COMPUTATION OF OPEN ELECTRON LENSES BY COUPLED FINITE ELEMENT AND BOUNDARY INTEGRAL METHODS.

For the computation of open rotationally symmetric electron lenses coupled finite element and boundary integral methods can be used, in which we do not have to specify the potential on an artificially chosen boundary. Inside a rectangular boundary enclosing all coils and the magnetic circuit is the energy functional evaluated in terms of vector potential at vertices of a topologically regular mesh by the finite element method supposing the potential to be a linear function in each finite element. In the external infinite region an integral equation using free space Green's function is formulated.