18. When electrons
pass
cavities
at
the
right
time,
they
sustain
oscillations.
a. Figure 94 shows how the electrons keep the magnetron oscillating.
When an electron arrives at a cavity gap (Part A of Figure 94), it hits the
cavity RF field. If the electron hits the field when the field is at peak
value, and is also moving in the same direction as the electron, the
electron is slowed down. As the electron slows down, it gives up some of
its energy to the RF field.
The added energy keeps the field from dying
out. This means that as long as the electrons hit the fields at the right
time, the cavities keep oscillating.
Figure 94.
How the RF Fields are Reinforced.
b. Part B of Figure 94 shows the electron an instant later. Now the
electron is out of the RF field and is gaining speed. At the same time, the
RF fields are reversing direction.
By the time the electron reaches the
next cavity (Part C of Figure 94), the fields have reversed. That means the
electron reaches the RF field of the next cavity when the field is maximum
and in the same direction as the electron.
So the same thing happens as
before: the electron is slowed down by the field, and gives up some energy
to the field. So, you see that as long as the electrons from the cathode
hit the RF fields at the right time, oscillations continue.
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