Plasma property investigations during negative ion beam extraction in a half-sized ITER NBI ion source at ELISE test facility

The RF negative hydrogen/deuterium ion source at the ELISE test facility is half the size of the ITER negative ion source for neutral beam injection (NBI). ELISE demonstrated 90 % of the ITER NBI requirement for the extracted negative ion current density in hydrogen for 600 s pulse for the first time, limited only by the available RF power. However, the electrons inevitably co-extracted with the negative ions increase in time and can limit the source performance. The majority of the extracted negative ions are surface produced by the conversion of atoms and positive ions on low-work function surfaces, achieved by cesium evaporation in the ion source. The temporal increase in the co-extracted electrons is associated with the cesium dynamics during long pulses close to the extraction region. This motivates a detailed investigation of the temporal behavior of the cesium and the effect on the plasma parameters and the co-extracted electrons at ELISE. For this purpose, a Langmuir probe, optical emission spectroscopy and tunable diode laser absorption are used to resolve plasma properties over the pulse length and correlate them with the co-extracted electron current density. It is shown that the cesium density decreases at the beginning of the pulse, which causes a change in the plasma potential structure in front of the extraction region, resulting in an increase of the co-extracted electrons. Stabilizing the co-extracted electrons can be achieved by varying the temperatures of the plasma grid and the bias plate which sustain a low value of the work function, resulting in a smaller slope of the co-extracted electrons’ temporal increase.