Development of an r.f. driven negative hydrogen ion source

A large r.f. driven negative hydrogen ion source has been constructed. R.f. negative ion sources have the potential of continuous and efficient operation for next-step neutral beam injection systems. The developed r.f. negative ion source is large for this fusion application. The r.f. plasma generator is a metal-walled multi-cusp bucket source with tent magnetic filter and has dimensions of 30 × 30 cm2 cross-section and 19 cm depth. An internal induction coil is used as an r.f. antenna. The negative ion accelerator, which consists of three grids, produces nine beamlets with an energy of 60 keV. The r.f. power amplifier system generates 2 MH – 30 kW for 1 s. The r.f. plasma was produced successfully under impedance matching condition. An ion current density of 150 mA cm−2 was obtained and the electron temperature was high, 5–10 eV, in the antenna vicinity. In the chamber center which is the negative ion production region, the electron temperature is still high, more than 3 eV. As a result, a negative ion current was not detected at an extraction energy of 1.2 keV.