MEASUREMENT OF THE ABERRATIONS OF AN ELECTROSTATIC QUADRUPOLE PROBE FORMING LENS SYSTEM

A Russian antisymmetric quadruplet of precision electrostatic quadrupole lenses is used to focus a MeV proton probe for ion beam analysis of geological specimens. The aberrations of the individual lenses have been measured with the grid shadow method and the lenses have been found to be remarkably free from parasitic sextupole contamination, commonly seen in magnetic quadrupole lenses. The most significant parasitic multipole component in the individual quadrupole lenses is octupole, possibly contributed by an imbalance in the voltages of the pair of high voltage power supplies connected to the positive and negative poles. The experimental shadow patterns of the quadruplet reveal aberration similar in nature to spherical aberration, but of the opposite sign to theory. This is consistent with the aberration being dominated by parasitic octupole components in the individual quadrupole lenses. The grid shadow method also revealed that the present quadruplet, as constructed, was free from parasitic aberration due to rotational misalignments of the individual quadrupoles.