The output of any klystron (regardless of the number of cavities used) is developed by velocity
modulation of the electron beam. The electrons that are accelerated by the CATHODE pulse are acted upon
by rf fields developed across the input and middle cavities. Some electrons are accelerated, some are
decelerated, and some are unaffected. Electron reaction depends on the amplitude and polarity of the
fields across the cavities when the electrons pass the cavity gaps. During the time the electrons are
traveling through the drift space between the cavities, the accelerated electrons overtake the decelerated
electrons to form bunches. As a result, bunches of electrons arrive at the output cavity at the proper
instant during each cycle of the rf field and deliver energy to the output cavity.
Only a small degree of bunching takes place within the electron beam during the interval of travel
from the input cavity to the middle cavity. The amount of bunching is sufficient, however, to cause
oscillations within the middle cavity and to maintain a large oscillating voltage across the input gap. Most
of the velocity modulation produced in the three-cavity klystron is caused by the voltage across the input
gap of the middle cavity. The high voltage across the gap causes the bunching process to proceed rapidly
in the drift space between the middle cavity and the output cavity. The electron bunches cross the gap of
the output cavity when the gap voltage is at maximum negative. Maximum energy transfer from the
electron beam to the output cavity occurs under these conditions. The energy given up by the electrons is
the kinetic energy that was originally absorbed from the cathode pulse.
Klystron AMPLIFIERs have been built with as many as five intermediate cavities in addition to the input
and output cavities. The effect of the intermediate cavities is to improve the electron bunching process
which improves amplifier gain. The overall efficiency of the tube is also improved to a lesser extent.
Adding more cavities is roughly the same as adding more stages to a conventional amplifier. The overall
amplifier gain is increased and the overall bandwidth is reduced if all the stages are tuned to the same
frequency. The same effect occurs with multicavity klystron tuning. A klystron amplifier tube will deliver
high gain and a narrow bandwidth if all the cavities are tuned to the same frequency. This method of
tuning is called SYNCHRONOUS TUNING. If the cavities are tuned to slightly different frequencies, the
gain of the amplifier will be reduced but the bandwidth will be appreciably increased. This method of
tuning is called STAGGERED TUNING.
Q-15. What can be added to the basic two-cavity klystron to increase the amount of velocity modulation
and the power output?
Q-16. How is the electron beam of a three-cavity klystron accelerated toward the drift tube?
Q-17. Which cavity of a three-cavity klystron causes most of the velocity modulation?
Q-18. In a multicavity klystron, tuning all the cavities to the same frequency has what effect on the
bandwidth of the tube?
Q-19. The cavities of a multicavity klystron are tuned to slightly different frequencies in what method of
The Reflex Klystron
Another tube based on velocity modulation, and used to generate microwave energy, is the REFLEX
KLYSTRON (figure 2-9). The reflex klystron contains a REFLECTOR PLATE, referred to as the
REPELLER, instead of the output cavity used in other types of klystrons. The electron beam is modulated
as it was in the other types of klystrons by passing it through an oscillating resonant cavity, but here the
similarity ends. The feedback required to maintain oscillations within the cavity is obtained by reversing
the beam and sending it back through the cavity. The electrons in the beam are velocity-modulated before
the beam passes through the cavity the second time and will give up the energy required to maintain