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.

LBL-12255Hydrogen ion species and discharge characteristics have been compared for two different magnet geometries in a multicusp ion source. One magnet configuration indicated that the H; ion percentage in the extracted beam could be reduced by eliminating the high energy ionizing electrons near the ion extraction region. To accomplish this and maintain the desire features of both magnet geometries, a magnetic filter was installed near the source + exit. With this combined arrangement, we found that the H ion percentage and discharge condition were much improved and a more uniform profile across the extraction plane was achieved. The atomic species fraction can then be further enhanced by the addition of low energy (-16 eV) primary electrons into the source plasma chamber.

LBL-14955 +The characteristics of a multicusp source that can produce H 2 or + D 2 ion fraction greater than 80% is described.