GRID SHADOW PATTERN-ANALYSIS OF ACHROMATIC QUADRUPOLE LENSES

An achromatic triplet of quadrupole lenses has been designed as a probe forming lens system for a 2.55 MeV H+ beam. The parasitic aberrations of the three magnetic and three electrostatic lens components have been measured with the grid shadow method. The lenses were found to have up to 1.3% parasitic sextupole components and up to 3.4% parasitic octupole components, as well as some skew multipole components assumed to be similar in magnitude to the former components. The electrostatic lenses, constructed with a novel design, had parasitic components comparable in magnitude to the magnetic lenses which were constructed with a conventional design. In achromatic mode, the most significant parasitic component appears to be octupole, possibly resulting from a misalignment between the superimposed magnetic and electrostatic lenses. The achromatic quadrupole lenses were found to be an order of magnitude less sensitive to changes in the beam energy away from the operating energy compared to the equivalent magnetic or electrostatic quadrupole lenses.