A selectable Coulomb aperture in charged particle system comprises a non-magnetic conductive plate with a plurality of holes therein. The plurality of holes has variant sizes or diameters to select different beam currents of primary beam in the charged particle system. The charged particle system may include a charged particle source for emitting a primary beam, a condenser lens for receiving the primary beam and condensing the primary beam, an objective lens for receiving the primary beam and focusing the primary beam on a surface of a specimen. The selectable Coulomb aperture is positioned between the charged particle source and the condenser lens.