Traveling wave tube

A traveling wave tube (TWT) is an electronic device used to amplify high-power radio frequency signals.

The TWT was invented by Rudolf Kompfner in a British radar lab during World War II, and refined by Kompfner and John Pierce at Bell Labs. Both of them have written books on the device.^[1][2] In 1994, A.S. Gilmour wrote a modern TWT book^[3] which is widely used by U.S. TWT engineers today, and research publications about TWTs are frequently published by the IEEE.

  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 a wire helix that stretches from the RF input to the RF output, the electron beam finally striking a collector at the other end. A directional coupler, which can be either 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 helix 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 current in the helix 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 helix (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 helix. An attenuator placed on the helix, usually between the input and output helicies, prevents reflected wave from travelling back to the cathode.

The bandwidth of a broadband TWT can be as high as three octaves, although tuned (narrowband) versions exist, and operating frequencies range from 300 MHz to 50 GHz. The voltage gain of the tube can be of the order of 70 decibels.

A TWT has sometimes been referred to as a traveling wave amplifier tube (TWAT),^[4][5] although this term has fallen out of use.

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Coupled-cavity TWT

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. Conceptually, 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 15 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.

Uses

TWTs are commonly used as amplifiers in satellite transponders.

They are also used extensively in radar, particularly in airborne fire-control radar systems, and in electronic warfare and self-protection systems. Typically a control grid is introduced between the electron gun and the slow-wave structure to allow pulsed operation.

A TWT integrated with a regulated power supply and protection circuits is referred to as a traveling wave tube amplifier (TWTA).

See also

• Traveling wave tube amplifier
• Distributed Amplifier

Other types of microwave power tubes include:

• Magnetron
• Klystron
• Crossed-field amplifier
• Backward wave oscillator

References