Chapter Seven: The High-Voltage System (continued)

The process begins with a low voltage being applied to the filament which causes it to heat up (filament voltage is usually 3 to 4 VAC, depending on the make and model). Remember, in a magnetron, the filament is also the cathode. The temperature rise causes increased molecular activity within the cathode to the extent that it begins to "boil" off or emit electrons. Electrons leaving the surface of a heated filament wire might be compared to molecules that leave the surface of boiling water in the form of steam. The electrons, however, do not evaporate. They float just off the surface of the cathode, waiting for some momentum.

Electrons, being negative charges, are strongly repelled by like 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 diode and capacitor. (4000 VDC 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 leave the vicinity of the cathode with vigor, and accelerate straight toward the positive anode—But not for long.

As the electrons hasten toward their objective, they encounter the powerful magnetic field. The effect of the two permanent magnets, positioned so that their magnetic field is applied parallel to the cathode, tends to deflect the speeding electrons away from the anode as described earlier. Figure 7-4D (previous page) shows the effect of the magnetic field on the path of the electron. 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 that eventually reaches the anode.