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Traveling Wave Tubes (TWTs)
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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:

Several pages relating to microwave tubes:

Of more historical interest, with interesting photos:

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