19. Finally, the electrons reach the anode.
The electrons reach the anode only after they have taken a spiral
path and given up some energy to the oscillating RF fields. Some electrons
never reach the anode, but instead they return to the cathode.
Electrons
that return to the cathode are lost. But when they hit the cathode, they
cause it to get hotter.
Most magnetrons therefore either have their
filament voltage turned off or turned down after they have started
oscillating
If the filament voltage were kept at the full starting value,
the combination of filament voltage heat and the heat from the returned
electrons would damage the tube.
20. Summarizing electron movement.
a. Electrons strike
the
anode block and shock excite
the resonant
cavities.
b. The RF fields in the cavities oscillate in such a way that alternate
anode segments are of the same polarity and 180 degrees out of phase with
their neighboring cavities.
c. Then the electrons pass the RF fields in phase and add their energy
to the fields.
d. The fields keep oscillating until the voltage applied to the cathode
is removed and the electrons stop flowing.
e. These four steps briefly state the operation of the magnetron. The
most critical part of the operation is getting the electrons to pass the RI
fields at the right time to keep the magnetron oscillating.
21. Cathode voltage and magnetic field make electrons pass at the right
time.
a. You have already seen how the voltage across the diode and the
magnetic field of the magnet affect the electrons.
The correct values of
voltage and magnetic force make the electrons travel in the right path.
That path is the one where the electrons pass the RF fields so as to add
their energy to them. If they don't add their energy in the right way, the
magnetron will not oscillate at all, or it will operate erratically. This
means then if the voltage or magnetic field change from the correct values,
one of the following trouble symptoms will occur:
(1) The magnetron will not oscillate at all.
(2) The magnetron will oscillate at the wrong frequency.
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