This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.
 

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Figure 3605 shows a quadrupole design. The optical axis, along which the electrons travel, is into the page. indicates the direction of the electron beam. The blue lines represent the magnetic field lines, while the red line represents the Lorentz force on the electrons. Figure 3605 also shows the quadrupole provides a simple focusing effect in one azimuthal direction while provides simultaneous defocusing in the perpendicular direction. Therefore, the quadrupole can be used to focus the beam into a line (called line focus).

Figure 3605. The quadrupole design.

In PEELS systems, a series of quadrupoles  (normally four) are used to control both the dispersion and the width of the spectrum at the detector.