Klystrons are used to generate and amplifymicrowaves. They are electron tubes with resonators. A klystron consists of an electrode system with cathode, which emits the electrons, and the anode, which receives the electron flow.
The electron flow of the klystron is accelerated by the anode voltage and its speed is modulated by the applied RF voltage. This input voltage is applied through a cavity resonator where it forms a standing wave. The electric field of the standing wave controls the speed of the electron current, which is decelerated and then accelerated again. The control effect is thus intensified.
In klystron oscillators, one uses feedback and couples part of the output signal back to the first resonator. Thus a free oscillation is achieved.