A boundary element analysis of magnetic shielding for electron microscopes

This paper describes a boundary element analysis of magnetic shieldings for electron microscopes. Since the thickness of the shielding layer is considerably small compared with its overall size, numerical analysis of electromagnetic fields inside the layer leads to an ill‐conditioned matrix. This problem can be overcome by analytical evaluation of the interior electromagnetic field, which yields the impedance boundary condition (IBC) valid for static and eddy current fields, which expresses the relationship between the electromagnetic fields on both surfaces of the layer. In this paper the magnetic fields around a shielding layer are analyzed by the boundary element method under the IBC on the shielding layer. Two‐dimensional and axisymmetric magnetic fields are analyzed to evaluate the shielding efficiency of shielding immersed in an ac magnetic field. It is shown that magnetic disturbances can be reduced to less than one‐hundredth inside a shielding consisting of double shielding layers.