The Wien filter: History, fundamentals and modern applications

Many references on historic and modern papers dealing with the Wien filter are given. Basic optic properties, advantages and disadvantages as well as applications are summarized. Then uncommon systems consisting of Wien filters with different azimuthal orientations, i.e. orthogonal deflection sections, are considered. The corresponding paraxial ray equation and the eight fundamental trajectories of such systems are introduced and the computation of the aberrations up to the third rank is briefly covered. As an example an electron monochromator with four Wien filters is discussed in detail. Finally, a novel mass separator with two Wien filters for a low-voltage focussed ion beam column with a liquid metal ion source is treated.