Wien filter

One of the in-column type energy filters. The "Wien filter" uses magnetic and electric fields perpendicular to each other for getting energy dispersion, though the omega filter and the alfa filter use only magnetic fields. The Wien filter is used as a monochromator by incorporating it in the illumination system of a TEM. Since its energy dispersion for a high-voltage 200 kV electron beam is small, the electron beam is introduced into the filter before the beam is accelerated or after the beam is decelerated to several 100 V. In this case, its energy dispersion is ～10 μm/eV. There is a limitation on applying a high voltage to the Wien filter due to discharge between electrodes. Thus, the filter is used for an incident electron beam at a voltage of ～10 keV or less. The advantage of the Wien filter is that the electron trajectory in the filter is parallel to the optical axis or forms a straight line.

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