Cavity magnetron

A cavity magnetron is a high-powered vacuum tube that generates coherent microwaves. They are commonly found in microwave ovens, as well as various radar applications.

Construction and operation


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. 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. 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). 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 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. Where precise frequencies are needed, other devices such as the Klystron are used. 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.

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 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. 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. 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.

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. 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&mdash;around 10 cm (3 GHz) rather than 150 cm&mdash;(200 MHz) available from tube-based generators of the time. It was known that a multi-cavity resonant magnetron had been developed in 1935 by Hans Hollmann in Berlin. However, the German military considered its frequency drift to be undesirable and based their radar systems on the klystron instead. It was primarily for this reason that German night fighter radars were not a match for their British counterparts.

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. 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. By September, the Massachusetts Institute of Technology had set up a secret laboratory to develop the cavity magnetron into a viable radar. Two months later, it was in mass production, and by early 1941, portable airborne radar were being installed into American and British planes.

An early 6 kW version, built in England by the GEC Research Laboratories, Wembley, London, was given to the US government in September 1940. It was later described as "the most valuable cargo ever brought to our shores" (see Tizard Mission). At the time the most powerful equivalent microwave producer available in the US (a klystron) had a power of only ten watts. 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.

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. 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. 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.

Since then, many millions of cavity magnetrons have been manufactured; some for radar, but the vast majority for microwave ovens. 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.

Health hazards


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. This heating can in turn lead to a higher incidence of cataracts in later life. 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&mdash;these ceramics often appear somewhat pink or purple-colored (see the photos above). 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. Single or chronic exposure can lead to berylliosis, an incurable lung condition. In addition, beryllia is listed as a confirmed human carcinogen by the IARC; therefore, broken ceramic insulators or magnetrons should not be directly handled.