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Traveling Wave Tubes (TWTs)
The concept
The Traveling-Wave Tube (TWT) is an amplifier
of microwave energy. It accomplishes this through the interaction
of an electron beam and an RF circuit known as a slow wave
structure. The term "slow wave" comes from the fact that the
RF wave velocity as it travels down the circuit is much less
than that of light in free-space. As the electron beam travels
down this interaction region an energy exchange takes place
between the particles and the rf circuit wave. As an example
if one was to apply five watts of rf energy to the input of
a TWT rf circuit, they may find one hundred watts at the output
rf terminal. In this way the linear beam device, TWT or Klystron
for instance, provides gain to the applied signal.
There are three basic components to any TWT,
or linear beam, device. They are the electron gun, the slow
wave circuit, and collector. Any or all of these major components
can range from the very complicated to the simplistic in design.
The choice is based upon performance requirements and customer
specifications.
Central to the operation of a linear beam
device is the electron gun. It is from this device that the
electron beam is generated. The source of the electrons is
a component known as the cathode. This component is typically
heated to anywhere from 760 to 1100 degrees Celsius, depending
on the nature of the cathode, and via thermonic emission and
the application of a high voltage bias the electrons are drawn
down the tube. This voltage, known as the cathode voltage,
may range in value from several thousands of volts to several
hundreds of thousands.
There are numerous types of cathodes that
have been used in microwave tubes since the early days
of the magnetron. These tubes employed what are known as
oxide coated cathodes. This is where a mix of carbonates,
usually barium, strontium, and calcium, are sprayed onto
a nickel surface and once heated break down into oxides
and provide the source of electrons for the vacuum device.
TWTs manufactured at the Williamsport operation utilize
the oxide cathode and have been very successful in the
field. Tubes with this particular source of beam electrons
have demonstrated life performances in excess of one hundred
thousand hours of continuous operation!
The second major component of the TWT, or
linear beam device, is the slow wave structure (SWS). This
assembly can be likened to a bandpass filter in the classic
microwave engineering sense. Over a particular band of frequencies,
which can range as high as two or more octaves (one octave
equals a doubling of frequency), the SWS support the RF signal.
There are numerous types of slow wave structures, helical,
coupled-cavity, ring-and-bar and many other types in this
class. The frequency at which the device operates controls
the geometry, or size of the structure. Also RF power handling
capabilities become important when selecting which type of
SWS to use.
The RF wave then travels down the SWS and
an interaction, or energy exchange, takes place between it
and the electron beam. One of the most important features
of the SWS is that it must control the velocity of the RF
wave such that it matches that of the beam. This is a characteristic
known as sycronisium and is very important to the operation
of the device. There are various theories that over the years
have been implemented to describe the exchange process. The
reader is refereed to J.R. Pierces [1] book for a particularly
good description of the beam wave interaction.
After the energy has been extracted to the
circuit the beam enters a region, or assembly, known as the
collector. The function of this device is exactly as the name
implies and it collects the spent beam. There are various
collector configurations used in linear beam devices. Some
of these include single-stage grounded collectors and multiple
stage collectors. The driving concept behind the selection
of collector used is efficiency and power supply considerations.
For instance in the TV broadcast market were power consumption
is a premium, multi-stage depressed collectors provide significant
savings in energy. Some tubes with this type of collector
configuration have reached basic electronic efficiencies of
over 65 percent.
What we have attempted to describe here, in
the most introductory of terms, are the basic operating principles
and components of linear beam microwave tubes. It goes without
saying that the science and theory adequately describing their
characteristics is beyond the scope of this document. For
the interested reader we have given a list of reference sources
available on the Internet or in most college libraries.
Some reference texts:
EDD Authors contribute to IEEE publication
[1] J.R. Pierce, Traveling-Wave Tubes. New
York: D. Van Nostrand Company, Inc., 1950.
[2] R.G.E. Hutter, Beam and Wave Electronics
in Microwave Tubes. New York: D. Van Nostrand Company, Inc.,
1960
[3] J.C. Slater, Microwave Electronics. New
York: D. Van Nostrand Company, Inc., 1950
Internet Sites:
A good page for engineering basics:
• http://www.efunda.com/home.cfm
Several pages relating to microwave tubes:
• http://www-group.slac.stanford.edu/kly/
• http://tempest.das.ucdavis.edu/muri/projects.html
• http://www.ipr.umd.edu/
Of more historical interest, with interesting
photos:
• http://www.electricstuff.co.uk/
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