Magnetron
A magnetron is a high-powered vacuum tube that generates coherent microwaves.
Construction and Operation
A magnetron consists of a hot filament (cathode) kept at or pulsed to a high negative potential by a high voltage direct current power supply. The cathode is built into the center of an evacuated, lobed, circular chamber. A perpendicular magnetic field 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 about the rim of the chamber are cylindrical cavities. The cavities are open along their length and so connected 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). 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
The size of the cavities determine the resonant frequency and so the frequency of the emitted RF energy microwaves. The frequency is thus not precisely controllable, which is not a problem in many applications such as heating (where it does not matter) and radar (where the receiver can be synchronised with the nonprecision output). (Where precise frequencies are required, other devices such as the klystron are used.) The voltage applied and the characteristics of the cathode determine the power of the device.
Applications
Radar
In radar devices the waveguide is connected to an antenna, which may be a slotted waveguide or a conical feedhorn pointing into a parabolic reflector. The magnetron is operated with very short high intensity pulses of applied voltage, resulting in a short pulse of microwave energy being emitted. A small portion of this energy is reflected back to the antenna and the waveguide where it is directed to a sensitive receiver. With further signal processing the signal is ultimately displayed as a radar map on a cathode ray tube.
Heating
In microwave ovens the waveguide leads to a radio frequency transparent port into the cooking chamber. It is important that there be food in the oven when it is operated so that these waves are absorbed, rather than reflecting back into the waveguide where the intensity of standing waves can cause arcing. The arcing, if allowed to occur for long periods will destroy the magnetron.
A typical resonant frequency for microwave ovens is 2.45 GHz, resulting in radiation with a wavelength of 12cm. This wavelength is strongly absorbed by the contents of the cooking chamber, which become heated by their own dielectric loss. It is popularly believed that only the water molecules absorb the heat by vibrating, but this is is a gross over simplification. All materials exhibit heating by microwaves to a differing degree.
History
Simple two-pole magnetrons were developed in the 1920s but gave relatively low power outputs. The cavity version 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 rather than 150 cm - available from generators of the time. In 1940, at Birmingham University in the UK, Professor John Randall and Doctor Harry Boot produced a working prototype of the cavity magnetron, and soon managed to increase its power output 100-fold.
An early version built by G.E.C. and given to the U.S. government in August 1940 was called "the most valuable cargo ever brought to our shores". Curiously, in order not to draw attention to the value of the package, it was not despatched under guard, but sent by regular parcel post. 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. Indeed, during the Battle of Britain, radar (then called Radio Direction Finding or RDF) was crucial to the outcome of the battle, though these early radars actually predated the cavity magnetron and gave only crude displays. The Germans were convinced that to achieve any defence of the skies, the British Royal Air Force (RAF) would have to fly standing patrols which would be expensive on both men and machines. Instead the RAF were able to remain on standby on the ground, and the Germans couldn't initially figure out why the RAF fighters were waiting behind the first cloud as they flew over Britain's shores.
Since then, many millions of cavity magnetrons have been manufactured; some for radar, but the vast majority for another application that was completely unanticipated at the time - the microwave oven.
See also:
