Achromatic quadrupole focusing systems for use with liquid metal ion sources

Abstract

We discuss the use of achromatic quadrupole lenses to overcome the limitation due to the energy spread of ions from liquid metal ion sources as they are currently focused with axially symmetric electrostatic lenses. Achromatic quadrupoles require both magnetic and electrostatic focusing elements. Each lens acts to focus in one plane and defocus in the other so that multiplet focusing systems are required to form an image of the source. Lens acceptance angles are limited by the bore of the electrostatic elements which in turn is limited by the necessary magnetic gradients required of the magnetic elements. The coefficients relating image distance to lens excitation of some of the elements of the quadrupole multiplets are found to be much larger than those in an axially symmetric electrostatic system. These factors are discussed in terms of the stability requirements of lens power supplies for a particular achromatic triplet system which is undergoing initial test.