Properties of the axial flux density distribution of magnetic electron lenses with a single cylindrical pole-piece

Abstract

The axial flux density distribution of single pole-piece magnetic electron lenses differ from those of conventional design. An experimental investigation has been carried out concerning the effect of the bore size on the shape of the field distribution of single pole-piece lenses. The size of the axial bore in the cylindrical pole-piece of an experimental lens was varied over a wide range and the axial flux density distribution was measured for each bore size. The characteristic features of the field distribution such as its peak value, its penetration inside the lens structure and its mathematical representation by an analytical function were studied in detail. The effect of the bore size on the magnetic asymmetry was also considered. Empirical equations concerning the properties of the field distribution are put forward. The results should be useful in the optimisation of lens design and in field distribution modelling.