The Magnetron Tube Used In Microwave Ovens

Electrons, being negative charges, are strongly repelled by other negative charges. So this floating cloud of electrons would be repelled away from a negatively charged cathode. The distance and velocity of their travel would increase with the intensity of the applied negative charge. Momentum is thus provided by a negative 4000 volts DC, which is produced by means of the high-voltage transformer and the doubler action of the high-voltage diode and capacitor . (4000 volts is an average. The actual voltage varies with make and model.) A negative 4000 volt potential on the cathode puts a corresponding positive 4000 volt potential on the anode. Needless to say, the electrons blast off from the cathode like tiny rockets. They accelerate straight toward the positive anode, or, at least they try to.

As the electrons hasten toward their objective, they encounter the powerful magnetic field of two permanent magnets . These are positioned so that their magnetic fields are applied parallel to the cathode. The effect of the magnetic fields tends to deflect the speeding electrons away from the anode, as described in page one . The illustration to the right shows the combined effect of the electric and the magnetic fields on the electrons' trajectory. Instead of traveling straight to the anode, they curve to a path at almost right angles to their previous direction, resulting in an expanding circular orbit around the cathode, which eventually reaches the anode.

The whirling cloud of electrons, influenced by the high voltage and the strong magnetic field, form a rotating pattern that resembles the spokes in a spinning wheel, as shown in Figure 4 . The interaction of this rotating space-charge wheel with the configuration of the surface of the anode produces an alternating current flow in the resonant cavities of the anode. This is explained as follows. As a "spoke" of electrons approaches an anode vane (or the segment between the two cavities), it induces a positive charge in that segment. As the electrons pass, the positive charge diminishes in the first segment while another positive charge is being induced in the next segment. Current is induced because the physical structure of the anode forms the equivalent of a series of high-Q resonant inductive-capacitive (LC) circuits. The effect of the strapping of alternate segments is to connect the LC circuits in parallel.

Next: Resonant Circuits... (soon)