Vacuum tubes we still (have to) use: The traveling wave tube, Part 2

We live in a world of solid-state active devices that have made most vacuum tubes obsolete, but a few types remain as they are still indispensable.

The previous part of this article introduced the traveling wave tube and its basic operation, along with some attributes. This part delves into some related issues.

Q: What’s the history of the TWT?

A: Credit for the development is generally given to Rudolph Kompfner, an Austrian working in England during World War II. He worked on another important microwave VED, the klystron, and devised the TWT in 1942. Although formally trained as an architect, he later switched to physic and engineering (see Reference 16 for some details on his fascinating and innovative career).

Q: Are there variations of the basic TWT design?

A: Of course and each variation enhances one or more operating attributes (linearity, frequency, efficiency) but at the expense of others. Reference 12 is a good place to start. For example, the original helical version described above has broadband characteristics, while another version uses a coupled cavity in place of the helix for higher-power operation. Some designs are optimized for continuous operation while others are designed for pulsed operation.

Q: Vacuum tubes burn out, as all VEDs do; is reliability an issue with TWTs?

A: Yes, and no. For many years, they had a reputation for being relatively unreliable (whether due to premature burn-out or other failure mechanism) or eventual wear out such as any tube would have. But they were stilled used, as the best or only component for the application.  In recent years, however, their basic reliability has improved greatly, driven by space- and mil/aero-based needs. The lifetime of a quality TWT is greater than 20,00 hours (between two and three years) or more. Keep in mind that SSPAs don’t last forever, either, and wear out for a variety of reasons, including thermal stress, which TWTs are better at handling.

Q: SSPAs and TWTs compete to some extent; what are their areas of difference and of overlap?

A: Although the situation is not static (it never is in technology) (Figure 1) gives an idea of where each is best suited with respect to frequency versus power. Reference 17 discusses some the areas where TWTs are still the best choice, as well as a comparison with SSPAs from a component and system-level perspective (note that it is written by a TWT vendor, so there may be some bias). Figure 2 shows a comparison of roughly equivalent TWTs and PAs with respect to various parameters, while References 18 and 19 provide some additional insight. Some vendors offer both types of components, and so can provide a reality unbiased guidance (assuming they have suitable components in their portfolio, of course).

Satellite use of TWTs greatly outnumbers that of SSPAs (Figure 3).

Q: Speaking of vendors, who makes TWTs?

A: Among the vendors are Quarterwave Corp, Thales Electron Devices, dB Control, Instruments for Industry, L3Harris Technologies, Crane Aerospace & Electronics, and Teledyne Microwave Solutions, and there are others, some of whom have TWTs targeting only highly specialized applications niches within the broader TWT market.

This article has briefly explored two important but less-known, specialty vacuum electron devices, the photomultiplier tube, and the traveling wave tube. Market researchers estimate that microwave tubes, including TWTs, are a $1 to $2 billion market annually, Reference 20. Those who say “tubes” are dead” are wrong, as these tubes are still vital and irreplaceable components for specific application niches, at least for the foreseeable future.

References

1. EE World, “Magnetron, Part 1: Application and operating principles”
2. EE World, “Magnetron, Part 2: History and future”
3. Wikipedia, “Photoelectric effect”
4. Khan Academy, “Photoelectric effect”
5. Encyclopaedia Britannica, “Photoelectric effect”
6. Scientific American, “Einstein’s Legacy: The Photoelectric Effect”
7. Florida State University, “Concepts in Digital Imaging Technology: Photomultiplier Tubes”
8. Hamamatsu Photonics, Photomultiplier Tubes: Basics and Applications,
9. Hamamatsu Photonics, Data sheet Hamamatsu R3896
10. EE World, “What are RF waveguides? Part 1: context and principles”
11. EE World, “What are RF waveguides? Part 2: implementation and components”
12. Radar Tutorial/EU. “Traveling Wave Tube” (very good description and images)
13. Elsevier Science Direct, “Traveling Wave Tube”
14. Tutorials Point, “Travelling Wave Tube” (readable, somewhat simplified)
15. Microwaves 101, “Traveling Wave Tubes” (has close-in photos of custom made, non-commercial TWTs)
16. Engineering and Technology History Wiki, Rudolf Kompfner
17. Microwave Journal, Military Microwaves Supplement, “TWTAs Still Dominate High-Power and mmWave Applications”
18. dB control/Heico, “What’s Better – TWTAs or SSPAs?
19. Greek Microwave Group, “TWTs vs SSPAs”
20. Microwave Journal, “ABI Research Finds Microwave Tube Market Still Strong at Over $1B for 2018”
21. Semantic Scholar, “Communication satellite power amplifiers: current and future SSPA and TWTA technologies”