Observations of electron emission sites on broad area electrodes by an electron emission microscope

To observe field electron emission sites which lead vacuum gaps to electrical breakdown an electron emission microscope EEM.has been developed. The microscope consists of three unipotential lenses, that is, objective, intermediate and projection lenses. A plane electrode of the objective lens facing the cathode has a 0.5 mm diameter pinhole which extracts field emitted electrons, and works as an anode. This plane anode form enables the observation of emission sites on a broad area cathode. By using this EEM changes of emission site aspects on pit-free electropolished Al cathodes before and after 500 breakdowns were observed. Images recorded showed that field emission occurs at several isolated sites rather than one site. Also revealed is that the number of emission sites after 500 breakdowns did not increase significantly. q1999 Elsevier Science B.V. All rights reserved.