Magnetron

Thursday, June 2, 2011

The magnetron is a high-powered vacuum tube which generates microwaves using the interaction of a stream of electrons with a magnetic field. A cavity magnetron is composed of a hot cathode with a high continuous or pulsed negative potential by a high-voltage, direct-current power supply. The cathode is built into the center of an evacuated, lobed, circular chamber. A magnetic field parallel to the filament is imposed by a permanent magnet. The magnetic field causes the electrons, attracted to the relatively positive outer part of the chamber, to spiral outward in a circular path rather than moving directly to this anode. The electromagnetic energy created from a magnetron can travel at the speed of light and is the same type of energy used in radio and television broadcasting.

There are cylindrical cavities around the ring of the magnetron. These cavities are open along their length and connect the common cavity space. As electrons sweep past these openings, they induce a resonant, high-frequency radio field in the cavity, which in turn causes the electrons to bunch into groups. A portion of this field is extracted with a short antenna that is connected to a waveguide (a metal tube usually of rectangular cross section). The waveguide directs the extracted RF energy to the load, which may be a cooking chamber in a microwave oven or a high-gain antenna in the case of radar. The magnetron is a self-oscillating device requiring no external elements other than a power supply.

In radar devices, the magnetron is operated with very short pulses of applied voltage, resulting in a short pulse of high power microwave energy being radiated. As in all radar systems, the radiation reflected off a target is analyzed to produce a radar map on a screen.