An ion source is configured for electron ionization and produces coaxial electron and ion beams. The ion source includes an ionization chamber along an axis, a magnet assembly configured for generating an axial magnetic field in the ionization chamber, an electron source, and a lens assembly configured for directing the ion beam out from the ionization chamber along the axis, reflecting the electron beam back toward the electron source, and transmitting higher energy ions out from the ion source while reflecting lower energy ions toward a lens element for neutralization.