An ion source modified by an additional electrode, designated as a scraper electrode, positioned between the anode and extractor electrode, having an aperture slightly less than the source plasma aperture, and adapted to carry a pre-determined voltage in reference to the anode. It has been found that the scraper electrode intercepts and removes ions at the periphery of the beam cross-sectional area and this is the region that contains ions that contribute most greatly to the increased emittance. The central, non-intercepted cross-sectional area therefore has a much reduced emittance. The voltage on the scraper electrode should be such as to not disturb or distract the electric field between anode and extractor electrode.
A plasma ion source includes a discharge chamber in which a plasma is produced by plasma generator, with an acceleration electrode being disposed adjacent to the discharge chamber in order to extract ions from the produced plasma. A deceleration electrode is disposed adjacent to the acceleration electrode to decelerate the extracted ions, and a ground electrode is disposed adjacent to the deceleration electrode. An insulator container is disposed so as to surround the discharge chamber and the respective electrodes, and a shield ring electrode of ground potential is disposed in the vicinity of the deceleration electrode and along an inner wall surface of the insulator container in order to prevent any discharge from arising across the deceleration electrode and the ground electrode.
An electron gun is used with a mirror electrostatic field to produce zero or near zero velocity electrons by forming a turning point in their trajectories. A gas capable of attaching zero or near zero velocity is introduced at this turning point, and negative ions are produced by the attachment or dissociative attachment process. Operation may be continuous or pulsed. Ions thus formed are extracted by a simple lens system and suitable biasing of grids.
