Spatial uniformity of negative ion production in volume negative ion source.

A spatial uniformity of negative ion production has been studied experimentally in a tandem-type volume negative ion source, where the plasma is divided into two regions by a magnetic filter, i.e., a driver region and an extraction region near the plasma grid (PG). A plasma density profile and the negative ion beam profile were measured. By the Langmuir probe measurement, a strong nonuniformity of the plasma density was observed near the plasma grid when the plasma grid is biased electrically with respect to the anode. By changing the polarity of the bias voltage, the deviation of the plasma density profile changed to the direction of E x B. In the pure hydrogen operation, the extracted beam profile corresponded to the plasma density profile in the extraction region. In the Cs-seeded operation, on the other hand, the uniformity of the beam current density became uniform despite the nonuniformity of the plasma density profile in the extraction region. The uniformity of the negative ions corresponded to the plasma uniformity in the driver region. These results indicate that the negative ions are not produced by positive ions in the extraction region in the Cs-seeded operation. We conclude that the negative ions are produced mainly by neutral atoms produced in the driver region via the surface production on the PG. [ABSTRACT FROM AUTHOR]