Cavity magnetron

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The cavity magnetron is a high-powered vacuum tube that generates
microwaves using the interaction of a stream of electrons with a
magnetic field. The 'resonant' cavity magnetron variant of the earlier
magnetron tube was invented by John Randall and Harry Boot in
1940.[1] The high power of pulses from the cavity magnetron made
centimeter-band radar practical. Shorter wavelength radars allowed
detection of smaller objects. The compact cavity magnetron tube
drastically reduced the size of radar sets[2] so that they could be
installed in anti-submarine aircraft[3] and escort ships.[2] At present,
cavity magnetrons are commonly used in microwave ovens and in
various radar applications.[4]


Construction and operation
All cavity magnetrons consist of a hot cathode with a high (continuous
or pulsed) negative potential by a high-voltage, direct-current power
supply. 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.
Spaced around the rim of the chamber are cylindrical cavities.


Applications
Magnetron from a microwave oven with magnet
in its mounting box. The horizontal plates form a
heat sink, cooled by airflow from a fan
Radar
In radar devices, the waveguide is connected to an antenna. 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 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.
Lighting
In microwave-excited lighting systems, such as a sulfur lamp, a magnetron provides the microwave field that is
passed through a waveguide to the lighting cavity containing the light-emitting substance (e.g., sulfur, metal halides,
etc.)
History
The first simple, two-pole magnetron was developed in 1920 by Albert Hull[6] 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.