English: Microwave oscillator using a klystron tube, in 1944. The klystron invented in 1937 by the Varian brothers, is a microwave amplifying tube in which a radio wave is amplified by absorbing kinetic energy from a beam of electrons. It was widely used in radar sets during WW2 and today is used in microwave radio links and particle accelerators. The electron beam is generated by an electron gun at the right end of the tube and is attracted to a high voltage "collector" electrode at the left end. In the center it passes through two cavity resonators. The first has a standing wave of radio waves in it. This velocity modulates (alternately accelerates and retards) the electrons, so they form into "bunches". In the second cavity the bunches of electrons induce a standing wave of radio waves. This is because the oscillating electric field in the cavity has the proper phase that when each bunch enters, the polarity of the electric potential opposes the electrons, decelerating them. Thus the kinetic energy of the electrons is transferred to the electric field, amplifying it. In the oscillator the two cavities are connected by a short length of coaxial cable (center), feeding back energy from the second cavity into the first. The positive feedback causes spontaneous microwave oscillations to be generated. The microwaves can be drawn off through a second coaxial cable and used. Behind the klystron is a centrifugal cooling fan.