Axially symmetric composite electromagnetic mirror for perfect axial-aberration correction

Abstract

A new element of charged-particle optics is proposed and examined: an axially symmetric composite electromagnetic mirror employing a focusing magnetic field as well as an electrostatic one, the two fields overlapping. It is shown that the chromatic and spherical aberration coefficients of the mirror are opposite in sign to those found in conventional axially symmetric optics; their magnitude is twice the length of the paraxial particle trajectory as projected on the mirror axis of symmetry. As a result, the negative axial aberration coefficients can be varied over the wide range from a few millimeters to one meter with the mirror focal length and the particle energy being fixed. This property offers a way to fully compensate for the axial aberration of optimized ion or electron objective lenses in current use. A correction system incorporating the mirror is proposed. As compared with the sole correction arrangement used in electron optics, the system considered has about 1/5 times as many alignment degrees of freedom (mechanical and electrical) and should therefore display far greater stability; furthermore, it can work with ions as well as electrons.