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A simple dipole electrostatic mirror has been studied by Preikszas and Rose [Preikszas D. and Rose H.: “Correction properties of electron mirrors”. J. Electron Micr. 1 (1997), 1-9.]. They give for voltages 10 kV on the input electrode (the energy of the particles) and –2.5 kV on the mirror electrode the following parameters for 1:1 imaging, nonrelativistic computations and 5 mm radius of the mirror zo=-38.688 mm, Cs3=-3.08245 m, Cs5=84.815 km and Cc=93.73 m. They give also the chromatic aberrations of higher order. The
potential computation was done in a mesh of 32768 mesh points, having 194 points
in the vertical direction. In the right-hand half of the mirror constant mesh
step with 8 lines per mm was used. Trajectories in the mirror were computed with
TRASYS, the ZRP interpolation method was used. In the figure below we used 5
trajectories starting at z=-38.688 mm from 5 to 25 mrad. 
The function of mirror and the negative value of its spherical aberration are clear from the detail of the figure from the previous page showing the object/image plane of the mirror.
From the coordinates in the image plane we can deduce the spherical aberration coefficients of the mirror, with a good agreement with the values of Preikszas and Rose. |
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