Editing Cavity magnetron (section)

==Common features==
[[File:Magnetron cutaway drawing.png|thumb|upright=1.6|Cutaway drawing of a cavity magnetron of 1984.  Part of the righthand magnet and copper anode block is cut away to show the cathode and cavities.  This older magnetron uses two horseshoe shaped [[alnico]] magnets, modern tubes use [[rare-earth magnet]]s. ]]
All cavity magnetrons consist of a heated cylindrical [[cathode]] at a high (continuous or pulsed) negative potential created by a high-voltage, direct-current power supply. The cathode is placed in the center of an [[vacuum|evacuated]], lobed, circular metal chamber.  The walls of the chamber are the anode of the tube.   A [[magnetic field]] parallel to the axis of the cavity is imposed by a [[permanent magnet]].  The electrons initially move radially outward from the cathode attracted by the electric field of the anode walls.   The magnetic field causes the electrons to spiral outward in a circular path, a consequence of the [[Lorentz force]].  Spaced around the rim of the chamber are cylindrical cavities. Slots are cut along the length of the cavities that open into the central, common cavity space. As electrons sweep past these slots, they induce a high-frequency radio field in each resonant cavity, which in turn causes the electrons to bunch into groups.  A portion of the radio frequency energy is extracted by a short coupling loop that is connected to a [[waveguide (electromagnetism)|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 (radio)|antenna]] in the case of radar.

The size of the cavities determine the resonant frequency, and thereby the frequency of the emitted microwaves. However, the frequency is not precisely controllable. The operating frequency varies with changes in load [[wave impedance|impedance]], with changes in the supply current, and with the temperature of the tube.<ref name="Turner76"/> This is not a problem in uses such as heating, or in some forms of [[radar]] where the receiver can be synchronized with an imprecise magnetron frequency. Where precise frequencies are needed, other devices, such as the [[klystron]] are used.

The magnetron is a self-oscillating device requiring no external elements other than a power supply.  A well-defined threshold anode voltage must be applied before oscillation will build up; this voltage is a function of the dimensions of the resonant cavity, and the applied magnetic field. In pulsed applications there is a delay of several cycles before the oscillator achieves full peak power, and the build-up of anode voltage must be coordinated with the build-up of oscillator output.<ref name=Turner76>L.W. Turner,(ed),'' Electronics Engineer's Reference Book, 4th ed.'' Newnes-Butterworth, London 1976 {{ISBN|9780408001687}}, pp. 7-71 to 7-77</ref>

Where there are an even number of cavities, two concentric rings can connect alternate cavity walls to prevent inefficient modes of oscillation. This is called pi-strapping because the two straps lock the phase difference between adjacent cavities at π radians (180°).

The modern magnetron is a fairly efficient device. In a microwave oven, for instance, a 1.1-kilowatt input will generally create about 700 watts of microwave power, an efficiency of around 65%. (The high-voltage and the properties of the cathode determine the power of a magnetron.) Large [[S band]] magnetrons can produce up to 2.5 megawatts peak power with an average power of 3.75&nbsp;kW.<ref name="Turner76"/> Some large magnetrons are water cooled. The magnetron remains in widespread use in roles which require high power, but where precise control over frequency and phase is unimportant.
{{clear}}