A ribbon-shaped ion beam is modified using multiple coil structures on a pair of opposed ferromagnetic bars. The coil structures comprise continuous windings which have predetermined variations along the length of the bar of turns per unit length. In an example, one coil structure may have uniform turns per unit length along the bar, so that energizing the coil structures forms a magnetic field component extending across the gap between the bars with a quadrupole intensity distribution. A second coil structure may have turns per unit length varying to produce a hexapole magnetic field intensity distribution. Further coil structures may be provided to produce octopole and decapole magnetic field distributions. The coil structures may be energized to produce magnetic fields parallel to the bars which vary along the length of the bars, to twist or flatten the ribbon-shaped beam.