Figure 46. Varying electric field at drift tube gap causes velocity changes in beam electrons.
ELECTRON VELOCITY MODULATION
reason, the microwave (RF) voltage applied to the
input cavity causes a varying electric field (para 10c)
a. Because of the interaction between the
between the grids inside the input cavity.
electron beam and cavity electric fields, some of the
beam electrons accelerate and others decelerate.
c. Part A of figure 46 shows the half cycle
This process is known as velocity modulation. For
during which the RF field is in the same direction as
example, let's consider what happens to the electron
the beam electrons' movement. However, the RF
beam as it passes the first gap in the drift tube.
field exerts a force that causes a velocity decrease in
That's the gap surrounded by the input cavity. The
the beam electrons as they travel past the positive
diagrams in figure 46 will help you to understand
grid toward the negative grid.
how a varying electric field causes velocity changes
d. Every other half cycle, the input cavity
in the beam electrons.
grids will have the polarity as shown in A of figure
b. In each diagram (A and B of figure 46),
46. On alternate half cycles, the grids reverse
the dotted lines represent the grids at the tips of the
polarity as shown in B of figure 46. The RF field,
drift tube. This means that the electron beam passes
therefore, causes the beam electrons to accelerate.
through a pair of grids whenever it passes a drift
That's because the beam electrons go past the
The grids are made of the same
negative grid toward the positive grid.
conducting material as the drift tube. For this