Electromagnetic lens

An electromagnetic lens configured to modify a charged-particle beam of a charged-particle optical apparatus, the electromagnetic lens being provided with a passage opening extending along a longitudinal axis and allowing passage of the charged-particle beam, the electromagnetic lens comprising:

• a magnetic circuit assembly comprising at least one ring magnet and a yoke body;
• and
• a sleeve insert,
• said sleeve insert surrounding the passage opening and extending between a first end and a second end thereof along the longitudinal axis, wherein the sleeve insert comprises at least one electrically conductive electrode element, said at least one electrically conductive electrode element being configured to be applied a respective electric potential via power supplies so as to generate an electrostatic field within the passage opening,
• said yoke body comprising an outer yoke shell and an inner yoke shell, arranged circumferentially around the longitudinal axis and comprising a magnetically permeable material, wherein the inner yoke shell is arranged surrounding at least a central portion of the sleeve insert, and the outer yoke shell surrounds the inner yoke shell and the sleeve insert,
• said at least one ring magnet arranged circumferentially around the inner yoke shell and arranged between the inner and outer yoke shells, said at least one ring magnet comprising a permanent magnetic material being magnetically oriented with its two magnetic poles towards the inner yoke shell and the outer yoke shell, respectively,
• wherein in the magnetic circuit assembly, the inner yoke shell, the at least one ring magnet, and the outer yoke shell form a closed magnetic circuit but having at least two gaps located at respective axial ends of the inner yoke shell towards respectively corresponding portions of the outer yoke shell, wherein the magnetic circuit assembly is configured to generate a magnetic field reaching inwards into the passage opening and spatially overlapping with the electrostatic field generated by the at least one electrically conductive electrode element of the sleeve insert.