Twt theory pg one 6-23-20

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Traveling Wave Tubes (TWT): Theory of Operation
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A TWT performs as a wideband microwave amplifier. 
This wideband amplification feature is obtained by 
the use of an interaction circuit which is essentially a 
transmission line and does not usually contain any 
resonant 
All TWTs incorporate the basic components shown in 
Figure 1. These components include an electron gun 
(composed of a cathode, a control or modulating 
grid, and an accelerator), which produces an electron 
beam; an RF circuit (delay line), which propagates a 
microwave signal in a manner that permits interaction 
between the beam and the signal; an attenuator, 
which isolates the input and output sections to 
prevent oscillations; and a collector, which removes 
the unused beam energy.
In addition, a magnetic field along the axis of the 
device (not shown) is used to focus the electron 
stream. Although TWT's may differ widely in size and 
construction, depending on the application, the 
basic theory of operation is the same for all types.
The principle of operation of a TWT is strikingly 
simple. As illustrated here, an electron beam is emit-
ted from the cathode and accelerated toward the 
collector at the opposite end of the device. The elec-
trons of the beam are surrounded by an RF wave with 
a strong field component in the direction of the beam 
travel. If the velocities of the beam and the wave are 
nearly the same, interaction takes place.
IThe delay line slows the RF wave down to the veloci-
ty of practical beams. This line is constructed so that 
the field components are primarily longitudinal in the 
vicinity of the beam. If a helix is used as the delay line, 
the RF wave actually travels along the path of the 
helix, but the beam sees only the much longer and 
slower wave components in the longitudinal direc-
tion; that is in traveling around one turn of the helix, 
the wave proceeds forward only by one pitch. The 
resultant velocity is a fraction of the original wave 
velocity.
The Ring Loop TWTs are best known for an ability to 
amplify microwave signals simultaneously over a 
radar band of frequencies. A Ring Loop TWT is illus-
trated schematically in Figure 2, which also shows the 
power supplies to operate the TWT. This TWT 
consists of an electron gun (composed of a heater, 
cathode, and control anode), a helix (slow-wave 
structure), an attenuator, a collector, and an input and 
an output coupling element
Four sources of power are normally required to oper-
ate a Ring Loop TWT: heater power, regulated beam 
power, gain or modulation control power, and collec-
tor power. The electron beam in some TWTs is 
focused by electrostatic or electromagnetic methods, 
and in these situations, a power source is required to 
operate the beam focusing device.