Varian reflex klystron: Electrons emitted from the heated cathode travel through the cavity grids toward the repeller plate and returned back through the grids. Self-sustaining oscillations develop, the frequency can be changed by adjusting the repeller voltage or changing the physical dimensions by squeezing the plates together http://www.cybermike.net/reference/liec_book/Semi/SEMI_13.html

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Magnetrons Magnetrons are devices capable of creating high energy electromagnetic microwave radiation. A. W. Hull first investigated their behavior in 1921. They are composed of a hollow tube with a filament in the middle. The outer part of the tube acts as an anode while the filament is the cathode. When this filament is heated it releases electrons, which try to travel to the positive anodes on the outer tube. However, an external magnetic field passing through the tube keeps the electrons from reaching the anodes and forces them to move in circles around the cavities in the tube. This creates radiation at the resonance frequency of the tube.

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