the coolant, the thermostatic bypass opens. The coolant now flows through the heat transfer coils, where it is
cooled by circulated air before it is returned to the klystron drift tube.
b. The alarm circuits within the heat exchanger are used to monitor the temperature and level of the
Section VI. TRAVELING-WAVE TUBES
a. The traveling-wave tube (TWT) has been with us since 1946. Originally, it required almost perfect
operating conditions because it was temperamental in its performance. The temperamental features have been
overcome and the unusual operating characteristics of this tube have permitted the development of new electronic
b. Traveling-wave tubes are made in several sizes to meet almost every power requirement. Because of
their high-gain and broadband characteristics, new communication equipment is being developed.
a. The TWT (fig. 18) has an electron gun
similar to the gun used in the klystron. The cathode
has a parabolic shape to give the initial focusing to
the electron stream emitted from it. The accelerating
anode does very little to focus the electron stream,
Figure 18. Traveling-wave tube.
but it does increase the velocity of the electrons.
The control grid controls the number of electrons in the electron stream.
b. It is the helix (loosely wound coil) that makes this tube different from all other tubes. The electron
beam is passed through the center of the helix and is eventually captured by the collector anode. The collector
anode is operated at a comparatively high voltage and causes the electrons to be continually accelerated. The
helix is operated at the same dc potential as the collector anode.
c. The beam of electrons passing through the helix presents the same problem as the electrons in the
drift tube of the klystron; that is, the natural repelling forces of the electrons tend to scatter the beam. To keep the
electrons in a tightly focused beam, the whole tube is surrounded with a magnetic focusing coil (fig. 19). The size
of the electron beam is controlled directly by the current through the magnetic coil. The higher the coil or
solenoid current, the tighter the beam. If the magnetic field of the solenoid were lost for an instant, the electron
beam would spread, intersect the helix, and destroy the traveling-wave tube.
d. The physical structure of the tube must have an RF input and output. In figure 19, the RF input and
output are transformer-coupled to and from the helix. Notice the attenuator between these couplers. The
attenuator is a ferrite isolator that prevents the output signal from returning to the input coupler and causing