• A linear beam electron tube which utilizes velocity modulation to amplify microwaves, or to generate microwave oscillations. A two-cavity klystron, for instance, incorporates an electron gun, a buncher cavity, and a catcher cavity. In the buncher cavity, the electron beam is modulated by high frequency radio waves, bunching occurs in the drift space between the cavities, and the energy in the bunches is transferred to the catcher cavity, causing it to resonate. The output is taken from the catcher cavity. A reflex klystron has one cavity, and there are also klystrons with three or more cavities.