1. Background and classification

Over the years, electron tube technologies have penetrated many areas of industry and science. They use a beam of high-energy electrons, accelerated by a high voltage (from one to several hundred kilovolts), which pass through a vacuum envelope. This description encompasses devices that are not covered here: X-ray sources ("Coolidge tubes"), brightness amplifiers (or "image intensifiers": IIL and IIR), photomultipliers (PM), electron sources, and of course cathode ray tubes (CRT: Cathode Ray Tube) and particle gas pedals, both large ("light sources" like SOLEIL) and small (radiotherapy equipment, for example). The ones we're interested in here are electron tubes where, through a variety of processes, part of the beam's energy is converted into a radiofrequency (RF) electromagnetic wave. In the remainder of this article, the term "electron tube" will be reserved for this family of components,...