Two-stage dodecapole aberration corrector for charged-particle beam

An aberration corrector for a charged-particle beam, comprising:

• two stages of magnetic dodecapole elements, each of the dodecapole elements having first through twelfth poles arranged in this numerical order;
• and
• exciting coils mounted at the poles, respectively, of each of the dodecapole elements,
• wherein the exciting coils of the (4n+1)th poles having a common polarity and the exciting coils of the (4n+4)th poles having a different common polarity are connected in a first series circuit (where n=0, 1, 2) with a first reversible power supply and produce magnetic fields which are identical in absolute value but alternately opposite in polarity, and
• wherein the exciting coils of the (4n+3)th poles having a common polarity and the exciting coils of the (4n+2)th poles having a different common polarity are connected in a second series circuit with a second reversible power supply and produce magnetic fields which are identical in absolute value but alternately opposite in polarity,
• whereby selection of the polarities of the first and second reversible power supplies enables multipole elements to produce magnetic fields of 3-fold symmetry in two orientations 60 degrees apart or multipole elements to produce magnetic fields of 6-fold symmetry in two orientations 30 degrees apart.