SEM STUDY OF STAINLESS STEEL ELECTRODES BEFORE AND AFTER ELECTRICAL BREAKDOWN IN ULTRA-HIGH VACUUM.

Two stainless steel needle cathodes prepared in different ways were examined in a scanning electron microscope before and after electrical breakdown in ultra-high vacuum. On one very carefully prepared cathode no protrusions could be found, which could explain field enhancement factors of the order of 10 to 100, as normally occur. On the other cathode a few non-metallic protrusions were present. After a few breakdowns four types of crater were found on the cathodes, three of which were situated at places where prior to breakdown, protrusions, inclusions or scratches and holes had been detected. After many breakdowns the rims of craters can act as field enhancing projections and the erosion occurred by preference at these places. 17 refs.