Magnetron Cathodes

The multicavity magnetron imposes exceedingly severe requirements upon its cathode. Spectacularly high electron emissions have been observed under pulsed conditions. Cathode parameters are obtained by interpreting, in terms of the emission equations, data secured using rectangular microsecond pulses. Emission densities exceeding 50 amperes per square centimeter, limited only by sparking, have been attained, and several forms of current-versus-voltage characteristic may be identified. Sparking may be precipitated by phenomena originating either in the cathode or anode. Two competing mechanisms for cathode-initiated sparking have been established experimentally. The resistance of the oxide coating results in a pulse-temperature rise during the flow of high currents. Resistance values of 1 to 100 ohm-centimeters squared, deduced from measurements of this effect, agree with those obtained in experiments performed with cathodes having probes embedded within the coating. The secondary emission of oxide-coated cathodes varies with temperature in an exponential manner from approximately 4 to 7 at room temperature to 100 at 850 degrees centigrade for a cathode activated to the optimum. Magnetrons have been operated solely by secondary emission as a consequence of the back-bombardment of the cathode. Measurements have revealed that 3 to 20 per cent of the output power may be dissipated in this manner. Correlations have been established between magnetron performance and cathode emission. The high-current-mode boundary depends upon the available thermionic emission. Undesired emission from certain regions of the cathode may introduce losses.