We turn now to a quantitative and qualitative analysis of several conventional vacuum tubes and microwave tubes in common use. The conventional vacuum tubes, such as triodes, tetrodes, and pentodes, are still used as signal sources of low output power at low microwave frequencies. The most important microwave tubes at present are the linear-beam tubes (0 type) tabulated in Table 9-0-1. The paramount 0 - type tube is the two-cavity klystron, and it is followed by the reflex klystron. The helix traveling-wave tube (TWT), the coupled-cavity TWT, the forward-wave amplifier (FWA), and the backward-wave amplifier and oscillator (BWA and BWO) are also 0 -type tubes, but they have nonresonant periodic structures for electron interactions. The Twystron is a hybrid amplifier that uses combinations of klystron and TWT components. The switching tubes such as krytron, thyratron, and planar triode are very useful in laser modulation. Although it is impossible to discuss all such tubes in detail, the common operating principles of many will be described.