Recent developments at IPP on evaporation and control of caesium in negative ion sources

The neutral beam injection system for ITER requires an ion source based on negative hydrogen and deuterium ions. The production of negative ions relies mainly on the conversion on a cesiated surface with a low work function. The influence of Cs on the work function and the optimal layer thickness is discussed. A list of Cs properties relevant for the ion source is compiled. The Cs oven used at IPP Garching is introduced and its modification with a valve is described, test results are presented. The standard of knowledge at IPP of the Cs dynamics in the plasma source is discussed. With respect to the surface bonding of Cs two methods to control the Cs are presented: the temperature-controlled source chamber and the heated plasma grid. The success of coating the inner source walls with molybdenum is mentioned. Further Cs diagnostics applied and explored at IPP are presented.