Effect of argon seeding on the negative ion yield of the Kamaboko III ion source

It has been previously reported that the addition of argon to a hydrogen plasma in an RF driven ion source can substantially increase (up to a factor 4) the extracted and accelerated negative ion (H−) current (W. Kraus et al., Development of large radio frequency negative-ion sources for nuclear fusion, Rev. Sci. Instrum. 73(2) (2002)). Realizing such an increase in the filamented arc discharge negative ion sources used for neutral beam injection systems would have significant benefits. Unfortunately the reported studies of argon addition to filamented sources have not shown a similar gain, but so far these have been carried out with arc powers and plasma densities far from those typical of the plasma in the negative ion sources used on neutral beam injectors (N. Nishiura et al., Cooling effect of hydrogen negative ions in argon gas mixture, Rev. Sci. Instrum. 73(2) (2002); N. Curran et al., The effect of the addition of noble gases on H- production in a dc filament discharge in hydrogen, Plasma Scources, Sci. Technol. 9 (2000)). The Kamaboko III ion source operates at the pressure and plasma density close to those anticipated in the ion source proposed for the ITER neutral beam injectors. Measurements have been made of the plasma density, electron temperature and the negative ion yield as a function of the argon seeding rate. The plasma parameters are determined with a fast spatially scanning Langmuir probe system. The effect on the H− yield is determined from the effect on the current extracted and accelerated from the source. Data will be presented for source filling pressures between 0.1 and 0.5 Pa of hydrogen, additions of argon from 0 to 30%, and a discharge power of 38 kW. All these data are collected in pure volume non-caesiated discharges. Some increase in the extracted H− yield is measured for small percentage additions of argon, (0–20%), but only at the highest H2 pressure used, 0.5 Pa.