Extraction of volume-produced H- ions from a multicusp source

H- ions formed by volume processes are extracted from a multicusp ion source. It is shown that a permanent magnet filter together with a small positive bias voltage on the plasma grid can produce a very significant reduction in electron drain as well as a sizable increase in H- ions available for extraction. A further reduction in electron current is achieved by installing a pair of ceramic magnets at the extraction aperture. The combined arrangement improves the ratio of extracted H- ion current to electron current to about unity