The cavity magnetron was used in RADAR during WW2 ~ see related link below .

strapping in cavity magnetrons is used to prevent mode jumping. the rising sun cavity structure may be used to ease strapping.

The working part of a microwave is a cavity magnetron. In essence it converts electricity into radiation of the microwave wavelength. The radiation then excites the atoms in the food and it becomes hot. The Cavity magnetron was also a vital invention in the history of radar, without it the Allies might have lost the second World War.

Microwaves are generated at the atomic level, based on the frequency of the EM radiation that it emits. This is conducted through radiation that is absorbed, reflected, attenuated, or refracted.

A klystron, magnetron or travelling-wave tube will generate microwaves. So will a vircator and an inductive output tube. These are just some of the vacuum tubes used to generate microwaves. In a microwave oven, the magnetron, which is a type of cavity resonator, is the device of choice.