The extraction of Hminus;ions from a volume source is greatly improved by the presence of a weak magnetic field parallel to the plasma electrode that magnetically insulates this electrode from the bulk plasma. When biased positive, the plasma electrode depletes the electron population in the neighboring region. To maintain plasma neutrality, negative ions from the main plasma replace the electrons in this region. Thus a high fraction of negative ions builds up in front of the plasma electrode. The implications of these phenomena for negative ion beam formation are discussed.
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