Abstract
Dispenser cathodes removed from life-test diodes having test periods of 580 h to 52000 h and unused dispenser cathodes have been examined by several surface analytical techniques. Surface topography has been characterized by scanning electron microscopy, and surface chemistry of cathodes activated in situ has been studied using Auger electron spectroscopy and X-ray photoelectron spectroscopy. The aim of these studies was to reveal information regarding potential long-life emission degradation. It was found that significant amounts of tungsten are lost from the cathodes in life tests giving rise to various morphological features. Residual deposits of dispenser impregnant are found on the surface and within the surface pores. Microscopic surface spots, ≲ 0.1 μm diameter, having low secondary emission coefficients are found to develop with life-test age. Auger electron spectroscopy shows that freshly activated cathodes, at operating temperatures, demonstrate relatively stable surface chemistry so long as the probing electron beam parameters are held constant. However, local changes in chemical composition accompany changes in beam flux. During the in situ activation procedure, the initial surface barium oxide undergoes significant chemical transformation. Barium on the activated surface appears to exist in a stable metallic form. No major chemical changes in the activated surface were found during 250 h at operating temperatures, as observed by Auger electron spectroscopy or X-ray photoelectron spectroscopy.
Published Version
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