Transmission Optics of Focused Ion Beams Used in Mass Spectrometry

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Abstract
Current density profiles and angular spreading of low intensity positive ion beams are measured with scanning slits in an evacuated beam analyzer. Experimental measurements are made of spherical aberration and transverse thermal velocity at a crossover focused by an ion gun. Spherical aberration is determined by an extrapolation technique which apparently has not been used before. Beam constants were evaluated for an ion gun used with V‐type, flat ribbon, and multiple filaments. Each type of surface ionization filament produces a beam of elliptical cross section. Expressions are given for the variations of current density in a plane normal to and moved along the beam axis in field‐free image space, and for the beam fraction transmitted through a rectangular aperture. Application of the results to estimate the degree of voltage and mass discriminations is briefly discussed.