Evaluation of a Novel Design for an Electrostatic Quadrupole Triplet Ion Beam Lens

We describe the design and evaluation of an electrostatic quadrupole triplet lens constructed to focus ion beams of up to 200 keV in energy. The lens was built to be used in an apparatus for fundamental sputtering studies. These studies are motivated in part by a desire to understand the influence of low-energy physiochemical processes on surfaces and atmospheres exposed to the solar wind in the inner Solar System. The lens is very compact and incorporates a feature to induce octupole fields that can correct for spherical and other octupole-order aberrations. Two methods were used to evaluate the lens: observation of the focused beam spot on a specially fabricated target while systematically varying lens voltages, and the grid-shadow technique. The latter demonstrated that octupole-order aberrations were completely corrected in one direction when the lens quadrupoles were operated individually with appropriate octupole excitations. This research was made possible by a grant from the National Science Foundation through the Physics Research Experience for Undergraduates (REU) Program at the University of North Texas. Additionally, funding was provided by the Ronald E. McNair Post-baccalaureate Achievement Program at the University of North Texas.