A Critical Assessment of the Finite Element Method for Calculating Magnetic Fields in Electron Optics

Abstract

The present investigation is concerned with the determination of the magnetic field distribution in magnetic electron lenses by means of the finite element method. A critical comparison is made of the
available finite element computer programs. Necessary modifications
have been made in certain programs, useful comments and general data format were written for two recently developed programs.

Special attention has been paid to those factors that affect the
efficiency of the finite element method in camputer aided design of
magnetic electron lenses. These include the correct choice of
boundary conditions. Another important factor investigated is the
influence of mesh layout for the subsequent finite element
calculations.

By recomputing several examples of lenses found in the literature,
discrepancies of up to 30% were reduced, resulting in excellent
agreement with the associated experimental results, simply by
rearranging the mesh layout in a more convenient way. Through a more systematic application of the finite element method, several
misleading conclusions to be found in the literature were detected.
Some of these conclusions arose from the finite element method itself,as previously mentioned. Other misleading conclusions were also
discovered that were not due to the finite element method but to design errors in the lens itself.

A study has been made of the errors in lens design including the
inadequate thickness of iron shrouds etc.

Date of Award	Nov 1985
Original language	English
Awarding Institution	Aston University