For example, in a 20-kilowatt, 10-GHz klystron am-
plifier, approximately 1 kilowatt of heat is generated
by the circulating RF currents in the output cavity.
Since the cavity is approximately a 1-inch cube, it is
apparent that removing the heat is a formidable prob-
lem.
Another problem associated with cavity heating is
not immediately apparent. The resonant frequency of
a cavity depends on its physical size. The cavities are
made of metal, which expands as its temperature in-
creases. This effect tends to change the resonant fre-
quency of the cavities and, thereby, detune the tube.
As the tube detunes, the power output drops. This, in
turn, reduces the RF heating and allows the tube to
come back into tune. If this problem were not con-
sidered in the initial tube design, the resulting tube
would be unstable in its operation. This situation
exists in some tubes that are external cavities. These
external cavities are cooled by air, rather than by
liquid, and the cavity tuning is seriously affected by
the ambient air temperature.
All high-power klystrons are liquid-cooled,
including cavities and tuners. The cavities are main-
tained at a stable temperature by controlling the tem-
perature of the cooling liquid; therefore, thermal de-
tuning is no longer a problem.
Drift-tube heating is a serious problem in very-
high-power and medium-power, high-frequency kly-
strons. The drift tubes, which are inside the vacuum
envelope, are physically small, and it is difficult to
conduct the drift-tube heat into the region outside the
vacuum envelope. In some high-power tubes, it is
necessary to bring the cooling liquid inside the vac-
uum envelope and around the drift tubes to remove
the heat.
In some high-power, high-frequency systems, it is
necessary to cool the output waveguide. A 10-GHz
waveguide carrying a 5-kilowatt signal becomes too
hot to-touch in normal ambient air. Fortunately, wave-
guides may be cooled easily by soldering copper
tubing along the sides of the guide and running cool-
ing liquid through the tubing.
Systems that use blowers for cooling usually have
an airflow switch; if the blower fails, the switch opens
and removes power from appropriate power supplies.
Systems that use liquid cooling normally distribute
the liquid into a large number of paths, since the flow
requirements are quite dissimilar. Each path has a
low-flow interlock. If one of the liquid cooling cir-
cuits becomes plugged, the low-flow interlock opens
and removes power from the system.
Distilled water is the best medium for cooling
klystron amplifiers. Some very-high-power amplifiers
specify that only distilled water may be used. Un-
fortunately, water freezes at a temperature that could
be encountered under normal operating conditions.
Many low- and medium-power klystrons permit the
use of ethylene glycol and water as the cooling liquid.
However, since ethylene glycol reacts with certain
types of metals and hoses that might be used in the
system, special care must be taken in working on a
system that uses ethylene glycol. Only nonferrous
metals should be used in a cooling system for a kly-
stron amplifier.
NOISE IN KLYSTRON AMPLIFIERS.—
Volumes have been written about noise in microwave
systems. However, this chapter covers only the high
points. The output of a klystron amplifier contains
harmonics primarily because the output cavity is
excited by bunches of electrons that pass through the
output gap once every cycle. Since the driving energy
supplied to the output cavity is not continuous, but
occurs in quick pulses, it is evident that the output
current may not be purely sinusoidal. Therefore, the
output contains harmonic components.
In general, the harmonic output of a klystron am-
plifier is largest when the tube is operated at or above
saturation. Harmonic content decreases when the tube
is operated below saturation. Also, harmonics in the
output may be reduced by the use of harmonic filters.
Another source of distortion is the nonlinearity of
the klystron. If the RF signal is amplitude modulated,
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