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All cavity magnetrons consist of a cathode with a high
(continuous or pulsed) negative potential created 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, a consequence of the Lorentz force. 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. This principle of
cavity resonator is very similar to blowing a stream of air across the open top
of a glass pop bottle. A portion of the field is extracted with a short
antenna that is connected to a waveguide .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.
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The magnetron is called a "crossed-field" device
in the industry because both magnetic and electric fields are employed in its
operation, and they are produced in perpendicular directions so that they
cross. The applied magnetic field is constant and applied along the axis of the
circular device illustrated. The power to the device is applied to the center
cathode which is heated to supply energetic electrons which would, in the
absence of the magnetic field, tend to move radially outward to the ring anode
which surrounds it.
Electrons are released at the center cathode by the
process of thermionic emission and have an accelerating field which moves them
outward toward the anode. The axial magnetic field exerts a magnetic force on
these charges which is perpendicular to their initially radial motion, and they
tend to be swept around the circle. In this way, work is done on the charges
and therefore energy from the power supply is given to them. As these electrons
sweep toward a point where there is excess negative charge, that charge tends
to be pushed back around the cavity, imparting energy to the oscillation at the
natural frequency of the cavity. This driven oscillation of the charges around
the cavities leads to radiation of electromagnetic waves, the output of the
magnetron.
The sizes of the cavities determine the resonant frequency,
and thereby the frequency of emitted microwaves. However, the frequency is not
precisely controllable. The operating frequency varies with changes in load
impedance, with changes in the supply current, and with the temperature of the
tube. 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 synchronized with the build-up of oscillator output.
The 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.).