2-10
Q-9. What is the effect upon an electron that enters the buncher gap when the potential across the
grids is at 0 volts?
Q-10. What determines the placement of the catcher cavity?
MICROWAVE TUBES
Microwave tubes perform the same functions of generation and amplification in the microwave
portion of the frequency spectrum that vacuum tubes perform at lower frequencies. This section will
explain the basic operation of the most widely used microwave tubes, including klystrons, traveling-wave
tubes, backward-wave oscillators, magnetrons, and crossed-field amplifiers. The variations of these tubes
for use in specific applications are so numerous that all of them cannot be discussed in this module.
However, general principles of operation are similar in all of the variations so the explanations will be
restricted to the general principles of operation.
The Basic Two-Cavity Klystron
Klystrons are velocity-modulated tubes that are used in radar and communications equipment as
oscillators and amplifiers. Klystrons make use of the transit-time effect by varying the velocity of an
electron beam in much the same manner as the previously discussed velocity-modulation process. Strong
electrostatic fields are necessary in the klystron for efficient operation. This is necessary because the
interaction of the signal and the electron beam takes place in a very short distance.
The construction and essential components of a TWO-CAVITY KLYSTRON are shown in figure
2-7A. Figure 2-7B is a schematic representation of the same tube. When the tube is energized, the cathode
emits electrons which are focused into a beam by a low positive voltage on the control grid. The beam is
then accelerated by a very high positive dc potential that is applied in equal amplitude to both the
accelerator grid and the buncher grids. The buncher grids are connected to a cavity resonator that
superimposes an ac potential on the dc voltage. Ac potentials are produced by oscillations within the
cavity that begin spontaneously when the tube is energized. The initial oscillations are caused by random
fields and circuit imbalances that are present when the circuit is energized. The oscillations within the
cavity produce an oscillating electrostatic field between the buncher grids that is at the same frequency as
the natural frequency of the cavity. The direction of the field changes with the frequency of the cavity.
These changes alternately accelerate and decelerate the electrons of the beam passing through the grids.
The area beyond the buncher grids is called the DRIFT SPACE. The electrons form bunches in this area
when the accelerated electrons overtake the decelerated electrons.
