An advanced boundary element method for the calculation of magnetic lenses

The magnetic held in unsaturated magnetic lenses can be calculated by the s uperposition of partial fields, generated by the coils and by surface curre nts, flowing on the boundary of the yoke. The latter are initially unknown and found by the solution of an integral equation which is derived from the electromagnetic boundary conditions. A new and highly improved numerical t echnique was developed for the solution of this Fredholm equation, as its e ssentially asymmetric structure was explicitly considered. Tests have shown that this procedure is considerably more accurate than the method presente d at the CPO-3 in Toulouse (Kasper and Stroer, Nucl. Instr. and Meth. A 298 (1990) 1, Optik 83 (1990) 93). This new technique is implemented as a part of the recently developed program-package EOS for the calculation of elect ron optical systems. (C) 2000 Elsevier Science B.V. All rights reserved.