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This is an example based on demonstration lens data DEM2 (used in LENS manual), deflector data DEMZ (used in DEFL manual), that were used to illustrate the computation of optical properties in the manual of COMBI. It shows the computation of a magnetic lens with two-stage magnetic deflection system as used in a scanning electron microscope. The geometry of the lens and deflectors and the deflection fields are obvious from the results of MMD below.
The
field interpolation was the slice method. Obviously the 5 mrad is too high
starting angle, because the beam would get 0.5 mm from the axis. The
deflection of the central trajectory obtained in TRASYS is: X=-0.0411521
mm, Y=0.0380084 mm. In order to see the aberration figures, we need a larger bunch of particles. In the last figure below we see close to the Gaussian image plane the central undeflected beam (green curve, we see the effect of spherical aberration) and two beams deflected by 0.25 and 0.5 mm (red and blue curve, respectively). In order to compare the deflected beam with the central beam, paraxial deflection distance was subtracted. Initial conditions were selected so that the paraxial beam aperture of the inner beam is 5 mrad, the outer beam has 10 mrad in the image plane. The central trajectory allows us to determine the deflection distortion. The aberration is obviously dominated by deflection coma. The aberration coefficients are then in a good agreement with the results of COMBI computations.
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