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The article represents a review of direct extraction sources of negative hydrogen ions. During the past year or so interest in this method of producing high intensity beams of negative ions has increased considerably because of new developments resulting in currents of more than 20 mA. The first part of the article deals with inelastic collisions between particles which lead to the creation or destruction of negative ions. The knowledge of such elementary processes is relevant because the intensity of extracted H− currents is determined primarily by the equilibrium between the competing processes of creation and destruction. Attempts by several authors to analyze theoretically the production of H− ions in the plasma of a discharge will be explained subsequently and their results compared with experiments. Four types of direct extraction sources will be described next: duoplasmatrons, hollow discharge duoplasmatrons, Penning sources, and magnetron sources. Details of their design and beam characteristics will be given, as well as suggestions for further improvements.

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Maintaining the proper low work function surface in steady-state high-current-density negative ion sources is difficult, due to intense ion bombardment of the surface. Experiments were performed in which liquid cesium was forced through a porous molybdenum converter in order to obtain a low work function, while at the same time allowing the surface to be cooled. In a small steady-state hollow cathode discharge source, the H− yield from a porous molybdenum converter was five fold higher than from a solid molybdenum converter which relied on the conventional method of cesium coverage by vapor deposition.

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