magnetron

A microwave tube in which electrons generated from a heated cathode are affected by magnetic and electric fields in such a way as to produce microwave radiation used in radar and in microwave ovens.

[MAGNE(T) + –TRON.]

The oldest of a family of crossed-field microwave electron tubes wherein electrons, generated from a heated cathode, move under the combined force of a radial electric field and an axial magnetic field. By its structure a magnetron causes moving electrons to interact synchronously with traveling-wave components of a microwave standing-wave pattern in such a manner that electron potential energy is converted to microwave energy with high efficiency. Magnetrons have been used since the 1940s as pulsed microwave radiation sources for radar tracking. Because of their compactness and the high efficiency with which they can emit short bursts of megawatt peak output power, they have proved excellent for installation in aircraft as well as in ground radar stations. In continuous operation, a magnetron can produce a kilowatt of microwave power which is appropriate for rapid microwave cooking. See also Electron tube.

The magnetron is a device of essentially cylindrical symmetry. On the central axis is a hollow cylindrical cathode. The outer surface of the cathode carries electronemitting materials, primarily barium and strontium oxides in a nickel matrix. Such a matrix is capable of emitting electrons when current flows through the heater inside the cathode cylinder.

At a radius somewhat larger than the outer radius of the cathode is a concentric cylindrical anode. The anode serves two functions: (1) to collect electrons emitted by the cathode and (2) to store and guide microwave energy. The anode consists of a series of quarter-wavelength cavity resonators symmetrically arranged around the cathode.

A radial dc electric field (perpendicular to the cathode) is applied between cathode and anode. This electric field and the axial magnetic field (parallel and coaxial with the cathode) introduced by pole pieces at either end of the cathode provide the required crossed-field configuration.

A type of vacuum tube used as the frequency source in microwave ovens, radar systems and other high-power microwave circuits. In radar, magnetrons can be used as the signal source to feed the power stage of the radar transmitter, typically a klystron tube.

A magnetron is a type of diode vacuum tube in which the filament also serves as the cathode. Rather than being used as a diode, the physical structure of the elements causes the magnetron to act as an oscillator in the UHF or microwave frequency ranges. This is accomplished through the placement of permanent magnets within a series of resonating chambers that are tuned to the center frequency of the oscillation. These resonating chambers comprise the anode of the magnetron.

In operation, as the electrons are released from the cathode, they are directed toward the anode. However, since the electrons are influenced by the magnets in the resonant cavities, they flow circularly through the resonant chambers and set up an oscillation based upon their speed through the chambers. One or more chambers contain collecting antennas that let the generated RF signal flow outside the tube. See klystron and diode.

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The noun has one meaning:

Meaning #1: a diode vacuum tube in which the flow of electrons from a central cathode to a cylindrical anode is controlled by crossed magnetic and electric fields; used mainly in microwave oscillators