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Dependence of Au- Production upon the Target Work Function in a Plasma-Sputter-Type Negative Ion Source

A method to measure the work function of the target surface in a plasma-sputter-type negative ion source has been developed. The method can determine the work function by measuring the photoelectric current induced by two lasers (He-Ne, Ar+ laser). The dependence of Au- production upon the work function of the target surface in the ion source was studied using this method. The time variation of the target work function and Au- production rate were measured during the cesium coverage decrease due to the plasma ion sputtering. The observed minimum work function of a cesiated gold surface in an Ar plasma was 1.3 eV. At the same time, the negative ion production rate (Au- current/target current) took the maximum value. The negative ion production rate indicated the same dependence on the incident ion energy as that of the sputtering rate when the work function was constant.