A multipole magnetic lens for manipulating a beam of charged particles traveling along an optical axis, the lens comprising:
- Magnetic field generating means, for generating a magnetic field;
- A magnetic yoke, comprising a plurality of radial yoke portions disposed about said optical axis so as to guide and concentrate said magnetic field toward an interaction region proximal to the optical axis,
- Said yoke has a composite structure, comprising an outer primary portion and an inner secondary portion;
- Said secondary portion is mounted as a monolithic insert within said primary portion so as to be disposed around said interaction region;
- Said secondary portion comprises a plurality of yoke stubs that are arranged around said optical axis and that are rigidly interconnected by a connecting structure such that:
▪ Said yoke stubs align up and match with said radial yoke portions;
▪ Said connecting structure acts as a magnetic constriction, configured such that said magnetic field undergoes saturation therein, thereby causing magnetic flux to exit the yoke stubs and form a multipole field in said interaction region.