Investigation of negative ion energy distribution and extraction mechanism with a compact retarding field energy analyser in a large filament-arc source for neutral beam injectors

In large beam sources for neutral beam injectors operating at high energies, negative hydrogen ions are extracted from a caesiated plasma discharge. In such sources H- ions are generated from volume and surface processes. The surface production is enhanced by Cs deposition on the source walls and in particular on the plasma electrode of the accelerator, made of molybdenum and kept at high temperature to maximize the caesium effect. 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 analyser was constructed and installed at the upstream surface of the plasma grid of a filament-arc source. It was used to characterize the negative ions in the plasma source with and without caesium. The first electrode of the analyser has a conical shape, resembling the plasma grid apertures from which the beam is extracted. The measured saturation current was analysed and correlated with the source parameters and the negative-ion beam current. The ion energy distribution depends on the origin of the negative ions, 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 analyser 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 minimise the dependence of the transmission on the perveance and extend the dynamic range of the diagnostic.