Classification of charge particle lenses by dimensionless parameters determining the focusing length

For three types of electrostatic lenses the focusing length is proportional, accordingly, to the second-, or first-, or half-power of parameter representing the relation of a charged particle accelerating potential to a lens potential. Accordingly, for magnetic lenses the focusing length is proportional to the second-, or first-, or half-power of parameter representing the relation of the particle kinetic momentum to the electromagnetic component of its canonical momentum.