Electrostatic lenses with very small spherical aberration.

A class of electrostatic lenses with extremely low spherical aberration is proposed. The axial potential distribution of these lenses has an asymmetric bell-shaped form with a lower potential on the image side. A lens with Cso∞/fo=0.51 is presented as an example. The maximum required electrode voltage is only five times larger than the initial accelerating voltage. At 5 mrad acceptance half-angle, 51 mm focal length, and magnification M=-3.3 the radius of the spherical aberration disk is 6 nm. [ABSTRACT FROM AUTHOR]