Aberration analysis of Wien filters and design of an electron energy-selective imaging system

Focusing-imaging, dispersive and aberration performances of Wien filters have been studied. The first order chromatic aberration and second order geometric aberration coefficients for Wien filters not satisfying the stigmatic focusing condition have been derived. An aberration correction method has been proposed using the symmetry of fields and fundamental paraxial rays to correct all the second order aberrations. A design of an energy-selective imaging system using the symmetry and consisting of four identical cascading Wien filters with an octupole electrical magnetic structure is described, and the imaging properties of the energy-selective imaging system has been computed.