A traveling-wave tube (TWT) is a specialised vacuum tube that is used in electronics to amplify radio frequency (RF) signals to high power, usually as part of an electronic assembly known as a traveling-wave tube amplifier (TWTA).
The bandwidth of a broadband TWT can be as high as one octave, although tuned (narrowband) versions exist, and operating frequencies range from 300 MHz to 50 GHz. The power gain of the tube is on the order of 70 decibels.
The device is an elongated vacuum tube with an electron gun (a heated cathode that emits electrons) at one end. A magnetic containment field around the tube focuses the electrons into a beam, which then passes down the middle of an RF circuit (wire helix or coupled cavity) that stretches from the RF input to the RF output, the electron beam finally striking a collector at the other end. A directional coupler, usually a waveguide or an electromagnetic coil, fed with the low-powered radio signal that is to be amplified, is positioned near the emitter, and induces a current into the helix.
The RF circuit acts as a delay line, in which the RF signal travels at near the same speed along the tube as the electron beam. The electromagnetic field due to the RF signal in the RF circuit interacts with the electron beam, causing bunching of the electrons (an effect called velocity modulation), and the electromagnetic field due to the beam current then induces more current back into the RF circuit (i.e. the current builds up and thus is amplified as it passes down).
A second directional coupler, positioned near the collector, receives an amplified version of the input signal from the far end of the RF circuit. Attenuators placed along the RF circuit prevent the reflected wave from traveling back to the cathode.
Higher powered Helix TWTs usually contain beryllium oxide ceramic as both a helix support rod and in some cases, as an electron collector for the TWT because of its special electrical, mechanical, and thermal properties.
Invention, development and early use
The invention of the TWT is widely attributed to Rudolf Kompfner in 1942–1943, although Nils Lindenblad, working at RCA (Radio Corporation of America) in the USA did patent a device in May 1940 that was remarkably similar to Kompfner's TWT.:2 Kompfner independently invented the TWT, and built the first working TWT, in a British Admiralty radar lab during World War II. His first sketch of his TWT is dated November 12, 1942, and he built his first TWT in early 1943.:3 The TWT was refined by Kompfner, John Pierce, and Lester M. Field at Bell Labs.
By the 1950s, after further development at the Electron Tube Laboratory at Hughes Aircraft Company in Culver City, California, TWTs went into production there, and by the 1960s TWTs were also produced by such companies as the English Electric Valve Company, followed by Ferranti in the seventies.
On July 10, 1962, the first communications satellite, Telstar 1, was launched with a 2 W, 4 GHz RCA-designed TWT transponder used for transmitting RF signals to Earth stations. Syncom 2, the first synchronous satellite (Syncom 1 did not reach its final orbit), launched on July 26, 1963 with two 2 W, 1850 MHz Hughes-designed TWT transponders (one active and one spare).
Helix TWTs are limited in peak RF power by the current handling (and therefore thickness) of the helix wire. As power level increases, the wire can overheat and cause the helix geometry to warp. Wire thickness can be increased to improve matters, but if the wire is too thick it becomes impossible to obtain the required helix pitch for proper operation. Typically helix TWTs achieve less than 2.5 kW output power.
The coupled-cavity TWT overcomes this limit by replacing the helix with a series of coupled cavities arranged axially along the beam. This structure provides a helical waveguide, and hence amplification can occur via velocity modulation. Helical waveguides have very nonlinear dispersion and thus are only narrowband (but wider than klystron). A coupled-cavity TWT can achieve 60 kW output power.
Operation is similar to that of a klystron, except that coupled-cavity TWTs are designed with attenuation between the slow-wave structure instead of a drift tube. The slow-wave structure gives the TWT its wide bandwidth. A free electron laser allows higher frequencies.
A TWT integrated with a regulated power supply and protection circuits is referred to as a traveling-wave-tube amplifier (abbreviated TWTA and often pronounced "TWEET-uh"). It is used to produce high-power radio frequency signals. The bandwidth of a broadband TWTA can be as high as one octave, although tuned (narrowband) versions exist; operating frequencies range from 300 MHz to 50 GHz.
A TWTA consists of a traveling-wave tube coupled with its protection circuits (as in klystron) and regulated power supply Electronic Power Conditioner (EPC), which may be supplied and integrated by a different manufacturer. The main difference between most power supplies and those for vacuum tubes is that efficient vacuum tubes have depressed collectors to recycle kinetic energy of the electrons, so the secondary winding of the power supply needs up to 6 taps of which the helix voltage needs precise regulation. The subsequent addition of a linearizer (as for inductive output tube) can, by complementary compensation, improve the gain compression and other characteristics of the TWTA; this combination is called a linearized TWTA (LTWTA, "EL-tweet-uh").
Broadband TWTAs generally use a helix TWT, and achieve less than 2.5 kW output power. TWTAs using a coupled cavity TWT can achieve 15 kW output power, but at the expense of bandwidth.
A TWTA whose output drives an antenna is a type of transmitter. TWTA transmitters are used extensively in radar, particularly in airborne fire-control radar systems, and in electronic warfare and self-protection systems. In such applications, a control grid is typically introduced between the TWT's electron gun and slow-wave structure to allow pulsed operation. The circuit that drives the control grid is usually referred to as a grid modulator.
A TWT has sometimes been referred to as a "traveling-wave amplifier tube" (TWAT), although this term was never widely adopted. "TWT" has been pronounced by engineers as "twit", and "TWTA" as "tweeta".
- Distributed amplifier
- Klystron tube
- Crossed-field amplifier
- Backward wave oscillator
- Inductive output tube
- 1997 Industrial Assessment of the Microwave Power Tube Industry - US Department of Defense 
- Beryllium Oxide Properties
- US 2300052
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- Fire Direct Website. Accessed 2 July 2008
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- Memorial page, with photo of John Pierce holding a TWT
- Nyquist page, with photo of Pierce, Kompfner, and Nyquist in front of TWT calculations on blackboard
- TMD Travelling Wave Tubes, Information & PDF data sheets.
- Flash animation showing the operation of a traveling wave tube (TWT) and its internal construction