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

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 cmX30 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/cm(2) 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. (C) 1996 American Institute of Physics.