A magnetron is a radio-frequency oscillator
that produces a high power output at a high frequency. Magnetrons
are commonly used in microwave
ovens and radar
The magnetron is basically a diode with the plate (or anode) tied to ground potential and the cathode at a very high potential during the time it's operating.
An electron in free space normally would travel from the cathode to the plate. When a magnetic field is exerted on the electron, the electron moves at a right angle to both the magnetic field and the direction of the electron path.
The stronger the magnetic field is, the more curved the path taken by the electron becomes.
When an electron can leave the cathode, travel out to the plate (but does not touch the plate), then is curved back to the cathode, the magnetron has attained the critical strength required to curve an electron back to it's source. Magnetrons operate above the critical strength, and do not allow electrons to get near the plate. No plate current will be developed when the magnetron is operating above it's critical strength.
The cavity between the cathode and the plate acts as a resonant circuit. The plate provides the inductance and the space between the cathode and plate provides capacitance. Applying a negative voltage to the cathode forms a DC electric field, and as a result the magnetron is shock-excited into weak oscillations. The cathode also begins to emit electrons. The electrons in transit absorb energy from the DC electric field and deliver energy to the RF electric field. The action of the electrons in the magnetic field of the cavity provides a "kick" to the oscillations, and the oscillations are sustained. The action is similar to feedback in other types of oscillator circuits.
In actual use, magnetrons use several circular cavities surrounding the cathode. A cross-section of a magnetron would look similar to a single Honeycomb breakfast cereal piece, with the center denoting the cathode and the surrounding holes as cavities.
The RF energy is extracted from the magnetron using a probe in one of the cavities or by a tuned slot in a cavity. The tuned slot is the most widely used, especially in high-powered circuits.
The cavities of the magnetron are normally operated in a near-vacuum. Using a slot for extraction of RF energy requires a pressure seal that is invisible to RF energy, yet keeps the external atmosphere from entering the cavity. This seal is the weakest point of a magnetron, and limits the voltage breakdown and output power.
The voltages used with magnetrons, even those in common microwave ovens, are extremely high (in excess of 100,000 volts in some high-power radar systems). Never open the back of a microwave oven when it is plugged in. The microwave energy is balanced and tuned to keep the RF energy inside the unit. RF energy cooks from the inside out, and RF burns are extremely painful and difficult to treat.