An advanced boundary element method for the calculation of magnetic lenses

The magnetic field in unsaturated magnetic lenses can be calculated by the superposition of partial fields, generated by the coils and by surface currents, 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 technique was developed for the solution of this Fredholm equation, as its essentially asymmetric structure was explicitly considered. Tests have shown that this procedure is considerably more accurate than the method presented at the CPO-3 in Toulouse (Kasper and Strِer, 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 electron optical systems.