Effect of Extended Fringing Fields on Ion-Focusing Properties of Deflecting Magnets

This paper presents the result of calculations on ion deflection and focusing in magnets that have realistic, extended fringing fields rather than sharply cutoff fringing fields. The most important effects of the change from sharp cutoff to extended fringing fields are (a) displacements of the beam center line at entrance and exit (a ``zeroth-order'' effect), and (b) reduction of first-order z-direction (``vertical'') focusing. When the pole boundaries are curved or an effective curvature is caused by the proximity of pole-piece corners, there are further first-order effects on the trajectories. These effects can be minimized, and in practice made negligible, by correct centering of the beam relative to the pole-piece corners at entrance and exit.