Traveling-wave tube

(electronics) An electron tube in which a stream of electrons interacts continuously or repeatedly with a guided electromagnetic wave moving substantially in synchronism with it, in such a way that there is a net transfer of energy from the stream to the wave; the tube is used as an amplifier or oscillator at frequencies in the microwave region.

A microwave electronic tube in which a beam of electrons interacts continuously with a wave that travels along a circuit, the interaction extending over a distance of many wavelengths. Traveling-wave tubes can provide amplification over exceedingly wide bandwidths. Typical bandwidths are 10–100% of the center frequency, with gains of 20– 60 dB. Low-noise traveling-wave tube amplifiers serve as the inputs to sensitive radars or communications receivers. High-efficiency medium-power traveling-wave tubes are the principal final amplifiers used in communication satellites, the space shuttle communications transmitter, and deep-space planetary probes and landers. High-power traveling-wave amplifiers operate as the final stages of radars, wide-band radar countermeasure systems, and scatter communication transmitters. They are capable of delivering continuous-wave power levels in the kilowatt range and pulsed power levels exceeding a megawatt. See also Communications satellite; Electronic warfare; Radar; Space communications; Space probe.

In a forward-wave, traveling-wave tube amplifier (see illustration), a thermionic cathode produces the electron beam. An electron gun initially focuses the beam, and an additional focusing system retains the electron stream as a beam throughout the length of the tube until the beam is captured by the collector electrode. The microwave signal to be amplified enters the tube near the electron gun and propagates along a slow-wave circuit. The tube delivers amplified microwave energy into an external matched load connected to the end of the circuit near the collector. The slow-wave circuit serves to propagate the microwave energy along the tube at approximately the same velocity as that of the electron beam. Interaction between beam and wave is continuous along the tube with contributions adding in phase.

Periodic-permanent-magnet (PPM) focused traveling-wave tube.

The principle use of this technique is to create a voltage-tunable microwave oscillator. Typically it uses a hollow, linear electron beam and a helix circuit designed to emphasize the backward-wave fields. This represents the earliest type of voltage-tunable microwave oscillator. It is capable of generating power levels of 10–100 milliwatts with a tuning range of 2:1 in frequency. Its use has almost disappeared with the development of magnetically tuned microwave transistor oscillators using yttrium-iron-garnet (YIG) spherical resonators. See also Ferrimagnetic garnets; Magnetron; Microwave tube; Oscillator.