Travelling wave tube amplifier
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A TWTA or travelling-wave tube amplifier is an electronics device 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, and operating frequencies range from 300 MHz to 50 GHz. The voltage gain of the tube can be of the order of 40 decibels.
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The device is an elongated vacuum tube with an electron gun (a heated cathode that emits electrons) at one end. A solenoid coil wrapped around the tube focuses the electrons into a beam, which then passes down the middle of a wire helix that stretches the length of the tube, 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 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. 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 prevents any reflected wave from travelling back to the cathode.
Coupled Cavity TWTA
Helix TWTAs 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 TWTAs achieve less than 2.5 kW output power.
The coupled cavity TWTA overcomes this limit by replacing the helix with a series of coupled cavities arranged axially along the beam. Conceptionally, this structure provides a helical waveguide and hence amplification can occur via velocity modulation. A coupled cavity TWTA can achive 15 kW output power.
Operation is similar to that of a Klystron, except that coupled cavity TWTAs are designed with attenuation in the slow-wave structure to prevent oscillations. It is this that gives the TWTA its wide bandwidth.
Uses
TWTAs are commonly used in satellite transponders.
They are also used in radar. Typically a control grid is introduced between the electron gun and the slow-wave structure to allow pulsed operation.la:TWT