Investigation of Negative Ion Energy Distribution and Extraction Mechanism With a Compact Retarding Field Energy Analyzer in a Large Filament-Arc Source for Neutral Beam Injectors

In large beam sources for neutral beam injectors (NBIs) operating at high energies, negative hydrogen ions (NI) are extracted from a caesiated plasma discharge. H− ions are generated from volume and surface processes; surface production is enhanced by cesium deposition on the source walls and in particular on the plasma-facing electrode of the accelerator. The mechanisms of negative ion production and of extraction in the magnetic field present at the extraction apertures influence the formation of the single beamlets and their optics. A newly developed compact retarding field energy analyzer (CRFEA) was constructed and installed at the upstream surface of the plasma grid (PG) of a filament-arc source, the research negative ion source (RNIS) at the National Institute of Fusion Science. It was used to characterize the negative ions in the plasma source with and without cesium. The first electrode of the analyzer has a conical shape, resembling the PG apertures from which the beam is extracted. The measured saturation current was analyzed and correlated with the source parameters and the current of the negative-ion beam. The energy distribution of collected ions depends on the origin of the negative ions, and on possible energy exchange processes; but it is found to be also strongly influenced by the curvature of the meniscus (the plasma edge where the beam is formed). This compact retarding field analyzer proved to be effective for the study of the formation of a negative ion beam. The limits of the present design, and possible improvements are discussed, in particular to minimize the dependence of the transmission on perveance and extend the dynamic range of the diagnostic.