Model scandate cathodes investigated by thermionic-emission microscopy

Model thermionic cathodes with different oxide coatings have been surveyed using a combination of photoelectron emission microscopy and thermionic electron emission microscopy. The model cathodes are imaged directly at operating temperatures and the electron yield is measured with a Faraday cup located in the image plane. Cathodes with combinations of Ba, Sc, BaO, Sc2O3, and pulsed-laser-deposited Sc2O3 on W substrates have been examined. BaO deposited on Sc2O3 is the most stable cathode combination. Measurement of the W-Sc2O3-BaO cathode shows that the electron yield enhancement produced by BaO and Sc2O3 is a two-step process which requires that the BaO be on top of the Sc2O3. BaO does not need to be in direct contact with the W substrate. (C) 2011 American Vacuum Society. [DOI: 10.1116/1.3589442]