By W. C. Wiley and I. H. McLaren. Review of Scientific Instruments 26(12), 1150–1157 (December 1955). DOI: doi.org/10.1063/1.1715212
Page 1150: ion source, evacuated drift tube and collector; later sections analyze initial-position/velocity broadening and two-field ion-gun focusing, then compare the analysis with recorded spectra.