[en] Calculations of the electric potential and of Cs⁺-ion trajectories explain the axially enhanced erosion of the sputter cathode in the Wisconsin Source of Negative Ions by Cesium Sputtering (SNICS) and similar sources. The effect arises from purely electrostatic focusing of fast Cs⁺ ions that are produced on a hot refractory-metal surface. Calculations which include the effects of a 1-mA Cs⁺ beam suggest that space charge is of only minor importance in this type of ion source. Possible improvements in the efficiency of the ionizer that arise from modifications to the source geometry are also discussed