2-8
Figure 2-5C.—Buncher cavity action. ELECTRON #2 VELOCITY UNCHANGED.
Figure 2-5D.—Buncher cavity action. ELECTRON #3 ACCELERATED.
Figure 2-5E.—Buncher cavity action. ELECTRONS BEGINNING TO BUNCH, DUE TO VELOCITY
DIFFERENCES.
The velocity modulation of the beam is merely a means to an end. No useful power has been
produced at this point. The energy gained by the accelerated electrons is balanced by the energy lost by
the decelerated electrons. However, a new and useful beam distribution will be formed if the velocity-
modulated electrons are allowed to drift into an area that has no electrostatic field.
As the electrons drift into the field-free area beyond the buncher cavity, bunches continue to form
because of the new velocity relationships between the electrons. Unless the beam is acted upon by some
other force, these bunches will tend to form and disperse until the original beam distribution is eventually
reformed. The net effect of velocity modulation is to form a current-density modulated beam that varies at
the same rate as the grid-signal frequency. The next step is to take useful power from the beam.
The current-modulated (bunched) electron beam in figure 2-6A and B is shown in various stages of
formation and dispersion. A second cavity, called a CATCHER CAVITY, must be placed at a point of
