A cavity magnetron is a type of vacuum tube that generates microwaves, which are short wavelength radio waves. Streams of electrons are manipulated by magnetic fields, causing an emission of microwaves. The first cavity magnetron was developed by two British physicists in 1940 and led to the development of portable radar units. Today, these devices are still used in radar equipment, and as the generator of the electromagnetic waves that every microwave oven uses to cook food.
A cathode is mounted in the center of a hollow cylinder-shaped anode. The central cavity, which is also cylinder shaped, is connected to further smaller cavities in the outer wall of the cylinder, and a large magnet is mounted at either end. The magnetic fields cause the electrons to spiral around the cathode as they travel to the anode rather than moving in a straight line. The interaction of the electron flow with the magnetic field causes an electric resonance field within the cavities in the outer cylinder wall, which emits microwaves. A cavity magnetron can be designed in such a way as to emit microwaves of a desired frequency.
The development of the cavity magnetron was one of the most important advancements in technology in the 20th century. Its implementation allowed for portable radar units to be mounted on ships and airplanes, as well as making ground-based radars much more efficient and effective. This improved radar technology is widely credited for giving the Allies a significant advantage over the Axis powers in this area during World War II.
In 2011, cavity magnetrons are still widely used for radar systems, although newer technologies using alternate transmitters are beginning to supplant them. This is due to the fact that the very nature of the oscillating electric fields that emit the microwaves make it difficult to tune them to precise narrow frequency bands, as well as to other considerations, such as human proximity to powerful electromagnetic radiation when these systems are mounted in airplanes and naval vessels. Existing radar systems are generally not replaced or upgraded, and for this reason, cavity magnetrons will continue to be in service in radar equipment for many years or even decades.
Microwave ovens use cavity magnetrons, although much smaller and less powerful than those used for radar equipment. In a microwave oven, the microwaves are directed into the cooking box, and are distributed by a diffuser, which helps the foods to cook more evenly. Most microwave ovens use a cavity magnetron that emits a maximum of 2500 watts or so in the case of large commercial models, while very powerful cavity magnetrons used for radar applications can produce millions of watts.