Simulation of electron trajectories of Wien filter for high-resolution EELS installed in TEM

Electric and magnetic field distributions followed by electron trajectories of Wien filters with eight poles were calculated numerically. Both the electric deflection and magnetic deflection fields had exactly the same distribution even at the fringing field region. This confirms the achievement of a perfectly straight optical axis of the Wien filter (the Wien condition). The stigmator component of electric field distribution is different from the deflection field distribution at the fringing region. This difference causes residual astigmatism. However, the astigmatism can easily be corrected by a slight increase of the quadrupole field strength. It is possible to reduce the number of power supplies by connecting a few electrodes at the same voltage. This connection causes the appearance of third-order aberrations due to the octupole electric field component. Further reduction of the power supplies is possible by connecting the vertical poles to the cramp electrically. In this case, however, an additional accelerating potential appears, and this potential causes a slight deviation from the Wien condition. The reduction of power supplies does not give so serious an effect on the energy resolution because the beam is distorted but its size is not enlarged excessively.