time. The reason is that the velocity of electron
happens only when the electron bunch passes the
number 1 decreased, the velocity of electron number
output gap during the time that the output cavity RF
2 did not change, and the velocity of electron
voltage is negative.
number 3 increased. Thus, the faster electrons
overtake the slower electrons and form a bunch
h. For the maximum number of beam
within the region of the output cavity. Because the
electrons to pass the output cavity, there must be
output cavity RF voltage is negative, the electron
some means for confining or restraining the
bunch surrenders a large amount of energy to the
electrons along the axis of the drift tube. Thus, the
output cavity. This energy is the klystron's RF
multicavity klystron requires an electron deflection
output.
or suppressing element to prevent the beam electrons
from colliding with the drift tube wall.
e. After passing the output cavity gap the
partly spent electrons terminate at the collector
1-49.
MAGNETIC DEFLECTION OF THE
dissipating their remaining energy.
ELECTRON BEAM
f. The principle shown in figure 49 can be
a. As in a cathode ray tube, either
applied to billions of electrons. Although the three
electrostatic or magnetic deflection can be used to
electrons shown in figure 49 bunch only near the
control the electron beam in the klystron. We will
output cavity, in reality the electrons may bunch and
limit our discussion to magnetic deflection because
debunch repeatedly before reaching the output
it is the most successful of the two methods.
cavity.
When the beam electrons bunch and
debunch, they are undergoing density modulation.
b. The type of magnetic deflection used is
The number of times that they bunch depends on the
usually electromagnetic deflection. The reason is
distance between cavities and the amplitude and
that it provides a means of adjusting the magnetic
frequency of the RF voltage within the cavity.
field strength by adjusting the current through the
coils. Figure 50 shows a cross-sectional view of the
g. The klystron operates at its highest
three-cavity klystron with its magnetic field coil
efficiency when the output cavity RF voltage causes
assembly. The field coil assembly encases the entire
tightly bunched electrons to loose velocity. This
length of the cavity-drift tube assembly.
Figure 50. Multicavity klystron's magnetic field coil assembly.
344 L1
48
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