Quiet operating regime of the microwave oven magnetron described as a negative feedback process

Summary form only given. The microwave oven magnetron under CW operating conditions persists in operating in a regime that is characterized by very low levels of both "close-in" and "far-out" noise and a cathode emission condition that is temperature limited. Outside of this regime the tube is very noisy and the cathode is at a high temperature, well removed from the temperature limited condition. Within the quiet regime, the internal electron flow exhibits a negative feedback process that adjusts the amount of backbombardment of the cathode to keep the emission from the cathode temperature limited over very wide variations of the operating parameters, including magnetic field, and coupling to the external load. The cathode increases in temperature only if more emission is required from it, following the Dushman expression for emission as a function of cathode temperature. The feedback nature of the behavior is evidenced by the tradeoff relationship between heating the cathode by backbombardment and from an external power source. If the anode current is kept constant, any increase in external heater power is offset by a reduction in backbombardment power to keep the cathode at the same temperature.