Thermionic Emission from Oxide Cathodes: Retarding and Accelerating Fields

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Abstract
An experimental investigation of the thermionic emission from oxide‐coated cathodes is presented in which the applied voltage range extends from the retarding potential region through zero field and up to an average accelerating field of 50,000 volts per cm. The retarding potential range yields information concerning the energy distribution of the electrons emitted, and indicates an excellent agreement between theory and experiment for high retarding fields. Near zero field, cathode inhomogeneity or patch effect may be responsible for the deviations found. Consideration is given to the influence of a possible reflection effect, but theoretical calculation indicates that with the large area cathode used, the simultaneous reflection at the emitter and the collector surfaces so alter the observed current voltage relationship, that the existence or non‐existence of a reflection effect cannot be determined by the experiment. Results shown in the accelerating field region up to 50,000 volts per cm are accounted for on the bases of two assumptions which are (1) patch effect for small accelerating fields and (2), the intensification of the field at sharp points expected in view of the inherent roughness of the cathodes studied. The deviation of the experimental results from those predicted by the Schottky mirror image theory is explained over the whole range of observation by these two assumptions.