Electron trajectory calculation of a stigmatic-focus Wien filter for electron energy loss spectroscopy II

Electron optical characteristics of a Wien filter were studied to construct a high resolution electron energy loss spectrometer. The spacing of the electrodes of the filter must be nearly equal to that of the magnet for minimizing the deviation from the Wien condition at the fringing region. However, as the spacing of the electrodes becomes large, a harmful second order inhomogeneity of the electric field appears. The effect of the inhomogeneity on the electron trajectory can be compensated by changing the shape of the magnet pole-faces. Based on these results, a filter which enables a stigmatic focus with a 1.6 μm dispersion for 1 eV energy difference and makes electron beams parallel to the optical axis at the exit of the filter was designed.