Correlation of Cs flux and work function of a converter surface during long plasma exposure for negative ion sources in view of ITER

Negative hydrogen ion sources for NBI systems at fusion devices rely on the surface conversion of hydrogen atoms and positive ions to negative hydrogen ions. In these sources the surface work function is decreased by adsorption of caesium (work function of 2.1 eV), enhancing consequently the negative ion yield. However, the performance of the ion source decreases during plasma pulses up to one hour, suggesting a deterioration of the work function. Fundamental investigations are performed in a laboratory experiment in order to study the impact of the plasma on the work function of a freshly caesiated stainless steel surface. A work function of 2.1 eV is achieved in the first 10 s of plasma, while further plasma exposure leads to the removal of Cs from the surface and to the change of the work function: a value of around 1.8–1.9 eV is measured after 10–15 min of plasma exposure and then the work function increases, approaching the work function of the substrate (≥4.2 eV) after 5 h. The Cs removal must be counteracted by continuous Cs evaporation, and investigations performed varying the Cs flux towards the surface have shown that a Cs flux of at least 1.5 × 1016 m−2s−1 is required to maintain a work function of 2.1 eV during long plasma exposure at the laboratory experiment.