COMPRESSION OF FIELD-EMISSION ANGULAR-DISTRIBUTION USING A CATHODE SHIELD

A conventional field-emission electron gun consists of a pointed catho de and an extraction electrode at a positive potential which produces a strong electric field at the cathode surface. Electrons are field-em itted from the cathode, pass through an axial hole in the extractor el ectrode: and must then be accelerated to full energy. The field emissi on current density at the cathode surface is very sensitive to changes in the electric field strength. The distribution of electric field on the end of the cathode can be changed by an additional nearby electro de if its size is not large compared to the cathode radius. A cylindri cal shield electrode has been proposed which surrounds the cathode, an d for convenience can be at the same potential as the cathode. This co nfiguration has been examined theoretically using second-order finite element analysis. The results of this analysis show that the change in distribution of the electric field on the cathode will depend upon th e relative dimensions of the cathode and the shield, and, for a signif icant change in emission distribution, the shield radius must not be l arger than approximately five times the cathode radius. The addition o f the shield leaves the virtual source size unchanged, so the brightne ss of the source is approximately proportional to the axial current de nsity. It has been suggested that combined cathodes and shields can be fabricated by ion milling to form a pointed cathode in the end of a s hield. (C) 1995 American Vacuum Society.