Cesium injection into a large rf-driven hydrogen negative-ion source

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Abstract
A large rf-driven hydrogen negative-ion source has been developed for a neutral beam injection (NBI) system in the fusion application. The rf plasma generator is a metal-walled multicusp bucket source, the dimensions of which are 30 cm×30 cm in cross section and 20 cm in depth. An induction coil antenna is fully immersed in the rf plasma generator, and is used for coupling the rf power into plasma. A strong external magnetic filter is generated in front of the plasma electrode for negative-ion production, and a filter field strength of more than 1000 G cm is required to reduce the electron temperature in the extraction region as low as 1 eV. Precisely 5.5 mA of a negative-ion current was obtained from a single aperture, corresponding to 4.1 mA/cm2 of current density. A small amount of cesium was supplied to the rf plasma generator from a cesium oven. As a result, a lowering of the operational gas pressure and improvement of the negative-ion production efficiency were observed. The optimum bias voltage was reduced by the cesium injection, which may be related to the reduction of the plasma potential.

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