Effects of linear fringe fields on the focusing of a parallel electron beam in a symmetric magnetic spectrometer

Abstract

A uniform-field magnetic spectrometer, of pole boundaries designed to focus monoenergetic electrons emitted by a point source, without aberrations in a median plane is considered. Electron trajectories in the median plane of the spectrometer are calculated. Electron equations of motion are solved numerically on a computer using the Runge-Kutta method. The dependence of aberrations on the width and strength of the fringe fields and on the equivalent angular divergence of the electron beam is determined. The aberrations place a lower limit on the energy resolution of the spectrometer, which is in the range of 0.005-0.113 eV for the spectrometer geometries investigated at 100 keV and changes very little with beam energy. The structure of dynamic focus is obtained and compared with the previously investigated point source case.