Microwave electron tubes - Traveling Wave Tubes

Whether in electron tubes or particle gas pedals, there are two main interaction processes between an electron beam and an electromagnetic wave propagating along a linear structure, enabling an exchange of energy: the beam interacts either with a standing wave, as in a klystron, or with a traveling wave. The difference between these two processes lies in the condition of synchronism between the beam and the wave that enables energy exchange. As their name suggests, TWTs (Traveling-Wave Tubes) belong to the second category, and for a reason that will become apparent later in this article, these devices are referred to as slow-wave tubes. TWTs differ from klystrons in that their bandwidth is greater (from 10% to over two octaves), and they are well suited to amplifying radar or communication signals above 1GHz. The operating principles are explained, first in outline, then with the help of Pierce's theory: the operation of a TWT is not intuitive and, in the author's opinion, the exposition of this theory is necessary. The current and achievable performances of helix tubes are deduced, and the main fields of use are introduced, with particular emphasis on space telecommunications; linearity performance, single- or multi-carrier, with or without linearizer, will be considered. In a second section, metal line tubes are described. These tubes can achieve high power levels (up to a few hundred kilowatts) in the low band (below 10 GHz), and are available up to and even beyond 100 GHz. We'll take a look at the state of research in the millimeter bands, which has been very active since around 2005.

At the end of the article, readers will find a glossary of important terms used in the article, as well as a table of acronyms and a table of symbols used.