An ultrahigh‐vacuum (UHV) differentially pumped low‐energy (50–3000 eV) ion beam system for theinsituirradiation of specimens in a UHV atom‐probe field‐ion microscope (FIM) was designed and constructed. The ion beam system consisted of a Finkelstein‐type ion source, an Einzel lens, and a magnetic mass analyzer. The ion source was connected to the analyzer chamber by small apertures which resulted in differential pumping between the ion source and the analyzer chamber; during a typicalinsituirradiation of a specimen in the atom‐probe FIM the total pressure was maintained at ≈10−7Torr. In the case of helium ion irradiation the optimum ion‐current density was ≈0.5 &mgr;A cm−2for 300‐eV He+ions at the atom‐probe FIM specimen. After the completion of a helium ion irradiation the pumpdown time from 5×10−7to ≈3×10−10Torr in the atom‐probe FIM chamber was 0.5 h.