Plasma characteristics of a large rf-driven negative hydrogen ion source

A large rf-driven negative hydrogen ion source has been developed for a neutral beam injection (NBI) system in future experimental fusion devices. Dimensions of the rf plasma generator are 30 x 30 x 20 cm(3). An induction coil antenna is fully immersed into the plasma generator. The rf-driven plasma with an electron density of 1.5 x 10(12) cm(-3) was generated at 28 kW rf power at a frequency of 2 MHz. The electron temperature in the extraction region decreases as the line-integrated magnetic filter field strength increases. An electron temperature in the extraction region of 1eV, which is suitable for generating negative ions, was obtained at a line-integrated filter field strength of 1030 G cm. Negative hydrogen ions are extracted from a single aperture of 13 mm in diameter in a pure-volume operation. An extracted negative ion current of 5.5 mA was obtained at 13.1 mTorr gas pressure and 15 kW rf power. This negativeion current corresponds to a power efficiency of 5.0 (mA/cm(2))/(W/cm(3)), which is superior to that in filament-are negative ion sources.