Traveling-wave tubes are high-power, microwave amplifiers used in radar radio transmitters. They are characterized by high-grain, large bandwidth and high-operating voltages. In addition to the radio frequency input and output connections, the majority of traveling-wave tubes are fitted with a body electrode, a cathode or electron gun, a collector, an interaction region, and means of focusing the electrons into a linear beam.
The core of the traveling-wave tube (TWT) is what is known as the delay-line structure, which is shown schematically as a helix in the picture below. Around 90% of all high-power TWTs utilizes the coupled-cavity approach, since it has excellent electrical characteristics, such as bandwidth, impedance, and mode structure. Other characteristics are mechanical simplicity, ruggedness, and versatility of its scale, with frequency, power, and bandwidth.
To proper traveling-wave tube performance, the focusing of the electron beam is very important, as is the case with the klystron and other common methods of focusing. The most commonly used methods in high-power tubes are the PPM methods and selenoidal focusing. In selenoidal focusing, the coils are typically foil-wound as an integral part of the tube to hold mechanically-induced variations in the magnetic field (to considerably less than 1%).
One of the TWT main drawbacks is its relatively low efficiency. There are two ways that this problem has been dealt with; the first one involves depressing the collector, operating with the collector at a potential below that of the body; while the second solution is the so-called velocity synchronization, which is achieved by taking the last portion of the periodic structure and jumping or increasing its potential by a relatively small amount.
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| Above, longitudinal section view of a traveling-wave tube. |