Use of auger spectroscopy in the evaluation of thermionic cathodes

The ability of Auger spectroscopy to identify chemical species existing on a surface has been used to evaluate the properties of "good" and "poor" impregnated tungsten cathodes used in high-power microwave wave tubes. The surfaces of "good" and "poor" cathodes were studied by means of Auger spectroscopy and work function measurements and the results interpreted to analyze failure modes in cathodes removed from TWT's because of poor emission characteristics. Most of the poor cathodes evaluated in this program were obtained from fabricated electron guns that had been employed and discarded from the 200-W TWT tubes developed for the Communication Technology Satellite (CTS) program. The results of these measurements have shown there are at least two types of failure modes that one observes with poor cathodes. They are 1) chemical contamination of the cathode surface and 2) low partial layer barium coverage of the cathode surface.