Fusion Message Board

In this space, visitors are invited to post any comments, questions, or skeptical observations about Philo T. Farnsworth's contributions to the field of Nuclear Fusion research.

Subject: Re: Off subject: TWT (Travelling Wave Tube)
Poster: Jim Lux

On , Jim Lux wrote:

Does anybody have a clue?

A TWT (also called a TWTA (A=Amplifier) when combined with a matching power supply, as is usually the case) is, as Richard pointed out, a microwave amplifier. They are not always broad band, however (particularly the "coupled cavity" variety).

How they work: An electron gun shoots a beam of electrons down the center of a (usually) helical delay line (or a series of tuned cavities, same effect, higher efficiency, lower bandwidth). . RF power is fed into the delay line at the gun end. The electron beam and RF interact in such a way as to increase the RF power in the delay line. At the far end, the RF power is extracted from the delay line, the beam hits a "collector" and turns into heat.

The delay line is set up so the propagation velocity of the RF is the same as the speed of the electron beam. The RF field causes the electrons to get bunched (think of the local added + or - field on top of the overall accelerating field. electrons that get ahead get retarded slightly, electrons that get behind get accelerated. . Then, at the end of the section of delay line, the phase velocity is retarded slightly, and the energy flows out of the clumps of electrons into the delay line, and thence to the output.

Not too complex in theory, mind bendingly complex to make work in practice as everything interacts with everything. Typical NRE time to make a new design (from "stock" parts and designs in an existing tube) is probably >12 months.

They are exceedingly popular as microwave amplifiers on comm sats and on land microwave links.

No coherent effects here (i.e. it isn't a laser)