Magnetic shielding of an accelerator beam using passive ferromagnetic material

A particle beam can be shielded from external magnetic fields by ferromagnetic shields, up to a certain field strength depending on the saturation characteristics of the used shielding material. The limits of passive shielding, in terms of external flux densities and local orientations of the field, are studied. The alternative solutions, i.e. the cancelation of the external field by either actively powered coils or by using a passive superconducting shield, are also discussed. The active and passive shielding can also be combined. As an example the proposed shielding of the HERA electron beam at the HERA-B experiment site at DESY is described.