From AMS Glossary
A power amplifier tube used to amplify weak microwave energy (provided by a radio- frequency exciter) to a high power level for a radar transmitter.
A klystron is characterized by high power, large size, high stability, high gain, and high operating voltages. Electrons are formed into a beam that is velocity modulated by the input waveform to produce microwave energy. A klystron is sometimes referred to as a linear beam tube because the direction of the electric field that accelerates the electron beam coincides with the axis of the magnetic field, in contrast to a crossed-field tube such as a magnetron. Klystrons provide a coherent transmitted signal appropriate for Doppler radar and pulse-compression applications. They are used in many operational radars, for example, NEXRAD (Next Generation Weather Radar) and TDWR (Terminal Doppler Weather Radar).