Analysis of the factors determining the extraction of negative ion beams from plasma sources

Abstract

Based on the modified Poisson–Vlasov method this study presents an analysis on the negative hydrogen-ion-beam extraction from plasma sources. Both the extraction cell and the values of the current densities of the extracted electron and ion beams considered are from experiments carried out regarding ITER. The role of different factors—potential of the electrodes, plasma parameters before the extraction, initial velocity distribution of the extracted particles, space-charge effects of the co-extracted electrons and details of the shape of the electrodes—determining the beam quality is analysed. It is shown that optimum mutual effects of external and space-charge fields leading to the formation of slightly concave meniscus acting together with the extraction electrode as a weak convexo-convex lens ensure formation of well-converged beams. The results outline (i) strong relation of the potentials of the electrodes to the plasma parameters before the extraction and (ii) the influence of the initial velocity distribution of the extracted particles as well as of the negative charge accumulation in the extraction region due to the electron beam.