Abstract
In order to study behaviour of ion beams precisely in a electromagnetic sector type mass analyzer, characteristics of the 3 rd order aberration of the aperture angle are discussed from the view-point of geometrical optics. It is proved that the position of the minimum image is affected mainly by the 3 rd order aberration, while the1st and the 2 nd have affects on the angle of tilt of the beam trajectory and the width of the minimum image, respectively. The effect of the 2 nd order aberration is compensated by that of the 3 rd and the resolving power may increase considerably, if the central beam enters into the uniform field with a small angle to the normal of the field boundary. The critical small angle should be determined from the coefficient of the 3 rd order aberration. This effect was verified by some experimental results.
