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The evolution of the electron emission of impregnated cathodes has been measured over many thousands of hours of operation at various temperatures in diodes and traveling-wave tubes. Certain physical methods were used to investigate the correlation between these results and the mechanisms of cathode operation: X-ray fluorescence electron microprobe analysis has given a new access to the study of the consumption of the active materials (barium and calcium) at various stages of cathode life. While barium depletes according to the t1/2 law, an approximate t1/3 law found for calcium could result from a two-step process in the barium and calcium chemistry. The formation and desorption of synthetized emissive layers on a nonimpregnated porous tungsten plug in a Becker-type test diode is one way to investigate the properties of emissive surfaces. The behavior of layers synthetized at low temperatures onto active impregnated cathodes suggests a surface equilibrium depending on the chemical nature of the substrate. Emission charts are proposed to compare, over a large range of temperatures, the different types of cathodes, in particular, the characteristics of uncoated and coated ones.
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Palluel et al. (1980) studied this question.
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