16. Electrons that strike the anode excite the cavities.
a. You know that an oscillator tuned circuit must be excited.
This
really means that energy must be introduced into the tuned circuit so that
it can vary at the resonant frequency of the tuned circuit. The method of
exciting a magnetron tuned circuit is shown in Figure 92. There you see the
direction of current flow, the RF E fields, and RF H fields.
b. The electrons emitted from the cathode take a curved path in
traveling to the anode.
The first electrons that strike the anode block
cause current to flow in the anode, and the RF fields are built up within
the cavities and interaction space.
The fields then vary at the natural
resonant frequency of the cavities.
You have already seen this method of
excitation in resonant cavities. When cavities are excited like this we say
they are shock-excited.
Figure 92.
How Magnetron Cavities are Excited.
c. Another thing you learned when studying resonant cavities was that
the fields may take different patterns in the cavities. These patterns are
called modes of operation. There are many modes of operation possible in a
magnetron, but only one mode, called the pi mode, gives the most power
output and the highest efficiency.
The fields for pi mode operation are
shown in Figure 92.
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