H⁻ density profile and response to applied bias and extraction voltages in H⁻ source

Characteristics of negative-hydrogen ion (H⁻) density in the vicinity of plasma grid (PG) which is a boundary electrode between plasma and beam were experimentally investigated in cesium-seeded H⁻ source. The H⁻ density was measured with Cavity Ring Down method (CRD). Our CRD system has been upgraded from fixed line measurement to movable one which provides a profile measurement of the H⁻ density. The H⁻ density above the PG aperture is lower than that above the PG metal surface, and this density structure become to disappear in further region from the PG surface. The H⁻ density decreases with positive bias voltage where an arc discharge chamber is higher potential than the PG. On the other hand, the H⁻ density does not largely change with negative bias voltage. Reduction of the H⁻ density was observed when a beam extraction voltage is applied. The reduction occurs in the case of lower bias voltage close to plasma potential. The extraction voltage influences H⁻ density to a greater degree than bias voltage in low bias voltage region.