An Inexpensive Energy Filter Lens for Large Diameter Electron Beams

The theory behind the use of a rotationally symmetric hyperbolic electrostatic field as an energy filter lens for electrons is discussed. The design of such a lens for use in eliminating inelastically scattered electrons from the output beam in a low energy electron scattering experiment is described. A filter lens designed, built, and tested by the authors is described. It has an entrance aperture of 1.27 cm diameter, an angular acceptance of ±2°, and a measured energy cutoff width of (0.27±0.02)%.