Spatial variation of negative ion density in a volume H- ion source

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Abstract
The density distribution of negative hydrogen ions along the axis of a hybrid multicusp H- ion source was measured, at different pressures, by using the photodetachement technique. It was found that in the region close to the plasma electrode (first electrode of the extractor) the negative ion density is strongly influenced by the stray magnetic field of the extractor magnets. This stray magnetic field divides the source into two distinct regions : a magnetic field-free region, in which the H- density is determined by volume production, and the region close to the plasma electrode, where the negative ion concentration is enhanced by diffusion from the surrounding plasma due to the unbalanced losses of electrons on the positively biased plasma electrode. For certain values of the bias potential the increase of H- density next to the plasma electrode is very large, leading to high extracted H- currents

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