Simultaneous compensation of 2nd-order parasitic aberrations in both principal sections of an achromatic quadrupole lens doublet

Abstract

Experimental measurements are made of the aberrations of a doublet of achromatic quadrupole lenses. Second-order terms are altered and eliminated. Adjustment is guided by a theoretical model containing dipole and hexapole terms. Fixed hexapole terms of unknown magnitude arising from the inherent imperfect symmetry of lens construction cause the parasitic aberrations. Variable hexapole terms, caused by 2-pole electric excitations, are used to achieve compensation. The variable terms are adjusted to null the total hexapole field of each lens, leaving a small dipole field. Thus compensation of parasitic aberration is achieved at the expense of a small fixed image shift, which may be ignored or compensated by a beam-sweeping system. When a doublet with bore of 1 mm and working distance 10 cm is used to focus 1.2 MeV protons, aberrations less than 0.5 μm over an aperture of 0.4 × 0.5 mm are obtained. Alternative compensation methods require us of 5 separated lenses, 2 quadrupole and 3 hexapole. The field in each such hexapole lens is adjustable in magnitude and azimuth, requiring 6 parameters to be adjusted. A single such lens does not produce exact compensation. A method of compensation using the quadrupole lens structure is superior because construction and mechanical alignment of the 3 separate lenses is not required, and because only 4 parameters must be adjusted.