Energy distribution of post-accelerated electrons field-emitted from carbon fibres

An electron gun system with post-acceleration is described suitable to operation with a carbon fibre field-emission tip. The system was tested in an electron optical bench with a vacuum pressure of about 10⁻⁶ torr. The electron current was most stable if the current to the extraction electrode was minimized. The half width of the energy distribution of the electrons accelerated up to 30 keV was 215 meV for small currents (≈10⁻⁹A). The half width of the distribution increases with growing emission current. This increase may be attributed to instabilities in the surface structure due to ion bombardment and to the circumstance that more than one emission centre contributes to the electron current.