Patent Number: 
Section: claims

1. Lens system for a plurality of charged particle beams, comprising:a lens body with a first pole piece, a second pole piece and a plurality of lens openings for the respective charged particle beams;a common excitation coil arranged around the plurality of lens openings for providing a respective first magnetic flux to the lens openings; anda compensation coil arranged between the lens openings for providing a respective second magnetic flux to at least some of the lens openings so as to compensate for an asymmetry of the first magnetic flux, wherein the compensation coil is arranged such that axial extension of each of the lens openings is outside the compensation coil. 2. Lens system according to claim 1, wherein the compensation coil is arranged around a magnetic stub, the magnetic stub providing an essentially gapless connection between the first pole piece and the second pole piece. 3. Lens system according to claim 1, wherein the compensation coil is arranged such that all lens openings are outside of the compensation coil when viewed from an optical axis defined by the lens openings. 4. Lens system according to claim 1, wherein during operation the compensation coil carries a current in the opposite direction of a current carried by the excitation coil. 5. Lens system according to claim 1, wherein the lens openings are arranged in a two-dimensional arrangement. 6. Lens system according to claim 1, wherein the lens openings are arranged as an array having at least two rows and at least two columns. 7. Lens system according to claim 1, wherein the lens openings have, at least in one direction, a distance with respect to each other of less than 90 mm. 8. Lens system according to claim 1, wherein each one of the lens openings defines a respective optical axis and wherein during operation a lens field for each one of the openings has at least one plane of symmetry, the at least one plane of symmetry containing the respective optical axis. 9. Lens system according to claim 1, being symmetrical with respect to a plane of symmetry (S1, S2) containing a respective center of at least one of the lens openings. 10. Lens system according to claim 1, being symmetrical with respect to at least two planes of symmetry (S1, S2). 11. Lens system according to claim 1, wherein the plurality of lens openings is at least four lens openings. 12. Lens system according to any claim 1, further comprising, for each of the plurality of lens openings, an adjustment coil arranged around the respective lens opening. 13. Lens system according to claim 1, wherein at least one of the first pole piece and the second pole piece are provided as a single body of magnetic material. 14. Lens system according to claim 1, wherein in the region of the lens openings, a gap separates the first pole piece from the second pole piece. 15. Multiple charged particle beam device, comprising:a charged particle beam source for generating a plurality of charged particle beams; anda charged particle beam column comprising a lens system according to claim 1. 16. Method for operating a charged particle beam device, comprising:generating a plurality of charged particle beams;guiding each of the charged particle beams through a respective one of a plurality of lens openings of a lens body;generating a current, in a first direction, in a common excitation coil arranged around the plurality of lens openings, thereby providing a respective first magnetic flux to the lens openings; andgenerating a current, in a second direction opposite to the first direction, in a compensation coil arranged between the lens openings, thereby providing a respective second magnetic flux to at least some of the lens openings and compensating an asymmetry of the first magnetic flux, wherein the compensation coil is arranged such that axial extension of each of the lens openings is outside the compensation coil. 17. Method according to claim 16, wherein the lens openings are arranged as an array having at least two rows and at least two columns. 18. Method according to claim 16, wherein the lens openings have, at least in one direction, a distance with respect to each other of less than 90 mm. 19. Method according to claim 16, wherein the compensation coil is arranged such that all lens openings are outside of the compensation coil when viewed from an optical axis defined by the lens openings.