Unipotential lens with electron-transparent electrodes

This paper reports results of the first test of a unipotential electron lens whose end electrodes are electron-transparent foils. This lens is unusual among electrostatic unipotential lenses in that it can have negative values for both its focal length and third-order spherical aberration coefficient. Preliminary measurements of these quantities have been made on a foil lens and are compared with theoretical values. Some of the practical limitations in using foils as lens elements are considered, and a potential use for this lens as part of a spherical aberration corrected doublet is discussed.