OPTICAL-PARAMETERS OF MEV ION MICROPROBES

Recent progress in design of high-energy microprobes for focusing ions has consisted of increased understanding of the aberrations of quadrupole lenses, and of methods for compensation of some of these aberrations. Electric rotation of the lenses, the effects of tilt in lenses with imperfect fields, and the excitation stability required in magnetic and achromatic lenses are described. Approximate analytic expressions are given for chromatic aberration in quadrupole doublets, as well as for the first-order, third-degree coefficients for achromatic quadrupole lenses. The size of second-order parasitic aberrations arising from imperfect symmetry is quantitatively estimated. The geometry of the hexapole fields responsible for these aberrations is discussed, and a method is presented for compensating them in quadrupole doublets. Experimental measurements of third-order coefficients are described.