From the 1950s onwards, extensive effort has been devoted to the development of field emission sources for use in electron guns. [e.g., DD53] Methods have been developed for generating on-axis beams, either by field-induced emitter build-up, or by selective deposition of a low-work-function adsorbate (usually Zirconium oxide – ZrO) into the flat apex of a (100) oriented Tungsten emitter.