A new concept positive (negative) surface ionization source equipped with a high porosity ionizer

A versatile, new concept, spherical-geometry, positive (negative) surface-ionization source has been designed, fabricated, and initial tests completed which can operate in either positive- or negative-ion beam generation modes without mechanical changes to the source. The highly permeable, composite Ir/C has an intrinsic work function of φ = 5.29 eV and can be used directly for the generation of positive-ion beams of highly electropositive elements. For negative-surface ionization, the work function is lowered by dynamic flow of a highly electropositive adsorbate such as Cs through the ionizer matrix. The results of initial testing indicate that the source is reliable, stable and easy to operate, with efficiencies for Cs⁺ conservatively estimated to exceed 60% and as high as ∼50% for F^- generation. However, the overall ionization efficiency for F^- formation and extraction is ∼1% due to the low efficiency for thermal dissociation of the carrier molecules (assumed to be CsF and AlF). The design features, operational principles, and initial performance of the source for generating Cs^- and F^-, when operated with Cs, are discussed in this article.