AN ACHROMATIC PROTON MICROBEAM OF 1.06 MEV ENERGY AND 0.1 MU-M LINEWIDTH

Protons from a Van de Graaff accelerator were focused with an achromatic quadrupole doublet with lenses of 60 mm length and 1.04 mm electric bore. The procedure for compensating second-order parasitic aberrations using the electrodes of the lenses is summarized, and parameters of parasitic hexapoles are found. Third-order aberrations were accurately measured and are shown to be small at the apertures used. First-order parasitic aberration was trimmed to zero simultaneously in both principal sections by applying an electric rotation voltage to the upstream lens. A test was conducted using a lens aperture set small enough to avoid residual second-order effects. As the object slits were narrowed, the focused beam line width decreased in a linear fashion from 1.0 to 0.1-mu-m.