The Magnetron Internal Feedback Mechanism A surprising and important finding made while investigating the suitability of the magnetron for SPS application was the internal feedback mechanism that automatically sets the temperature of the cathode for quiet tube operation. This temperature turns out to be the lowest temperature at which the thermionically emitting cathode can supply the needed anode current. But long cathode life is dependent upon low temperature operation, which is held to a minimum by the internal feedback mechanism described elsewhere [8]. The internal control of the cathode temperature works best when the external source of heating, the filament, is turned off and the control loop regulates the amount of electron back-bombardment to keep the filament at the desired temperature. This automatic control works well over a wide range of the operating parameters of the tube, including anode current, magnetic field, and coupling of internal microwave circuit to the external microwave circuit. The cathode temperature is sensitive only to anode current. The high signal-to-noise level is also obtained over a very wide range of parameters as shown in Fig. 9. In a later noise investigation it was found that the noise in a
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