Investigation of gaseous discharge ion sources for isotope separation on-line

Abstract

Gaseous discharge ion sources with circular end extraction have been investigated with the aim of meeting the special requirements of on-line isotope separation. The studies represent an attempt to increase lifetime, temperature and ionization efficiency of the ion source under stable discharge conditions and to reduce the ion current density of usually 1–100 mA/cm 2 of low intensity separator sources. The detailed investigation of a modified Nielsen Ion Source is described. The source yields in oscillating electron are operation reproducible ionization efficiencies of 32% for krypton and 45% for xenon. It was found that minor variations of geometry and potentials in a Nielsen-type ion source eliminated the discontinuity of the ionization efficiency at the arc threshold pressure. Ionization efficiencies up to 50% for xenon were obtained at ion source operation pressures down to 10 −5 torr, that is almost two orders of magnitude below the arc threshold pressure. The efficiency was practically independent of pressure variations between 2 × 10 −5 to 2 × 10 −4 torr. The resulting ion current density of typically 0.1 mA/cm 2 led to an improved line shape of the mass spectra, corresponding to a considerable reduction of the cross-contamination. Moreover, the line shape was found to be widely independent of ion source parameter variations.