The klystron is the device used, for example, in radars, in television satellites and in particle accelerators. Its function is to produce high-power electromagnetic waves at radio and microwave frequencies.

It is essentially a gun that produces a stream of electrons, accelerated by a high electric voltage. The electrons pass through a resonant cavity that modulates their velocity: some are accelerated, others slowed down, so that along the subsequent path the continuous stream turns into a stream of "bunches" of electrons (a magnetic field only serves to keep the beam focused). The bunches enter a second cavity where they excite radio-frequency or microwave standing waves. These waves are carried outside through a waveguide, which makes the signal available for transmission.