Caesium influence on plasma parameters and source performance during conditioning of the prototype ITER neutral beam injector negative ion source

Systematic Langmuir probe measurements were performed during the caesium (Cs) conditioning of the IPP prototype negative hydrogen ion source. Conditioning consists of slowly injecting caesium into the source in order to increase the extracted negative ion current and minimize the co-extraced electrons. This process usually takes several days. Measurements were taken near the plasma grid where the majority of the extracted negative ions are created. A clear transition between a poorly conditioned (volume dominated H− production) and a well conditioned (surface dominated H− production) source was seen. By following the plasma parameter variations, an overall decrease of the plasma potential, the floating potential and also the electron saturation current was observed. However, the ion saturation current remained constant. Consequently, near the plasma grid where negative ions are generated and extracted, a transition occurred in the space charge balance (quasineutrality): the space charge is balanced by electrons when the source is poorly conditioned and by negative ions when the source is well conditioned. Caesium influence on plasma parameters and source performance during conditioning of the prototype ITER neutral beam injector negative ion source2