis of the tube. The magnetic field caused the electrons to circle in bunches within the central cavity of the tube, inducing microwave oscillations in the 6 peripheral cylindrical cavities. The cavity magnetron was the first practical device for producing power at centimeter wavelengths. It enabled the development of radars that could produce clear images of distant objects by bouncing microwaves off their targets. These played a significant role in World War 2. Please note: this image has not been fully finished.
context_page_description: The cavity magnetron is a high-powered vacuum tube that generates microwaves using the interaction of a stream of electrons with a magnetic field while moving past a series of open metal cavities. Electrons pass by the openings to these cavities and cause microwaves to oscillate within, similar to the way a whistle produces a tone when excited by an air stream blown past its opening. The frequency of the microwaves produced, the resonant frequency, is determined by the cavities' physical dimensions. Unlike other vacuum tubes such as a klystron or a traveling-wave tube, the magnetron cannot function as an amplifier in order to increase 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.
  An early form of magnetron was invented by H. Gerdien in 1910. Another form of magnetron tube, the split-anode magnetron, was invented by Albert Hull of General Electric Research Laboratory in 1920, but it achieved a frequency of only 30 kHz. Similar devices were experimented with by many teams through the 1920s and 1930s.
context_section_description: 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. 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.
  In the US, Albert Hull put this work to use in an attempt to bypass Western Electric's patents on the triode. Western Electric had gained control of this design by buying Lee 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 at General Electric's Research Laboratories in Schenectady, 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.
  Hull's magnetron was not originally intended to generate VHF (very-high-frequency) electromagnetic waves.  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.  Žáček, a professor at Prague's Charles University, published first; however, he published in a journal with a small circulation and thus attracted little attention. Habann, a student at the University of Jena, investigated the magnetron for his doctoral dissertation of 1924. Throughout the 1920s, Hull and other researchers around the world worked to develop the magnetron. 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.
  While radar was being developed during World War II, there arose an urgent need for a high-power microwave generator that worked at shorter wavelengths, 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 by Hans Hollmann in Berlin. However, the German military considered the frequency drift of Hollman's device to be undesirable, and based their radar systems on the klystron instead.  But klystrons could not at that time achieve the high power output that magnetrons eventually reached.  This was one reason that German night fighter radars, which never strayed beyond the low-UHF band to start with for front-line aircraft, were not a match for their British counterparts. Likewise, in the UK, Albert Beaumont Wood detailed a system with "six or eight small holes" drilled in a metal block, identical to later production designs. 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. 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. 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 by James Sayers coupling ("strapping") alternate cavities within the magnetron which reduced the instability by a factor of 5–6. (For an overview of early magnetron designs, including that of Boot and Randall, see ) According to Andy Manning from the RAF Air Defence Radar Museum, Randall and Boot's discovery was "a massive, massive 
caption: The anode block which is part of the cavity magnetron developed by Randall and Boot
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