Adjustable Magnetic Lens Having Permanent-Magnetic and Electromagnetic Components

A charged particle lens configured to modify a charged-particle beam of a charged particle optical system, the lens being provided with a passage space extending primarily along a longitudinal axis and allowing the passage of a charged-particle beam, said lens including a magnetic circuit assembly comprising:

• at least one permanent magnet;
• and
• a yoke body;
• said yoke body being composed of at least two yoke components, of which a first yoke component realizes an inner yoke shell, arranged surrounding the passage space, and a second yoke component realizes an outer yoke shell which is arranged surrounding the inner yoke shell, said yoke components being arranged circumferential around the longitudinal axis and comprise highly magnetic permeable material;
• said at least one permanent magnet being arranged between the at least two yoke components and circumferentially around the inner yoke shell, said at least one permanent magnet comprising a permanent magnetic material being magnetically oriented with its two magnetic poles towards respective yoke components;
• wherein in the magnetic circuit assembly, the at least one permanent magnet and the yoke body form a closed magnetic circuit but having at least two gaps formed between respective axial faces of different yoke components, configured to direct a magnetic flux density coming from said at least one permanent magnet through the yoke body and in said gaps induce a magnetic field, which is reaching inwards into the passage space,
• the charged particle lens including an electromagnetic adjustment coil located between the inner and outer yoke shell and configured to be driven by an adjustable supply current, wherein the electric current in the electromagnetic adjustment coil is substantially running along a circumferential direction around the longitudinal axis, the electromagnetic adjustment coil being configured to modify the magnetic flux in the magnetic circuit to cause a variation in the magnetic flux density in at least one of the gaps.