Multi pole

The multi pole is an electron optical component for creating a symmetric magnetic field. It is used to correct the aberration of the objective lens.
The multi pole is manufactured in such a way that, soft magnetic metal rods are coiled with copper wire and are placed radially and symmetrically with respect to the optical axis of the electron beam (Fig.(a)). According to a desired symmetry of magnetic field, various multi poles are designed. That is, a quadra-pole (Fig.(b)), a hexa-pole (Fig.(c)), an octa-pole (Fig.(d)) and a dodeca-pole (Fig.(e)) are used for TEM. The strength of the magnetic field on the optical axis is varied by adjusting the current applied to the coils.
In the spherical aberration (Cs) corrector for the objective lens, a hexa-pole (Fig.(c)) or a dodeca-pole (Fig.(e)) is used to generate a three-fold symmetric magnetic field. There is also a Cs corrector composed of a quadra-pole (Fig.(b)) and an octa-pole (Fig.(d)).
It is noted that, in the chromatic aberration (Cc) corrector for the objective lens, a multi pole where an electric field is superposed on a magnetic field is used.

• Schematic of magnetic pole, coil, and magnetic field [B] in Quadra-pole
• Quadra-pole
• Hexa-pole
• Octa-pole
• Dodeca-pole

Related Term(s)